what is tissue .....explain

Introduction

The human body consists of around 50 trillion to 106 trillion individual structural working
units called cells (Marieb and Hoehn, 2008). Cells work together to ensure that homeostasis
is maintained.
Cells come in many different shapes, sizes and life spans; however, they can
be categorised
depending on their structure and functions. A group of cells that have a
similar structure and function are called tissue, and within the human body there are four
distinct types of tissue. Cells that provide a covering for organs and structures, for example,
are referred to as epithelial tissue, whereas cells that provide support for structures are
called connective tissue. Cells that govern body movement are muscle tissue, and cells that
help control homeostasis are nervous tissue. Most organs of the body contain a selection of
all four tissue types. The heart, for example, contains muscle tissue, is controlled by nervous
tissue, lined by epithelial tissue and supported by connective tissue. Tissue also has the
capacity to repair itself. This chapter examines
all four types of tissue and the process of
tissue
repair.

Epithelial tissue

Epithelial tissue is essentially a sheet of cells that covers an area of the body. Epithelial tissue
covers or lines body surfaces (i.e. skin), or it lines the walls and the organs within body cavities.
The major role of epithelial tissue is as an interface; indeed, nearly all the substances absorbed
or secreted by the body must pass through epithelial tissue. Broadly speaking, epithelial tissue
has six main functions:
•• absorption
•• protection
•• excretion
•• secretion
•• filtration
•• sensory reception.
Not all epithelial tissue carries out all six functions. In many areas of the body epithelial tissue
specialises in just one or two functions. Epithelial tissue in the digestive system, for example,
specialises in absorption of nutrients, whereas epithelial tissue within skin provides a protective
layer.
Epithelial tissue cells are closely bonded together in continuous sheets, which have an
apical
and a basal surface. The apical surface faces outwards, towards the exterior of the
organ it covers. Apical surfaces can be smooth, but most have hair‐like extensions called
microvilli. Microvilli dramatically increase the surface area of the epithelial tissue and therefore
increase its ability for absorption and secretion. Some areas, within the respiratory tract for
example, possess larger hair‐like extensions called cilia, which are also capable of propelling
substances. Lying close to the basal surface is a thin sheet of glycoproteins that acts as a
selective filter, governing which substances can enter epithelial tissue. Epithelial tissue is
innervated by neurones, but it has no blood supply as such. Rather than being served by a
network of capillaries, epithelial tissue receives a supply of nutrients from nearby blood
vessels.
Owing to its protective role, epithelial tissue needs to endure a great deal of abrasion
and environmental damage, and epithelial cells need to be very hardy and tough. This hardiness
is generated by their ability to divide and regenerate rapidly, resulting in the swift
replacement of damaged epithelial cells. However, this regenerative capacity is reliant upon
a plentiful supply of nutrients.
Epithelial tissue can be categorised into the following three distinct types:
•• simple
•• stratified
•• glandular.
Simple epithelium consists of a single layer of cells bound into a continuous sheet. Stratified
epithelium is also arranged into a continuous sheet but is thicker with numerous layers of cells.
Glandular epithelium forms the glands of the body.
All epithelial cells have six sides; indeed, under a microscope a cross‐section of epithelial
tissue
looks like a honeycomb. Epithelial cells can be subdivided further into the following three
different six‐sided shapes:
•• cuboidal
•• columnar
•• squamous.
As their names suggest, cuboidal and columnar epithelial cells are square and tall respectively,
whereas squamous epithelial cells are rather flat and scaly (see Figure 4.1). When examining the
many different types of epithelial cell it is easy to work out its size and shape by its name.
For instance, simple squamous epithelium is thin, flat and scale‐like.

Simple epithelium

Because simple epithelia consist of a single cellular layer it specialises in absorption, secretion
and filtration rather than protection.
Simple squamous epithelium is quite often very permeable and is found where the diffusion
of nutrients is essential. Capillary walls, the alveoli of the lungs and the glomeruli in the kidneys
are all lined with simple squamous epithelium, which facilitates the rapid diffusion of nutrients.
Simple squamous epithelium is also found within the heart and blood and lymph vessels. Simple
squamous epithelium found within the heart and blood and lymph vessels is called endothelium
Simple cuboidal epithelium specialises in secretion as well as absorption. Simple cuboidal
epithelium is found in the lining of the ovaries, the kidney tubules and the ducts of smaller
glands. It also forms part of the secretory portions of glands such as the thyroid and pancreas
(see Figure 4.3).
Simple columnar epithelium can be ciliated or non‐ciliated. As its name suggests, ciliated
simple columnar epithelium has cilia on its apical surface. It is found in areas of the body where
movement of fluids, mucus or other substances is required. Ciliated simple columnar epithelial
tissue, for example, lines the passageways of the central nervous system and helps propel cerebrospinal
fluid. It also lines the Fallopian tubes and helps move oocytes recently expelled from
the ovaries (see Figure 4.4). A common location for non‐ciliated simple columnar epithelium is
the lining of the digestive tract from the stomach to the rectum (see Figure 4.5). Non‐ciliated
simple columnar epithelium performs two broad functions. Some possess microvilli, greatly
increasing their surface area for absorption; others specialise in the secretion of mucus. Such
cells are referred to as goblet cells owing to their cup‐like shape. Simple columnar epithelial cells
are generally of equal size. However, in some instances simple columnar epithelial cells vary in
height, with only the tallest reaching the apical surface. This gives the illusion that the tissue has

