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Transport in Mammals

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Transport in Mammals

A recurring theme in biological systems is the surface area to volume
ratio. All cells require nutrients and most require oxygen as well.
Wastes also need to be removed.

With a small organism this demand can be met by simple diffusion over
the body surface but larger or very active organisms need a transport
system with a pump to ensure that the supply meets the demand of all
cells, even those deep within the body.

In mammals, the pump is the heart. Substances are carried in a
transport medium of the blood. The blood is contained within vessels,
with substances being released out of, or into the blood as it flows
through certain vessels called capillaries.


Blood vessels
-------------

Blood is carried within a closed transport system that is made up of
three types of vessel:
arteries, capillaries and veins.

Arteries carry blood away from the heart.

Capillaries are the site of the exchange of materials between the
blood and tissues. Veins take blood back into the heart.


Arteries

Blood pumped out of the heart is at a very high pressure, so the
structure of the arteries must be adapted to this. They can withstand
high pressure by having very thick walls made up of elastic fibres and
smooth muscle. These allow the wall to stretch as blood surges through
them so that they don't burst or rupture.

It also means that as the artery increases in diameter, the pressure
is reduced a little. After they 'give', the elastic fibres recoil back
inwards as the pressure falls. The artery decreases in diameter thus
raising the pressure a little.

The lowering of the pressure when it is high, and the raising of it
when it is lower produces some smoothing out of the flow of the blood.
It should be obvious though when you feel your neck or wrist that it
is by no means complete - you can still feel the pulses of the flow
some distance from your heart.

A large artery will split into smaller arterioles that then branch
further into many tiny capillaries. Arterioles have walls with a
similar structure to arteries but have a greater proportion of smooth
muscle and less elastic tissue. They do not have to withstand as high
a pressure as arteries and have the ability to contract because of the
smooth muscle and regulate the flow of blood to a tissue.


Capillaries

To work efficiently, the capillaries need to be small enough to be in
close proximity with small groups of cells and their walls need to be
thin enough to allow substances to move in and out of the blood.

To enable this there are tiny gaps between the cells making up the
wall of the capillary. These allow substances to leave the blood and
bathe the cells of the tissues. The fluid made up of plasma and
dissolved substances is called tissue fluid.


Tissue fluid

Tissue fluid is formed because of the high hydrostatic pressure of the
blood at the arteriole end of the capillary that pushes fluid out of
the blood.

The blood contains plasma proteins giving the blood a relatively high
solute potential (and therefore a low water potential), tending to
draw water into the blood. Since the hydrostatic pressure has a
greater effect than the solute potential at the arteriole end, the net
effect is that fluid leaves the capillary. No blood cells or large
proteins leave as they are too big to fit through the gaps.

At the venule end of the capillary, since fluid has been lost, the
hydrostatic pressure of the blood is lower and the solute potential is
higher. Because of this, fluid drains back into the blood. At this
stage, the useful materials such as amino acids and glucose will have
been taken up by the cells and the tissue fluid will now contain waste
substances such as carbon dioxide and urea.

About 90% of the fluid which leaks out of the capillaries seeps back
in, the remaining 10% is returned to the blood by the lymphatic system
and is called lymph. This system is made up of many blind-ending lymph
vessels, which allow tissue fluid to flow into them via one way
valves. These valves are large enough to allow proteins, which are too
big to get into the capillaries, into the lymph vessels. If tissue
fluid accumulates rather than be returned to the blood by the
lymphatic system, bloating or oedema is the result.

Blood consists of cells bathed in a liquid plasma. When this plasma
leaks out of the capillaries, it is called tissue fluid. This is
almost identical in composition to plasma but contains less protein
molecules and no red blood cells. White blood cells can escape the
capillaries into the tissue fluid. Lymph is virtually identical in
composition to tissue fluid and just has a different name due to its
different location.

This diagram shows the formation of tissue fluid:



Veins

The capillaries then join to form larger venules which themselves then
join to form veins.

