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Blood vessels and tissue fluid: what leaks out and what comes back questions
The structure of arteries, arterioles, capillaries, venules and veins related to function and to the pressure each carries; why capillary walls are one cell thick; the formation and return of tissue fluid from hydrostatic and oncotic pressures at the two ends of a capillary bed; the lymphatic system; and oedema explained through the same pressures.
6 original questions · 20 marks · the blood vessels and tissue fluid: what leaks out and what comes back notes · Animal transport and cardiovascular biology
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At the arteriole end of a capillary the hydrostatic pressure is 4.6 kPa and the oncotic pressure is 3.3 kPa. At the venule end the hydrostatic pressure has fallen to 2.3 kPa. Calculate the net pressure at each end, and calculate by how much the outward net pressure exceeds the inward one.
Mark scheme
- M1 net pressure is the hydrostatic pressure minus the oncotic pressure, and the oncotic pressure is still 3.3 kPa at the venule end because the proteins never left
- A1 arteriole end: 4.6 − 3.3 = 1.3 kPa net outwards
- A1 venule end: 3.3 − 2.3 = 1.0 kPa net inwards
- A1 the outward net pressure exceeds the inward one by 0.3 kPa, so more fluid leaves than returns directly
Explain how tissue fluid is formed at the arteriole end of a capillary bed and how most of it is returned at the venule end.
Mark scheme
- B1 at the arteriole end the hydrostatic pressure of about 4.6 kPa pushing outwards exceeds the oncotic pressure of about 3.3 kPa pulling inwards
- B1 water and small solutes are forced out through the gaps between the endothelial cells by ultrafiltration, while plasma proteins and red blood cells are too large to leave
- B1 hydrostatic pressure falls along the capillary to about 2.3 kPa, lost to friction against the wall and to the fluid that has already left
- A1 the oncotic pressure has not changed, because the proteins responsible are still inside, so it now exceeds the hydrostatic pressure and water returns down the water potential gradient
Explain how the elastic tissue in an artery wall keeps blood flowing between beats, and explain why it is the arterioles rather than the arteries that bring the mean blood pressure down.
Mark scheme
- B1 elastic tissue in the artery wall stretches during ventricular systole and stores energy as it does so
- B1 during diastole it recoils and pushes the blood on, so arterial pressure never falls to zero between beats and the flow is smoothed rather than arriving in slugs
- B1 arterioles are narrow and heavily muscled, and resistance to flow rises steeply as the radius falls, so halving the radius raises the resistance sixteen-fold
- A1 millions of arterioles in parallel take the mean pressure from about 90 mmHg down to about 35 mmHg and damp out the difference between systolic and diastolic
Explain why blocking the lymph vessels that drain a limb makes that limb swell.
Standing still for an hour on a hot day often makes a person's ankles swell. Suggest why.
A blood vessel has a thin wall, a very wide lumen and valves along its length. Identify the type of vessel, and give the pressure the blood in it typically carries.
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