Questions on blood vessels, tissue fluid and lymph

Question 1

Compare the open circulatory system of an insect with the closed circulatory system of a mammal, referring to where the fluid travels, the pressure reached and the control of where it goes.

Accepted Answer

B1 in the insect haemolymph is pumped by a dorsal vessel and then leaves it to bathe the organs directly in the haemocoel, whereas in the mammal blood stays inside vessels for the whole journey B1 in the insect the haemolymph is blood and tissue fluid at once, whereas in the mammal tissue fluid is a separate compartment filtered out through the capillary walls B1 pressure is low in the haemocoel because nothing contains the fluid, so delivery is slow, whereas the vessel walls of a closed system hold the pressure the ventricle generates, so blood moves far faster A1 flow cannot be directed in an open system because there is no vessel whose radius could be changed, whereas in a closed system vasoconstriction and vasodilation of arterioles redistribute blood between organs

Question 2

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.

Accepted Answer

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

Question 3

Using the A-level examination model of capillary exchange, 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.

Accepted Answer

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

Question 4

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.

Accepted Answer

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

Question 5

A capillary wall is a single layer of endothelium about 0.6 μm thick, with gaps between its cells and a lumen about 7 μm across, so red blood cells pass through pressed against the wall in single file. Together the capillaries of a bed have a far greater cross-sectional area than the artery supplying them. Explain how each of these features suits capillaries to exchange.

Accepted Answer

B1 the thin wall gives a very short diffusion pathway, so oxygen, glucose and other small solutes cross quickly between blood and tissue B1 gaps between the endothelial cells allow water and small solutes to filter out of the capillary as tissue fluid, while keeping the larger plasma proteins inside B1 the narrow lumen holds red blood cells right against the exchange surface, minimising the diffusion distance from the cell carrying oxygen to the tissue A1 the huge total cross-sectional area of all the capillaries together, compared with the aorta, slows the blood right down as it passes through, giving more time for exchange to take place

Question 6

In left-sided heart failure, blood backs up in the pulmonary veins. Suggest how this leads to fluid collecting in the alveoli of the lungs.

Accepted Answer

B1 blood backing up in the pulmonary veins raises the pressure in the capillaries of the lungs, since the blood cannot be moved on as quickly as normal B1 this raises the hydrostatic pressure along the whole length of these capillaries, so more fluid than usual is forced out into the surrounding tissue B1 the oncotic pressure pulling fluid back in is unchanged, so it can no longer draw back as much of the increased outflow A1 more fluid is therefore forced out than can be returned or drained by the lymphatic system, and it accumulates in the alveoli, impairing gas exchange

Question 7

A person on a diet severely short of protein develops a swollen abdomen, a condition called kwashiorkor. Explain, in terms of the pressures acting at a capillary bed, why this happens.

Accepted Answer

B1 too little dietary protein means the liver cannot make enough plasma proteins, such as albumin, so the concentration of protein in the blood plasma falls B1 plasma proteins are what generate the oncotic pressure, so this inward pull is reduced B1 the hydrostatic pressure pushing fluid out at the arteriole end is unchanged, so as much fluid as usual, or more, is still forced out A1 at the venule end the weakened oncotic pressure can no longer draw back as much fluid as normal, so fluid accumulates in the tissues faster than the lymphatic system can drain it, causing the swelling

Question 8

The aorta is classed as an elastic artery and the renal artery as a muscular artery. Explain the difference between the walls of the two vessels and how each wall suits the job that vessel does.

Accepted Answer

B1 the wall of the aorta is dominated by thick concentric elastic laminae with relatively little smooth muscle, whereas the wall of the renal artery has much more smooth muscle and only thin elastic layers B1 the aorta lies next to the heart and takes the full force of ventricular systole, so it stretches as blood is ejected and recoils during diastole, which maintains pressure between beats and smooths the pulse out A1 the renal artery lies further out where the pulse has already been smoothed, and contracting or relaxing its smooth muscle constricts or dilates the vessel, which adjusts how much blood is distributed to the kidney

Question 9

Explain why blocking the lymph vessels that drain a limb makes that limb swell.

Accepted Answer

B1 in the examination model about a tenth of the fluid leaving the capillary does not return at the venule end, and is normally collected by the blind-ended lymph capillaries B1 with the drainage blocked that surplus fluid has nowhere to go, so it accumulates in the tissue as oedema A1 lymph capillaries also take up plasma proteins that have escaped from the blood, so protein now builds up in the tissue fluid, raising its oncotic pressure and drawing back still less water

Question 10

Standing still for an hour on a hot day often makes a person's ankles swell. Suggest why.

Accepted Answer

B1 blood in the veins of the leg is returned largely by contraction of the surrounding skeletal muscle squeezing the veins, and standing still removes that muscle pump; or, in the heat, arterioles supplying the skin dilate, so more blood reaches the capillaries at a higher hydrostatic pressure B1 blood therefore pools in the leg veins and the hydrostatic pressure in the capillaries of the ankle rises A1 more fluid is forced out at the arteriole end than can be drawn back at the venule end or drained by the lymphatics, so fluid collects in the tissue

Question 11

Explain why a vein has a thin wall and a wide lumen, and explain how blood is kept moving back towards the heart despite the low pressure in it.

Accepted Answer

B1 blood in a vein is at a low pressure of only about 8 mmHg, so there is no need for a thick wall to withstand the pressure, and a thin wall and wide lumen offer low resistance to flow and hold a large volume of blood B1 semilunar valves along the length of the vein prevent blood flowing backwards A1 contraction of the surrounding skeletal muscle squeezes the vein and drives blood onwards towards the heart, which is why standing still without moving reduces venous return

Question 12

During exercise, blood flow to the gut falls while blood flow to the leg muscles rises. Suggest how arterioles bring this about.

Accepted Answer

B1 arterioles supplying the gut constrict (vasoconstriction), narrowing their lumen and raising resistance, which reduces the blood reaching the gut B1 arterioles supplying the leg muscles dilate (vasodilation), widening their lumen and lowering resistance, which increases the blood reaching them A1 because arterioles, unlike capillaries, have smooth muscle in their walls that can contract or relax, blood is redistributed to the tissues that need it most at that moment

Question 13

Describe how lymph is moved through the lymphatic system back to the bloodstream, given that it has no central pump.

Accepted Answer

B1 smooth muscle in the walls of the larger lymph vessels contracts rhythmically, helping to move the lymph along B1 contraction of surrounding skeletal muscle also squeezes the lymph vessels, and one-way valves stop the lymph sliding back A1 lymph eventually drains back into the bloodstream at the subclavian veins near the collarbone

Question 14

State what the classical A-level model of capillary exchange says happens at the venule end and what the current physiological model says instead, and state what each implies for the fluid returned by the lymphatic system.

Accepted Answer

B1 the classical A-level model says that hydrostatic pressure falls enough along the capillary that oncotic pressure exceeds it at the venule end, so most of the fluid that left is directly reabsorbed there B1 the current physiological model says that in most tissues at steady state, sustained venous reabsorption at the venule end is limited A1 so the lymphatic system returns only a surplus of about a tenth in the classical model, but most of the net filtered fluid in the current one

Question 15

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.

Accepted Answer

A1 it is a vein, or a venule merging into one A1 a low pressure of about 8 mmHg, accepting any value from 5 to 15 mmHg