Stratified epithelium

Unlike simple epithelia, stratified epithelia have many layers. The cells regenerate from below,
with new cells dividing on the basal layer pushing the older cells towards the surface. As stratified
epithelium is thicker, its principal function is protection.
Stratified squamous epithelium is the most common stratified epithelium and forms the
external part of skin (see Chapter 17). Stratified squamous epithelial tissue is keratinised,
toughened
by the presence of keratin, a special tough fibrous protein. Non‐keratinised stratified
squamous epithelial tissue lines wet areas of the body – the mouth, the tongue and the vagina
for example (see Figure 4.7). Only the outer layers of stratified squamous epithelium are actually
squamous in shape; the basal layers may be cuboidal or columnar.
Stratified cuboidal epithelium is found in the oesophagus, sweat glands and in the male
urethra (see Figure 4.8). Stratified columnar epithelium, however, is quite rare. Small amounts
can be found in the male urethra and in the ducts of some glands. Another common example of
stratified epithelium is transitional epithelium, which may have both squamous and cuboidal
cells in its apical surface. The basal surface may contain both cuboidal and columnar cells.
Transitional epithelium can withstand a great deal of stretch and is found in organs such as the
bladder, which is subject to considerable distension (see Figure 4.9)

Glandular epithelia

The glands of the body are formed by glandular epithelia. All glands are classified as endocrine
or exocrine. Glands that secrete their products internally are called endocrine glands. Endocrine
glands release hormones, regulatory chemicals for use elsewhere in the body (see Chapter 15).
Exocrine glands release their products onto the surface of epithelial tissue. Exocrine glands are
either unicellular or multicellular. Unicellular exocrine glands consist of a single cell type and the
main example is the goblet cell, which releases a glycoprotein called mucin. Once dissolved in
water mucin forms mucus, which lubricates and protects surfaces. Multicellular exocrine glands
are far more complex, coming in several shapes and sizes. Some exocrine glands are simple and
consist of a single branched duct, whereas others are more complex with multibranched ducts
(see Figure 4.10). However, they all contain two distinct areas: an epithelial duct and secretory
cells (acinus). Exocrine glands that are tubular in shape can be found within the digestive system
and stomach. Other exocrine glands are spherical and referred to as alveolar or acinar. The oil
glands within skin and mammary glands are two examples of spherical‐ or acinar‐shaped
exocrine
glands. Glands that are both tubular and acinar are referred to as tubulacinar.
The salivary
glands, for example, are tubulacinar.

Connective tissue

Connective tissue is the most abundant tissue in the human body. Its main functions are to bind
tissues together, reinforcement, insulation, protection and support. All epithelial tissue is
reinforced
by the connective tissue base it rests upon (see Figure 4.11). There are four types of
connective tissue:
•• connective tissue proper
•• cartilage
•• bone
•• liquid connective tissue.
Connective tissue is not present on body surfaces and, unlike epithelial tissue, is highly
vascular
and receives a rich blood supply.

The following types of cell are present in connective tissue

•• adipocytes
•• primary blast cells
•• macrophages
•• plasma cells
•• mast cells
•• leucocytes (white blood cells).
Adipocytes are fat cells. Within connective tissue adipocytes store triglycerides (fats). Primary blast
cells continually secrete ground substance and produce mature connective tissue cells. Each type of
connective tissue contains its own unique primary blast cells (see Table 4.1). Macrophages, plasma
cells and white blood cells form part of the body’s immune system. Their functions are as follows:
•• Macrophages engulf invading substances and plasma cells produce antibodies.
•• White blood cells are not normally found in significant numbers within connective tissue;
however, they do migrate into connective tissue during inflammation.
•• Mast cells produce histamine, which promotes vasodilatation during the body’s inflammatory
response.

the skeletal system

Introduction

Despite what seems to be a solid, dry, inert material, bone is in fact a complex living organism
that is being recreated constantly; bone is metabolically active. As old bone dies new bone is
being rebuilt. There are a number of complex activities occurring as bone is destroyed and
reformed. Bones are therefore living organs that are made up of a number of different tissues,
and this includes bone tissue.
The human skeleton, in contrast to other skeletons, is built to move erect as opposed to
walking
on all fours. The skeleton provides us with shape and the power to move, but it cannot
do this in isolation. It needs many other systems of the body for it to function properly – for
example, the nervous system and the muscles and for the body to move in its various and
complex
ways (the spine, for example, allows us to twist and bend); this is attributed to the joints
and their ability to articulate.
Like a house, the human body needs a framework, but the framework for the body is not
made of wood and steel as is the case with the house. The skeletal system is made up of bones,
ligaments and tendons. The human skeleton is built to take the hard knocks of life. It is an
engineering
wonder; for its weight, bone is nearly as strong as steel.
The skeleton produces blood cells. The bones also act as storage areas for minerals, vital for
blood clotting, nerve function and contraction of muscles. The bones begin to form in utero and
continue to grow into adulthood. Bones develop from cartilage, so infants are born with large
amounts of cartilage as well as having more bones than adults. As the child ages, the bones usually
fuse together and the child ends up with the normal adult number of bones. The bones of babies
are soft, but as more minerals are deposited they become harder – this is known as ossification.

