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Additional Support Materials i.e. animations, quizzes, pictures, worksheets |
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Artery
and Vein Tissue (picture)
(provided by: inner learning online) Transport
in Animals M.C Qu's Blood
in detail Structure
and Function of Blood Vessels in detail Concept
Map of Transport in Mammals
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Small
organisms don’t have a bloodstream, but instead rely on the simple diffusion
of materials for transport around their cells. This is OK for single cells, but
it would take days for molecules to diffuse through a large animal, so most
animals have a circulatory system with a pump to transport materials quickly
around their bodies. This is an example of a mass flow system, which
means the transport of substances in the flow of a fluid (as opposed to
diffusion, which is the random motion of molecules in a stationary fluid). The
transport of materials in the xylem and phloem of plants is another example of
mass flow. Mass flow systems work together with the specialised exchange systems
(such as lungs).
Humans have a double circulatory system with a 4-chambered heart. In humans the right side of the heart pumps blood to the lungs only and is called the pulmonary circulation, while the left side of the heart pumps blood to the rest of the body – the systemic circulation. The circulation of blood round the body was discovered by William Harvey in 1628. Until then people assumed that blood ebbed and flowed through the same tubes, because they hadn't seen capillaries.
Blood
entering via vessel: |
Organ |
Blood
leaving via vessel: |
Carotid
artery |
Head |
Jugular
Vein |
Hepatic
artery |
Liver |
Hepatic
vein |
Renal
artery |
Kidneys |
Renal
vein |
Superior
and inferior vena cava |
Heart |
Pulmonary
artery |
|
Veins
and Venules |
Capillaries |
Arteries
and Arterioles |
|
|
|
Function
is to carry blood from tissues to the heart |
Function
is to allow exchange of materials between the blood and the tissues |
Function
is to carry blood from the heart to the tissues |
Thin
walls, mainly collagen, since blood at low pressure |
Very
thin, permeable walls, only one cell thick to allow exchange of materials |
Thick
walls with smooth elastic layers to resist high pressure and muscle layer
to aid pumping |
Large
lumen to reduce resistance to flow. |
Very
small lumen. Blood cells must distort to pass through. |
Small
lumen |
Many
valves to prevent back-flow |
No
valves |
No
valves (except in heart) |
Blood
at low pressure |
Blood
pressure falls in capillaries. |
Blood
at high pressure |
Blood
usually deoxygenated (except in pulmonary vein) |
Blood
changes from oxygenated to deoxygenated (except in lungs) |
Blood
usually oxygenated (except in pulmonary artery) |
Arteries
carry blood from the heart to
every tissue in the body. They have thick, elastic walls to withstand the high
pressure of blood from the heart. The arteries close to the heart are
particularly elastic and expand during systole (heart muscles contracting) and
recoil again during diastole (heart muscles relaxing), helping to even out the
pulsating blood flow. The smaller arteries and arterioles are more muscular and
can contract (vasoconstriction) to close off the capillary beds to which
they lead; or relax (vasodilation) to open up the capillary bed. These
changes are happening constantly under the involuntary control of the medulla in
the brain, and are most obvious in the capillary beds of the skin, causing the
skin to change colour from pink (skin arterioles dilated) to blue (skin
arterioles constricted). There is not enough blood to fill all the body’s
capillaries, and at any given time up to 20% of the capillary beds are closed
off.
Veins
carry blood from every tissue in the body to the heart. The blood has lost
almost all its pressure in the capillaries, so it is at low pressure inside
veins and moving slowly. Veins therefore don’t need thick walls and they have
a larger lumen that arteries, to reduce the resistance to flow. They also have
semi-lunar valves to stop the blood flowing backwards. It is particularly
difficult for blood to flow upwards through the legs to heart, and the flow is
helped by contractions of the leg and abdominal muscles:
The
body relies on constant contraction of these muscles to get the blood back to
the heart, and this explains why soldiers standing still on parade for long
periods can faint, and why sitting still on a long flight can cause swelling of
the ankles and Deep Vein Thrombosis (DVT or “economy class syndrome”), where
small blood clots collect in the legs.
Capillaries
are where the transported substances actually enter and leave the blood. No
exchange of materials takes place in the arteries and veins, whose walls are too
thick and impermeable. Capillaries are very narrow and thin-walled, but there
are a vast number of them (108 m in one adult!), so they have a huge
surface area : volume ratio, helping rapid diffusion of substances between blood
and cells. Capillaries are arranged in networks called capillary beds
feeding a group of cells, and no cell in the body is more than 2 cells away from
a capillary.
Blood
is a specialised tissue containing a number of different living cell
types floating in a non-living watery liquid called plasma. 55% of blood
is made up of plasma and the balance is various blood cells that are suspended
in this watery matrix. This pale yellow liquid is made up of 90% water and 10%
dissolved substances.
