Circulation Revision Notes

Revision notes on the human circulatory system including all the main types of blood vessels, haemoglobin and the oxygen dissociation curve.

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  • Created on: 13-09-13 20:45
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Topic 3- Circulation
Over large distances, efficient supply of materials is provided by mass transport.
The Human Circulatory System
The 4 blood vessels of the heart are the pulmonary artery,
pulmonary vein, aorta and the vena cava (superior and
inferior). The superior vena cava returns blood from the head,
neck and arms. The inferior vena cava returns blood from the
lower body.
The hepatic artery delivers blood to the liver. The hepatic vein
returns blood from the liver to the heart via the inferior vena
cava.
The mesenteric artery delivers blood to the stomach and gut.
The used blood is then sent to the liver via the hepatic portal
vein where it passes through the liver and then returns to the
heart through the hepatic vein and the vena cava.
Blood Vessels
Arteries-
Have a thick wall made from muscle and elastic fibres.
Small lumen means that the pressure of blood in the arteries is high, which
enables the blood to be forced around the whole body.
Elastic tissue stretches and recoils in order to smooth out blood flow.
Thick wall to cope with high pressures as blood is pumped away from the
heart.
Endothelium is smooth to reduce resistance and allow blood to flow
smoothly.
Arterioles-
The total cross section of the arterioles (Y) is greater than that of the arteries
(X); therefore the pressure in the arterioles is less than the pressure in the
arteries. X < Y
More relative muscle than the artery as the arterioles control blood flow to
different parts of the body. When the smooth muscle contracts, the lumen of the arteriole narrows so blood flow to that
area is reduced. Relaxation of the arterioles causes dilation so blood flow is increased.
Blood pulses less in the arterioles than the arteries because elasticity of the arteries smooths out blood flow before it
reaches the arteriole.
Capillaries-
Capillary wall consists only of endothelium which is one cell thick, creating a short diffusion pathway which speeds up
diffusion of substances into and out of the blood.
Capillaries have a very large total surface area which means there is a high rate of diffusion into and out of the capillaries.
Slow rate of blood flow allows efficient diffusion between the capillary and the cells.
Veins-
Thin wall because the pressure of the blood in the veins is low.
Not as much muscle or elastic fibres as the blood flows smoothly through the veins so it doesn't need the elastic recoil to
be able to cope with the pressure surges.
Smooth endothelium to reduce friction and help blood flow faster and more smoothly.
Large lumen decreases the pressure of the blood further.
Valves in the veins prevent the backflow of blood.
Blood in the veins is kept moving by the small pressure from the blood coming from the capillaries, which pushes blood in
the veins forward. The blood cannot move back because of valves.
The blood at the bottom of the veins in the leg is almost static due
to the pressure caused by the large column of blood above. When
the leg muscles contract they squeeze on the veins and pressure

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Valves prevent it flowing back down.
The reduced pressure on the atria during atrial systole means blood flows up into the atrium because blood always
moves from a higher pressure to a lower pressure.
Tissue Fluid
High blood pressure at the arteriole end of the capillary forces water and molecules out of the capillary into the tissue
fluid.
Large substances and components such as platelets, cells and un-hydrolysed large molecules such as proteins cannot
exit the capillary.…read more

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The Effect of pH
Carbon dioxide and water produced by respiring cells react to make a hydrogen carbonate ion and a hydrogen ion (CO2 +
H2O HCO2 + H+). The hydrogen ion lowers the pH of the blood.
The more carbon dioxide the more hydrogen ions there are and therefore the lower the pH of the blood.
Lower pH means that oxygen dissociates more easily so that in active tissues, oxygen can be more easily delivered.…read more

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