Since at this stage, the pressure of the blood is low, blood needs to
be 'encouraged' to flow back to the heart. To prevent any backflow of
the blood (particularly important if blood is flowing against gravity)
there are valves in the veins. Also the veins pass through or very
close to muscles. When the muscles are active in contracting and
relaxing, the squeezing on the veins moves blood along but due to the
valves, only ever towards the heart.

As the pressure is so much lower in the veins than in the arteries,
there is little need for the elastic fibres and smooth muscle in the
walls.

Blood

Just over half of the blood volume is made up of a pale yellow fluid
called plasma. The rest of the blood is made up of cells (red blood
cells and white blood cells) and platelets.


Blood has several vital functions:

[IMAGE]

Transport - oxygen in the red blood cells, absorbed, digested food by
the plasma, excretory products by the plasma, hormones by the plasma
etc.

[IMAGE]

Defence - by the white blood cells (a.k.a. leucocytes).

[IMAGE]

Formation of lymph and tissue fluid.

[IMAGE]

Homeostasis.

Red blood cells


Also known as erythrocytes. These contain a pigment, haemoglobin,
which gives them their colour.

Red blood cells are made in the bone marrow (the liver in a foetus) of
many bones. They have a life span of about 120 days and are about 7mm
in diameter (very small).

Being like a biconcave disc in shape, the surface area to volume ratio
is very large. Oxygen can therefore diffuse very quickly into the cell
and because the cell is so small, quickly bind to a haemoglobin
molecule.

They lack organelles meaning that there is more room for haemoglobin.
Their size and flexible membrane also means that they can squeeze
through capillaries and transport oxygen extremely close to cells.


White blood cells


These cells all have a nucleus, most are much larger than red blood
cells and are spherical or irregular in shape.

There are two basic types of white blood cells; the granulocytes (they
have granular cytoplasm and lobed nuclei) and agranulocytes (the
cytoplasm appears smooth and the nucleus is rounded or horseshoe in
shape).

Made in:

Function:

Agranulocytes:

Monocytes

Red bone marrow

Phagocytic against bacteria and antibody-coated viruses

Lymphocytes

Spleen, lymph nodes

Produces antibodies

Granulocytes:

Neutrophils

Red bone marrow

Phagocytic and contain lysosomes to break down ingested bacteria

Eosinophils

Red bone marrow

Phagocytic. Works against allergens by making antihistamines

Basophils

Red bone marrow

Make antihistamines, make heparin (prevents unnecessary blood
clotting) and make serotonin (makes the capillaries more leaky so that
phagocytes can leave the blood and enter the site of infection


For more information, see the QuickLearn on Immunity.



Platelets


These are formed in the bone marrow and are fragments of larger cells.
They have no nucleus but reactions do take place in the cytoplasm.

They have a variety of role such as blood clotting and the production
of prostaglandins that regulate the degree of constriction or dilation
in blood vessels.

Blood clotting: Platelets stick to damaged cells on the inner surface
of blood vessels forming a plug. Unless the damage is small, platelets
are involved in a chain of reactions by releasing particular
chemicals. A soluble protein, fibrinogen (present in the plasma),
becomes an insoluble protein, fibrin, and this forms layers of fibres
across the wound. The mesh that this creates traps red blood cells and
platelets and a scab is formed.

This has two useful effects:

1. Blood does not leak out of he vessel.
2. It is less likely that an infectious organism will enter from
outside and cause harm.



Blood groups

The most commonly required blood-grouping system is the ABO system. It
concerns two antigens that can occur on the surface of red blood
cells. The antigens are called agglutinogens in this case and are:
agglutinogen A and agglutinogen B.

Plasma also contains antigens, called agglutinins in this case, and
they are agglutinin A and agglutinin B.

We shall call agglutinogens A or B and the agglutinins a or b.
If A and a come into contact, the red cells will clump together.
If B and b come into contact the red cells will clump together.

Therefore, in your blood you will not contain the agglutinogen and the
agglutinin of the same type.