The axial and appendicular skeleton

There are 206 named bones in the adult human skeleton. For classification purposes the skeleton
is divided into two parts: the axial skeleton and the appendicular skeleton. Both have their own
purposes

BOON of the axila 

Skull

Cranium
Face
Total
8
14
22
Hyoid 1
Auditory ossicles (bones) 6
Vertebral column 26
Thorax
Sternum
Ribs
Total
1
24
25
Total number of bones in the axial skeleton 80

The axial skeleton

The axial skeleton forms the central axis of the body and consists of 80 bones. This part of the
skeleton supports the head (including the bones in the ear), neck and the torso (this is also
referred to as the trunk). It consists of the skull, the vertebral column, the ribs and the sternum.
The 80 bones in the axial skeleton are noted in Table 5.1.

The appendicular skeleton

The bones of the appendicular skeleton are those bones of the upper and lower extremities –
the arms and the legs as well as the bones that attach them to the axial skeleton. There are
126 bones in the appendicular skeleton, and these bones are shown in Table 5.2.
See Figure 5.1 depicting the human skeleton.

Bone and its functions

The skeletal system – and this includes the bones of the skeleton, the ligaments, cartilage and
connective tissues that provide stability or attach the bones – has a number of key functions:
1. provides support
2. enables movement
3. stores minerals and lipids
4. protects the body
5. produces blood cells.

Support

Apart from bone and cartilage, all body tissue is soft, and without the skeleton the body would
be jelly‐like and would not be able to stand up. The way the bones are arranged provides the
body with its shape/form. The skeletal system provides structural support for the body, providing
a bony framework for the attachment of soft tissues and organ

THE BOON OF THE APPENDICULAR

Pectoral girdle

Clavicle

Scapula

Total

2

2

4

Upper limbs

Humerus

Ulna

Radius

Carpals

Metacarpals

Phalanges

Total

2

3

4

16

10

28

60

Pelvic girdle

Pelvic bone

2

Lower limbs

Femur

Patella

Fibula

Tibia

Tarsals

Metatarsals

Phalanges

Total

2

2

3

3

14

10

28

60

Total number of bones in the appendicular skeleton 126

Total number of bones in the adult human skeleton 206

Movement

The skeleton allows and enables movement. The bones act as levers, providing the transmission

of muscular forces. A number of bones can (through leverage, contracting and pulling) change

the extent and direction of the forces generated by skeletal muscles, through the work of the

tendons and the ligaments. These movements can be very intricate, such as the ability to write,

the ability to thread a needle (the coordination of fine movement), to gross movement, such

as the ability to change body posture. The skeleton with the interaction of muscles permits

breathing

to occur. Movement becomes possible through articulation

CELL DIVISION THE TRANSFERENCE OF GENS

The transference of genes

This section discusses how genetic information is transferred from cells to new cells, and also
from parents to children. The first thing to do is to look at how cells pass on genetic information
to new cells.
In order for the body to grow, and also for the replacement of body cells that have died, our cells
must be able to reproduce themselves, but in order for genetic information not to be lost,
they must be able to reproduce themselves accurately. They do this by cloning themselves.
In some prokaryotic organisms this occurs by binary fission, whereby the nucleus in a single
cell becomes elongated and then divides to form two nuclei in the same cell, each of which
carries identical genetic information. The cytoplasm then divides in the middle between the
two nuclei, and so two identical daughter cells result, each with its own nucleus and other
essential organelles.
However, humans, being much more complicated, have eukaryotic cells, which divide by
means of cell division, whereby the division of the nucleus occurs first of all, after which the division
of cytoplasm (known as cytokinesis) takes place. After this division, the new cells will grow
until they reach a stage when the process can be repeated.
Within this process of cell division, the process of transference of genes (or reproduction of
cells carrying genetic information) is divided into two stages: mitosis and meiosis.

Mitosis

This section commences by looking at the way that cells reproduce, particularly how they reproduce
their genetic material.
In humans, cell reproduction takes place using a complex process called mitosis, in which
the number of chromosomes in the daughter cells has to be the same as in the original
parent cell.
In the figures below, only a few of the chromosomes are depicted in order to improve the
clarity of the figures.
Mitosis can be divided into four stages:
•• prophase
•• metaphase
•• anaphase
•• telophase.
Before and after it has divided, the cell enters a stage known as interphase until the time comes
for the next cell reproduction.