The
four main components in blood are shown in the diagram below:
There
are dozens of different substances in blood, all being transported from one part
of the body to another. Some of the main ones are listed in this table:
Substance |
Where |
Reason |
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Oxygen |
Red blood cells |
Transported from lungs to all cells
for respiration |
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Carbon dioxide |
Plasma |
Transported from all cells to lungs
for excretion |
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Nutrients (e.g. glucose, amino
acids, vitamins, lipids, nucleotides) |
Plasma |
Transported from small intestine to
liver and from liver to all cells |
||||
Waste products (e.g. urea, lactic
acid) |
Plasma |
Transported from cells to liver and
from liver to kidneys for excretion |
||||
3 4 |
Plasma |
Transported from small intestine to
cells, and help buffer the blood pH. |
||||
Hormones |
Plasma |
Transported from glands to target
organs |
||||
Proteins (eg albumins) |
Plasma |
Amino acid reserve |
||||
Blood clotting factors |
Plasma |
At least 13 different substances
(mainly proteins) required to make blood clot. |
||||
Antigens and antibodies |
Plasma |
Part of immune system |
||||
Water |
Plasma |
Transported from large intestine and
cells to kidneys for excretion. |
||||
Bacteria and viruses |
plasma |
|
||||
Heat |
Plasma |
Transported from muscles to skin for
heat exchange. |
Erythrocytes
(red blood cells)
make up approximately 45% of blood. They are non-nucleated biconcave
discs that carry oxygen around the body. A pigment called haemoglobin
combines with the oxygen in the lungs to form oxyhaemoglobin. New red
blood cells (RBC) are produced in the red bone marrow of the ribs, sternum,
vertebrae, skull and long bones.
The
main function of RBC is the transport of respiratory gasses.
Oxygen is carried in RBC bound to the protein haemoglobin. A haemoglobin molecule consists of four polypeptide chains,
with a haem prosthetic group at the centre of each chain.
Each haem group contain one iron atom, and one oxygen molecule binds to
each iron atom. Therefore
haemoglobin can bind up to four oxygen molecules.
RBC have specific features (listed below) that make it efficient in
absorbing and transporting respiratory gasses:
They have a small size - They are much smaller than most other cells in the body. This means that all the haemoglobin molecules are close to the surface, allowing oxygen to be picked up and release rapidly.
Shape
- RBC are biconcave shapes discs. It
allows the cell to contain a lot of haemoglobin while still allowing
efficient diffusion through the plasma membrane
Organelles
– RBC do not contain either
nuclei or mitochondria. This
allows more space inside the cell for haemoglobin.
Leucocytes
(white blood cells)
are nucleated amoeboid cells, which are much larger than red blood cells and
protect us against disease. They have the ability of leaving the blood vessels
to invade diseased tissues. White blood cells (WBC) are arranged according to
whether they are granular or agranular. Granular WBC are large
cells containing a nucleus and granular cytoplasm. There are three kinds - neutrophils,
eosinophils and basophils. Agranular WBC have round or
kidney-shaped nuclei and cytoplasm that lacks any granules. There are two types
- lymphocytes and monocytes.
Granulocytes |
Thrombocytes
are bits of broken up blood
cells that help clot the blood when we cut ourselves and bleed. When we bleed,
platelets, chemicals and substances called clotting proteins (prothrombin) help
to form an insoluble 'plug' to seal off the bleeding point.
These
substances are all exchanged between the blood and the cells in capillary beds.
Substances do not actually move directly between the blood and the cell: they
first diffuse into the tissue fluid that surrounds all cells, and then
diffuse from there to the cells.
1.
At the arterial end of the capillary bed the blood is still at high
hydrostatic pressure, so blood plasma is squeezed out through the permeable
walls of the capillary. Cells and proteins are too big to leave the capillary,
so they remain in the blood.
2.
This fluid now forms tissue fluid surrounding the cells. Materials are
exchanged between the tissue fluid and the cells by all four methods of
transport across a cell membrane. Gases and lipid-soluble substances (such as
steroids) cross by lipid diffusion; water crosses by osmosis, ions cross by
facilitated diffusion; and glucose and amino acids cross by active transport.
3.
At the venous end of the capillary bed the blood is at low pressure,
since it has lost so much plasma. Water returns to the blood by osmosis since
the blood has a low water potential. Solutes (such as carbon dioxide, urea,
salts, etc) enter the blood by diffusion, down their concentration gradients.
4.
Not all the plasma that left the blood returns to it, so there is excess
tissue fluid. This excess drains into lymph vessels, which are found in
all capillary beds. Lymph vessels have very thin walls, like capillaries, and
tissue fluid can easily diffuse inside, forming lymph.
The
lymphatic system consists of a network of lymph vessels flowing alongside the
veins. The vessels lead towards the heart, where the lymph drains back into the
blood system at the superior vena cava. There is no pump, but there are numerous
semi-lunar valves, and lymph is helped along by contraction of muscles, just as
in veins. Lymph vessels also absorb fats from the small intestine, where they
form lacteals inside each villus. There are networks of lymph vessels at various
places in the body (such as tonsils and armpits) called lymph nodes where white
blood cells develop. These become swollen if more white blood cells are required
to fight an infection.
Remember
the difference between these four solutions:
Plasma
The liquid part of blood. It contains dissolved glucose, amino acids,
salts and vitamins; and suspended proteins and fats.
Serum
Purified blood plasma used in hospitals for blood transfusions.
Tissue
Fluid
The solution surrounding cells. Its composition is similar to plasma, but
without proteins (which stay in the blood capillaries).
Lymph
The solution inside lymph vessels. Its composition is similar to tissue
fluid, but with more fats (from the digestive system).
Last updated 20/06/2004