Blood Group:

Agglutinogen A:

Agglutinin
A:

Agglutinogen B:

Agglutinin:
B

A

[IMAGE]

[IMAGE]

[IMAGE]

[IMAGE]

B

[IMAGE]

[IMAGE]

[IMAGE]

[IMAGE]

AB

[IMAGE]

[IMAGE]

[IMAGE]

[IMAGE]

O

[IMAGE]

[IMAGE]

[IMAGE]

[IMAGE]

Blood transfusions

It is important to match blood correctly so that agglutinins in the
recipient don't clump the red blood cells of the donor.

In transfusions it is important to remember that the volume of blood
donated is relatively small compared to the volume of the recipients
blood. The agglutinins in the plasma from the donor are so diluted
that no harm is done. However the aggluinogens on the red blood cells
are not so diluted so harm can be done. These are the possible
transfusions ([IMAGE] is safe, [IMAGE]is not):

Recipient:

Donor:

A

B

AB

O

A

[IMAGE]

[IMAGE]

[IMAGE]

[IMAGE]

B

[IMAGE]

[IMAGE]

[IMAGE]

[IMAGE]

AB

[IMAGE]

[IMAGE]

[IMAGE]

[IMAGE]

O

[IMAGE]

[IMAGE]

[IMAGE]


Oxygen carriage
---------------

Oxygen does dissolve in plasma but the solubility is low and decreases
further if the temperature increases. The amount that could be carried
by the plasma therefore would be completely insufficient to supply all
cells.

There is a protein in the blood however that will carry 4 molecules of
oxygen. The protein is called haemoglobin (Hb) and is made up of 4
polypeptide chains, each with a haem group. Each haem group can pick
up 1 molecule of O2. The protein, being fairly small, could pass out
of the blood during ultrafiltration in the kidneys so, to ensure that
it is not lost, it is found within red blood cells.

Note: During this topic you will come across the term of partial
pressure of oxygen. It does not mean the pressure of the blood itself.
Essentially it is a measure of the concentration of oxygen. It is
written in shorthand as pO2 and is measured in kilopascals (kPa).

Inhaled air in the alveoli has a pO2 = 14kPa. The pO2 of resting
tissue = 5.3kPa (lower pO2 = lower O2 concentration due to
respiration) and the pO2 of active tissues = 2.7kPa. In either case,
blood arriving at the lungs has a lower pO2 than that in the lungs.

There is therefore a diffusion gradient and oxygen will move from the
alveoli into the blood. The O2 is then loaded onto the Hb until the
blood is about 96% saturated with oxygen. The Hb is now called
oxyhaemoglobin (HbO2).

Hb + 4O2 ® HbO8

Dissociation curves

The blood is then taken to tissues where the cells are respiring all
the time, using oxygen. The pO2 will be low. As the red blood cell
enters this region, the Hb will start to unload the O2, which will
diffuse into the tissues and be used for further respiration. Since
much of the Hb will have unloaded the O2, a much lower percentage of
the blood will be saturated with O2.

A graph of the percentage saturation of blood with O2, i.e. the amount
of HbO2 as opposed to Hb at different pO2 is shown below. It is called
an oxygen dissociation curve:



It is S-shaped because of the behaviour of the Hb in different pO2.

The first molecule of O2 combines with an Hb and slightly distorts it.
The joining of the first is quite slow (the flatter part of the graph
at the beginning) but after the Hb has changed shape a little, it
becomes easier and easier for the second and third O2 to join. This is
shown by the curve becoming steeper. It flattens off at the top
because joining the fourth O2 is more difficult.

Overall, it shows that at the higher and lower end of the partial
pressures, there isn't a great deal of change in the saturation of the
Hb, but in the middle range, a small change in the pO2 can result in a
large change in the percentage saturation of the blood.


The effect of pH - The Bohr effect

The amount of O2 carried and released by Hb depends not only on the pO2
but also on pH.

An acidic environment causes HbO2 to dissociate (unload) to release
the O2 to the tissues. Just a small decrease in the pH results in a
large decrease in the percentage saturation of the blood with O2.

Acidity depends on the concentration of hydrogen ions.