Interphase

Mitosis begins with interphase. This was often thought to be a resting period for the cell, but
we now know that the cell is actually very busy during this period getting ready for replication.
If we look at the cell cycle and suppose that one full cycle represents 24 hours, then the actual
process of replication (mitosis) would only last for about one of those 24 hours (Figure 3.9).
During the rest of the time the cell is undertaking DNA synthesis (i.e. producing DNA). During
this period of interphase the cell has to produce two of everything, not just DNA, but all the
other organelles in the cell (see Chapter 2), such as the mitochondria. In addition, the cell has to
go through the process of obtaining and digesting nutrition so that it has the raw materials for
this duplication and also for the energy that will power the various functions of the cell

interphase, the chromosomes in the nucleus are very difficult to see because they are

in the form of long threads. They need to be in this state to make it easier for them to be duplicated.
During the process of duplication, the cells have to ensure that there will be sufficient and
accurate genetic material for each of the two ‘daughter cells’. The strands of DNA separate and
reattach to new strands of DNA. Because of the selectivity of the bases as to which other base
they are able to join in this process, an exact replication of the DNA occurs (Figure 3.5).
In addition, extra cell organelles are manufactured or produced by the replication of existing
organelles. Also during interphase, the cell builds up a store of energy, which is required for the
process of division.

Prophase

The first stage after interphase is prophase. During prophase, the chromosomes become
shorter, fatter and more visible. Each chromosome now consists of two chromatids, each containing
the same genetic information (in other words, the DNA has exactly replicated itself during
interphase). These two chromatids are joined together at an area known as the centromere.
The two centrosomes move to opposite ends of the cell (the poles) and are joined together by
the nuclear spindle, which stretches from end to end (or pole to pole) of the cell. The centre of
the cell is now called the equator. Finally, the nucleolus and nuclear membrane disappear,
leaving the chromosomes within the cytoplasm.

Metaphase

During metaphase, the 46 chromosomes (two of each of the 23 chromosomes) each consisting
of two chromatids move to the equator of the nuclear spindle, and here they become attached
to the spindle fibres.

Anaphase

During anaphase, the chromatids in each chromosome are separated, and one chromatid
from each chromosome then moves towards each pole of the spindle.

Telophase

There are now 46 chromatids at each pole, and these will form the chromosomes of the daughter
cells. The cell membrane constricts in the centre of the cell, dividing it into two cells. The nuclear
spindle disappears, and a nuclear membrane forms around the chromosomes in each of the
daughter cells. The chromosomes become long and thread-like again.

Cell division

Cell division is now complete (Figure 3.10) and the daughter cells themselves enter the interphase
stage in order to prepare for their replication and division.
This process of cell division explains how we grow by producing new cells as well as replacing
old, damaged and dead cells.

Meiosis

Whereas mitosis is concerned with the reproduction of individual cells, meiosis is concerned
with the development of whole organisms (e.g. human beings).
The reproduction of a human being depends upon the fusion of reproductive cells (known as
gametes) from each of the parents. These gametes are:
•• spermatozoa (sperm) from the male;
•• ova (eggs) from the female.
Each cell of the human body contains 23 pairs of chromosomes (i.e. 46 in total). It is very
important that during the process of human reproduction the cell formed when the gametes
fuse has the correct number of chromosomes for a human being (23 pairs). Therefore, each
gamete must possess only 23 single chromosomes, because when gametes fuse during
reproduction all their chromosomes remain intact in the new life form. If each gamete had a
full complement of 46 chromosomes, then the resulting fused cell would possess 92 chromosomes
– or four copies of each chromosome rather than the two that a human cell should possess.
From then on, each succeeding generation would have double the number of chromosomes,
so that after several generations humans would have cells that possess millions and millions
of copies of the 23 chromosomes. To stop this happening, the gametes only possess one
copy of each chromosome, so that the resulting fused cell has 46 chromosomes, like
the parents.
Now you have two new terms to learn and understand: diploid and haploid cells.
•• Diploid cell: a cell with a full complement of 46 chromosomes (i.e. 23 pairs).
•• Haploid cell: a cell with only half that number of chromosomes (i.e. 23 single
chromosomes).
Gametes are therefore haploid cells, because they only possess one copy of each chromosome,
while all other cells of the body are diploid cells.
Gametes actually develop from cells with 46 chromosomes, and it is through the process
of meiosis that they end up with just

Mendelian genetics

So far this chapter has examined the biology of genetics, and now it is going to look at the role
of genetics in inheritance. This is very important because, as stated previously, what we are is
designated to a large extent by our genetic make-up – which is inherited from our parents. The
caveat ‘to a large extent’ is because as well as being a product of our genes we are also a product
of our environment – time, space, relationships, education, and so on.
So how do we inherit our genes from our parents? To understand this we have to return to the
1860s. In Brno (which is now a large town in the Czech Republic but was then a small, sleepy
town in Bohemia) there was a monastery, and in that monastery there lived and worked a monk
with a very inquiring mind. His name was Gregor Mendel and he worked in the monastery
gardens
where he put his mind to good use trying to perfect the ideal pea. As part of this work,
he experimented with cross‐breeding. Now, at that time, cross‐breeding went on everywhere –
on farms and in gardens; and of course, we humans cross‐breed as well. However, what was
different about Mendel was that not only did he experiment with cross‐breeding different peas,
but he also made notes on his experiments and observations. He introduced three novel
approaches to the study of cross‐breeding – at least novel for his time, because no one else was
doing this. Not only did he observe, but he experimented and observed. Having observed and
experimented he then used statistics. He ensured that the original parental stocks, from which
his crosses were derived, were pure breeding stocks (the use of statistics was not at that time
fully part of the tradition of biology).

chromosomes

Chromosomes

First of all, look again at the previous brief discussion of chromosomes to be found near the
beginning of this chapter. In actual fact, the chromosome does not consist of just DNA. Instead,
the nuclear DNA (also known as nucleic acid) of eukaryotes is combined with protein molecules
known as histones. Note that a eukaryote is any organism whose cells contain a nucleus and
other organelles enclosed within membranes (see Chapter 2 for more about the cell).
The DNA and histones together make up the nucleosomes contained within the cell nucleus.
This nucleic acid–histone complex is known as chromatin.
Now we run into a problem: if we unravelled all the nucleic acid from every cell in a human
adult body it would stretch to the Moon and back about 8000 times. So how do we manage to
package that number of DNA and histone molecules into our rather small bodies? The answer,
of course, is that we have to fold them so that they fit into each cell of the body – just like having
to fold clothes to ensure that they fit into a suitcase when going on holiday. And just as clothes
often will only fit in the suitcase if they are neatly folded, the same applies to the chromatin in
our cells. It cannot just be pushed in haphazardly – it would never fit and there would be a great
possibility of things going wrong.
So, in order to fit within our cells, the chromosomes twist on one another, then twist into
loops, before finally assuming the shape that is commonly recognised as a chromosome – the X
shape which is easily seen in a human cell (Figure 3.3) (Jorde et al., 2009).
Let us look in more detail at chromosomes. Each chromosome is made up of two chromatids
joined by a centromere. Looking at Figure 3.3, you can see that one half of the chromosome is
a chromatid, and where they join near the top of the X, that is the centromere.
In most humans, each nucleated cell (i.e. each cell with a nucleus) within the body has
46 chromosomes, arranged in 23 pairs (Figure 3.4). Of those 23 pairs, one pair determines
the gender of the person.
•• Females have a matched homologous (means ‘the same’) pair of X chromosomes.
•• Males have an unmatched heterologous (means ‘different’) pair – one X and one Y
chromosome.
•• The remaining 22 pairs of chromosomes are known as autosomes. In biology the word
‘some’ means body, so autosome means ‘self body’. Thus, ‘autosome’ can be defined as the
chromosomes that determine physical/body characteristics – in other words, all the
characteristics of a person that are not connected with gender.
The position a gene occupies on a chromosome is called a locus, and there are different loci
for colour, height, hair, and so on (‘loci’ is the plural of ‘locus’). Think of the locus as the address of
that particular gene on Chromosome Street – just like your address signifies that that is where
you live.Genes that occupy corresponding loci are called alleles. So, the gene for the same characteristic
on each of the two chromatids is an allele. Alleles are found at the same place in each of the two
corresponding chromatids, and an allele determines an alternative form of the same characteristic.
Remembering that one of your chromatids comes from your mother and the other corresponding
chromatid comes from your father may be of help in understanding this. As an example, think of
the colour of eyes. There is one particular gene that determines eye colour and it is found at the
same place on each of the two chromatids of one chromosome. One gene will come from the
father and the other from the mother. If parents of a child have different coloured eyes from each
other, perhaps the mother has green eyes and the father brown eyes, then the child may have
green or brown eyes, depending upon factors that will be discussed later in this chapter.
So each of these particular genes at that same point (or locus) on each chromatid determines
eye colour. This applies to every one of a person’s characteristics. A person with a pair of identical
alleles for a particular gene locus is said to be homozygous for that gene, while someone with
a dissimilar pair is said to be heterozygous for that gene

what is cell celullar compartment transport system fluid moment between compartment..

Introduction

Cells are the structural and functional units of all living organisms. Some organisms, such as
bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are
multicellular,
indicating that humans are made up of many cells (see Figure 2.1). They are the
smallest independent units of life with different parts (see Figure 2.2) that perform their own
function (see Table 2.1). For the cells to survive some fundamental chemical activities occur
within the cell. Some of these activities include cellular growth, metabolism and reproduction.
Each cell is an amazing unit of life; it can take in nutrients, convert these nutrients into energy,
carry out specialised functions and reproduce as necessary. Most amazingly, each cell stores its
own set of instructions for carrying out each of these activities.
Substances such as water, electrolytes and nutrients move in and out of a cell utilising a transport
system. There is constant movement of fluid and electrolytes between the intracellular and
extracellular compartments. The movement of fluid and electrolytes ensures that the cells
receive a constant supply of electrolytes, such as sodium, chloride, potassium, magnesium,
phosphates, bicarbonate and calcium, for cellular function. The cell consists of four basic parts:
•• cell membrane
•• cytoplasm
•• nucleus
•• nucleoplasm.

Cell membrane

Like all other cellular membranes, the plasma membrane consists of both lipids and proteins.

The fundamental structure of the membrane is the phospholipid bilayer, which forms a stable

barrier between two aqueous compartments. In the case of the plasma membrane, these

compartments

are the inside and the outside of the cell. Proteins embedded within the

phospholipid

bilayer carry out the specific functions of the plasma membrane, including selective

transport of molecules and cell–cell recognition.