H+ displaces O2 from the HbO2, thus increasing the O2 available to the
respiring tissues.

H+ + HbO2 ® HHb + O2

HHb is called haemoglobinic acid.

This means that the Hb mops up free H+. That way the Hb helps to
maintain the almost neutral pH of the blood. Hb acts as a buffer.

This release of O2 when the pH is low (even if the pO2 is relatively
high) is called the Bohr effect.


When does the pH decrease because of free H+ in the blood?

During respiration, CO2 is produced. This diffuses into the blood
plasma and into the red blood cells. Inside the red blood cells are
many molecules of an enzyme called carbonic anhydrase. It catalyses
the reaction between CO2 and H2O. The resulting carbonic acid then
dissociates into HCO3- + H+.
(Both reactions are reversible).


CO2

+

H2O

®

H2CO3

carbon dioxide

water

carbonic acid

H2CO3

®

HCO3-

+

H+

Carbonic acid

hydrogencarbonate ion

hydrogen ion

Therefore, the more CO2, the more the dissociation curve shifts to the
right:


Carbon dioxide transport
------------------------

About 85% of the CO2 produced by respiration diffuses into the red
blood cells and forms carbonic acid under the control of carbonic
anhydrase.

The HCO3- diffuses out of the red blood cell into the plasma. This
leaves a shortage of negatively charged ions inside the red blood
cells. To compensate for this, chloride ions move from the plasma into
the red blood cells. This restoration of the electrical charge inside
the red blood cells is called the chloride shift.

About 5% of the CO2 produced simply dissolves in the blood plasma.
Some CO2 diffuses into the red blood cells but instead of forming
carbonic acid, attaches directly onto the haemoglobin molecules to
form carbaminohaemoglobin. Since the CO2 doesn't bind to the haem
groups the Hb is still able to pick up O2 or H+.


Other effects
-------------

Carbon monoxide

If carbon monoxide is breathed in (for example from car exhaust
fumes), it binds irreversibly with haemoglobin to form
carboxyhaemoglobin. This means that the Hb cannot load and carry O2.

To make matters worse, Hb combines with CO about 250 times more
readily than it does with O2 so that, even if the CO concentration is
fairly low, it can cause death due to lack of O2 to the tissues.

Cigarettes produce CO-containing smoke, which means that a small
percentage of a smoker's blood is unable to transport O2.


Foetal haemoglobin

A foetus developing in the uterus must be able to load O2 from its
mother's blood. Due to the respiration occurring in the foetus' cells,
the pO2 is lower in the foetus blood than in the mother's blood. Some
will unload from the mother's HbO2 and diffuse across to the foetus.
However, because of the relatively small concentration difference, not
much O2 is passed across.

To maximise the amount of O2 that the foetus receives, it has
different haemoglobin - foetal haemoglobin. This has a higher affinity
for O2 than adult Hb (it combines more readily with O2) so the foetus
picks up enough O2. The dissociation curve shifts to the left.



Myoglobin

Skeletal muscle contains a pigment called myoglobin. It is very
similar to Hb but has a higher affinity for O2.

It will load with O2 as Hb unloads and will store the O2 in the muscle
until it is required. It only releases the O2 when the pO2 is very low
- when the Hb cannot supply O2 fast enough and the demand is great.
The dissociation curve shifts to the left.


Temperature

The higher the temperature, the less saturated the blood is with O2,
i.e. the more the HbO2 unloads the O2.

This situation might arise during exercise - heat is produced during
metabolic activity and during this time, the O2 supply will need to
increase. The dissociation curve shifts to the right:


Environment

Animals living where there is a shortage of oxygen, animals living at
high altitude need to be able to pick up O2 when it is at a very low
pO2. Their dissociation curves will look like those of foetal
haemoglobin and myoglobin - the curve shifts to the left.

If somebody were to climb quickly from sea level to high altitude,
they are likely to suffer from altitude sickness, which can be fatal.
However if the body is given time to adapt, most people can cope well
with high altitudes. Instead of taking up the normal 45% of blood
volume, the red blood cells can increase in numbers to take up 60%.

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