The cell membrane is a thin membrane that forms the outermost layer of a cell, and it is also

called the plasma membrane. This membrane ensures the boundary and integrity of the cell and

that its contents are separated from the surrounding environment. The cell membrane contains

a variety

of biological molecules, mainly proteins and lipids, which are involved in many cellular

functions, such as cellular communication and cellular transport. The cell membrane is made up

of a double layer (bilayer) of phospholipid (fatty) molecules with protein molecules interspersed

between them (see Figure 2.3). The cell membrane can vary from 7.5 to 10 nm (nanometres) in

thickness (Jenkins and Tortora, 2013).

The phospholipid bilayer consists of a polar ‘head’ end, which is hydrophilic (water loving),

and fatty acid ‘tails’, which are hydrophobic (water hating). The hydrophilic heads are situated on

the outer and inner surfaces of the cell, while the hydrophobic areas point into the cell membrane

(see Figure 2.3) as they are ‘water‐hating’ ends. These phospholipid molecules are arranged

as a bilayer with the heads facing outwards. This means that the bilayer is self‐sealing. It is the

central part of the plasma membrane, consisting of hydrophobic ‘tails’, that makes the cell membrane

impermeable to water‐soluble molecules, and so prevents the passage of these molecules

into and out of the cell (Marieb and Hoehn, 2013). However, substances need to enter and leave

the cells for the cells to survive and function; these are provided by special proteins, such as

integral and peripheral membrane proteins (see Figure 2.3).

The integral transmembrane proteins are attached to the cell membrane and they can form

channels that allow for the transportation of materials into and out of the cell. Examples of integral

transmembrane proteins include voltage‐gated ion channels, such as those that transport

potassium ions in and out of cells, certain types of T cell receptors, the insulin receptor, and many

other receptors and neurotransmitters. On the other hand, peripheral membrane proteins are

Functions of the cell membrane

Cell membranes serve several important functions:

•• They are selective semi permeable membranes, which means that some molecules can diffuse

across the lipid bilayer but others cannot. Small hydrophobic molecules and gases like oxygen

and carbon dioxide cross membranes rapidly. Small polar molecules, such as water and

ethanol, can also pass through membranes, but they do so more slowly. On the other hand,

cell membranes restrict diffusion of highly charged molecules, such as ions, and large

molecules, such as sugars and amino acids. The passage of these molecules relies on specific

transport proteins embedded in the membrane.

•• Membrane transport proteins are specific and selective for the molecules they move, and

they often use energy to catalyse passage. Also, these proteins transport some nutrients

against the concentration gradient, which requires additional energy. The ability to maintain

concentration gradients and sometimes move materials against them is vital to cell health

and maintenance. Thanks to membrane barriers and transport proteins, the cell can

accumulate nutrients in higher concentrations than exist in the environment and, conversely,

get rid of waste products (Figure 2.4).

•• Other transmembrane proteins have communication‐related jobs. These proteins bind

signals, such as hormones or immune mediators, to their extracellular portions. Binding

causes a conformational change in the protein that transmits a signal to intracellular

Cellular fluid compartments

The two principal body fluid compartments are intracellular and extracellular. The intracellular

compartment is the space inside a cell and the fluid inside the cell is called intracellular fluid

(ICF). The extracellular compartment is found outside the cell and the fluid outside the cell is

called extracellular fluid (ECF). However, the extracellular compartment is further divided into```

Intracellular fluid

•• The ICF is primarily a solution of potassium and organic anions, proteins, and so on.

•• The cell membranes and cellular metabolism control the constituents of this ICF.

•• ICF is not consistent in the body. It represents a collection of fluids from all the different cells

Extracellular fluid

•• The ECF primarily consists of NaCl and NaHCO3 solution.

•• The ECF is further subdivided into three compartments:

•• Interstitial fluid (ISF) consists of all the bits of fluid that lie in the interstices of all body

tissues. This is also a ‘virtual’ fluid (i.e. it exists in many separate small bits but is spoken

about as though it is a pool of fluid of uniform composition in the one location). The ISF

bathes all the cells in the body and is the link between the ICF and the intravascular

compartment. Oxygen, nutrients, wastes and chemical messengers all pass through the ISF.

ISF has the compositional characteristics of ECF (as mentioned above), but in addition it is

distinguished by its usually low protein concentration (in comparison with plasma). Lymph

is considered as a part of the ISF. The lymphatic system returns protein and excess ISF to the

circulation. Lymph is more easily obtained for analysis than other parts of the ISF.

•• Plasma is the only major fluid compartment that exists as a real fluid collection all in

one location. It differs from ISF in its much higher protein content and its high bulk

flow (transport function). Blood contains suspended red and white cells, so plasma has

been called the ‘ISF of the blood’. The fluid compartment called the blood volume is

interesting in that it is a composite compartment containing ECF (plasma) and ICF (red

cell water).

•• Transcellular fluid is a small compartment that represents all those body fluids that are

formed from the transport activities of cells. It is contained within epithelial‐lined spaces. It

includes cerebrospinal fluid, gastrointestinal tract fluids, bladder urine, aqueous humour

and joint fluid. It is important because of the specialised functions involved. The fluid fluxes

involved with gastrointestinal tract fluids can be quite significant.

•• The fluid of bone and dense connective tissue is significant because it contains about 15%

of the total body water. This fluid is mobilised only very slowly, and this lessens its

importance when considering the effects of acute fluid interventions.

Snapshot

Loss of fluid and electrolytes through burns

Boon Sew, a 48‐year‐old male, was brought in by ambulance to the emergency department after

being rescued from his burning house. He was asleep at night when a spark from the family fireplace

started a fire, leaving him trapped in his bedroom. By the time the fire rescue team arrived, he

had suffered severe burns and excessive smoke inhalation.

On arrival to the emergency department, he was unconscious. He had second‐degree burns

over 5% of his body and third‐degree burns over 20% of his body – both covering his thoracic and

abdominal regions and his right elbow. His vital signs were quite unstable: blood pressure was

53/35 mmHg; heart rate was 200 beats/min; and respiratory rate was 38 breaths/min. He was deteriorating

from circulatory failure. Two intravenous lines were inserted and fluids and electrolytes

were administered through each. His vital signs stabilised and he was admitted to the intensive

care unit.

Boon Sew regained consciousness the following day and was able to respond verbally. Once his

condition was stable and he was able to respond to treatment, he was transferred to the ward

where he continued to make a good recovery. He was then discharged into the community after

making a full recovery.

Fluid movement between compartments

The movement of fluid between the intracellular and the extracellular compartments is primarily

controlled by two forces:

•• hydrostatic pressure – the pressure exerted by the fluid;

•• osmotic pressure – the pressure that must be exerted on a solution to prevent the passage of

water into it across a selective permeable membrane.

Furthermore, the movement of fluid is dependent on solutes dissolved within the fluid; changes

in the concentration of solutes will affect fluid movement between compartments. Similarly,

changes in fluid volume will also affect fluid movement between compartments. An example of

fluid and solute movement that occurs in the body is when blood pressure (hydrostatic pressure)

forces fluid and solutes from the arterial end of the capillaries into the ISF space (see Figure 2.7).

Fluid and solutes return to the capillaries at the venous end as a result of the osmotic pressure.

Fluid also enters the lymphatic capillaries from the interstitial space as a result of the osmotic

pressure

in the lymphatic vessels.

The movement of fluid between the intracellular and extracellular compartments is the

result of hydrostatic and osmotic pressures. In a normal state of health, the hydrostatic pressure

in the intracellular compartment and the interstitial space is in balance and therefore

the fluid movement is minimal. However, changes in the osmotic pressure either in the intracellular

or extracellular compartments can affect fluid movement. As the capillary barrier is

readily permeable to ions, the osmotic pressure within the capillary is principally determined

by plasma proteins that are relatively impermeable. Therefore, instead of speaking of ‘osmotic’

pressure, this pressure is referred to as the ‘oncotic’ pressure or ‘colloid osmotic’ pressure

because it is generated by colloids. Albumin generates about 70% of the oncotic pressure.

This pressure is typically 25–30 mmHg. The oncotic pressure increases along the length of

Composition of body fluid

Composition of body fluid

The body fluid is composed of water and dissolved substances such as electrolytes (sodium,

potassium and chloride), gases (oxygen and carbon dioxide), nutrients, enzymes and hormones.

The total body water constitutes 60% of the total body weight, and water plays an important

part in cellular function (LeMone et al., 2011). Water is essential for the body as:

•• It acts as a lubricant, which makes swallowing easy.

•• It is also the major component of the body’s transport systems. The blood transports

nutrients, oxygen, glucose and fats to various tissues and cells. Also, the waste products of

cellular metabolism are removed, such as lactic acid and carbon dioxide. Via the urine, a

number of waste products are transported out of the body; for example, urea, phosphates,

sulfites, minerals, ketones from fat metabolism and nitrogenous waste from protein

breakdown.

•• It is needed for regulation of body temperature at 37 °C. When body temperature starts to

rise, blood vessels near the surface of the skin dilate to release some of the heat; the reverse

happens when body temperature starts to drop. Also, when body temperature rises, sweat

glands secrete sweat, which is 99% water. As the sweat evaporates, heat is removed from

the body.

•• It provides an optimum medium for the cells to function.

•• There are chemical reactions in the body which require water. A synthesis reaction

involves the joining of two molecules by the removal of a water molecule, and a

hydrolysis reaction involves a molecule being split into two smaller molecules with the

addition of water.

•• It breaks down food particles in the digestive system.

•• It provides lubrication for the joints as it is a component of synovial fluid. It is also a component

of tears, which lubricate the eyes, and of saliva to provide lubrication to food, which aids

chewing, swallowing and digestion of food. It also has protective roles, washing away

particles that get into the eyes, providing cushioning against shock for the eyes and the

spinal cord. It is also a component of amniotic fluid, which provides protection for the foetus

during pregnancy.

Effects of water deficiency

Deficiency of water in the body can affect various functions, and in severe conditions it might

also lead to death. Some of the problems associated with water deficiency include:

•• low blood pressure

•• increased clotting of blood

•• kidney dysfunction, leading to renal failure

•• severe constipation

•• multisystem failure

•• proneness to infection

•• electrolyte imbalance

Variation in body fluid content

Neonates contain more water than adults: 75–80% water with proportionately more ECF than

adults. At birth, the amount of ISF is proportionally three times larger than in an adult. By the age

of 12 months this has decreased to 60%, which is the adult value. Total body water as a percentage

of total body weight decreases progressively with increasing age. By the age of 60 years,

total body water may decrease to only 50% of total body weight in males, mostly due to an

increase in adipose tissue

Clinical considerations

Dehydration

Dehydration may be caused by restricted water intake, excessive water loss or both. The most

common cause of dehydration is failure to drink liquids. The deprivation of water is far more serious

than the deprivation of food. The average person loses approximately 2.5% of total body water per

day (approximately 1200 mL) in urine, in expired air, by insensible perspiration and from the

gastrointestinal tract. If, in addition to this loss, the loss through perspiration is greatly increased

Transport systems

Cells utilise two processes to move substances in and out of the cell: the passive and active

transport systems. When molecules pass in and out of a cell membrane without the use of

cellular energy it is called a passive transport system. This includes:

•• simple diffusion

•• facilitated diffusion

•• osmosis

•• filtration.

On the other hand, an active transport system requires energy to move substances in and out

of a cell. The active transport systems include:

•• active transport with the utilisation of adenosine triphosphate (ATP)

•• endocytosis

•• exocytosis.

Simple diffusion

The term simple diffusion refers to a process whereby a substance passes through a membrane

without the aid of an intermediary, such as an integral membrane protein (Figure 2.8). Water,

oxygen, carbon dioxide, ethanol and urea are examples of molecules that readily cross cell

membranes by simple diffusion. They pass either directly through the lipid bilayer or through

pores created by certain integral membrane proteins. Small non-polar molecules can diffuse

directly through the plasma membrane. One example of simple diffusion is the exchange of

respiratory gases between the cells of the alveolar sac and the blood in the lungs. The rate of

diffusion depends on several factors, and they are:

•• gases diffuse rapidly and liquids diffuse more slowly;

•• at high temperature, the rate of diffusion is much faster;

•• smaller molecules, such as glycerol, will diffuse much faster than larger molecules, like fatty acids;

•• surface area of the cell membrane over which the molecule can work;

•• solubility of the molecule;

•• concentration gradient.

Osmosis

Osmosis is a process where water moves from an area of volume of high water concentration to

a volume of low water concentration through a selective permeable membrane. A selectively

permeable membrane is one that allows unrestricted passage of water, but not solute molecules

or ions. The relative concentrations of water are determined in the amount of solutes dissolved

in the water. For example, a higher concentration of salt on one side of the cell membrane means

that there is less space for water molecules. Water then will move from the side where there is

the greater number of water molecules through the cell membrane to the other side of the cell

where there are fewer water molecules. This is known as osmotic pressure. The higher the concentration

of the solute on one side of the membrane, the higher the osmotic pressure available

for the movement of the water (Colbert et al., 2012).

The osmotic pressure can be too great and damage the cell membrane; therefore, it is important

for the cell to have a relatively constant pressure between the internal and external environment.

If the osmotic pressure on one side of the cell is greater than on the other side, changes to

the cell could take place resulting in cell damage. A red blood cell placed in a solution with a

lower concentration of solute will undergo haemolysis, and if placed in a fluid with a high concentration

of solutes the red blood cell will crenate (see Figure 2.10). On the other hand, if the

Filtration

Filtration is a process where small substances are forced through a semipermeable membrane

with the aid of hydrostatic pressure. One example of filtration within the human body is at the

capillary end of the blood vessels. With the aid of blood pressure, fluid and solutes are forced

out of the single‐layered cells of the capillaries into the ISF space. Large molecules, such as

proteins and red blood cells, remain in the capillaries. Another example of filtration that occurs

in the human body takes place in the kidneys. Blood pressure forces water and dissolved

waste products, such as urea and uric acid, into the kidney tubules during the formation of

urine (see Chapter 10).

active transport system

The main difference between the active and passive transport systems is that the active transport

system utilises cellular energy to move substances through a semipermeable membrane.

The energy is obtained by splitting ATP into adenosine diphosphate (ADP) and phosphate

(see Figure 2.11). Examples of active transport processes are active transport, endocytosis

and exocytosis.

Active transport

An active process is one in which substances move against a concentration gradient from an

area of lower to higher concentration. The cell must expend energy that is released by splitting

ATP into ADP and phosphate. ATP is a compound of a base, a sugar, and three phosphate

groups (triphosphate). These phosphate groups are held together by high‐energy bonds,

which when broken release a high level of energy. Once one of these phosphate bonds has

been broken and a phosphate group has been released, that compound now has only two

phosphate groups (diphosphate) and there is now also a spare phosphate group. This, in turn,

will join up with another adenosine diphosphate group, so forming another molecule of ATP

(with energy stored in the phosphate bonds), and the whole process continues recurring