Biology › Animal transport and cardiovascular biology › Blood vessels and tissue fluid: what leaks out and what comes back
Blood vessels and tissue fluid: what leaks out and what comes back
Five kinds of tube, each built for the pressure it carries, and one capillary bed where fluid is squeezed out at one end and pulled back at the other. Get the two pressures at each end straight and oedema, lymph and shock all fall out of the same explanation.
Before this The cardiac cycle and the pressures the ventricles generate · Osmosis and water potential · Proteins as large molecules that cannot cross membranes freely
Before you start
Arteries carry oxygenated blood and veins carry deoxygenated blood. This is true of nearly every vessel you will be asked about, which is exactly what makes it dangerous — it feels like a definition when it is only a pattern. The pulmonary artery carries deoxygenated blood to the lungs and the pulmonary vein brings oxygenated blood back, and the umbilical vessels of a fetus break it the same way round: the umbilical arteries carry deoxygenated blood out to the placenta and the umbilical vein brings oxygenated blood back. Artery means carrying blood away from the heart. That is the definition, and it is about direction, not cargo.
What you should be able to do
- Relate the structure of an artery, arteriole, capillary, venule and vein to its function and to the pressure it carries.
- Explain why a capillary wall is one cell thick and why the blood moves through it slowly.
- Account for the formation of tissue fluid at the arteriole end of a capillary bed using hydrostatic and oncotic pressure.
- Account for the return of most of that fluid at the venule end using the same two pressures.
- Describe what the lymphatic system does with the fluid that is not returned.
- Explain three different causes of oedema using nothing but hydrostatic and oncotic pressure.
Five tubes, each built for its pressure
Every vessel in the body is a compromise between three things: holding the pressure inside it, moving blood along efficiently, and letting things across the wall. Different vessels weight those three differently, and once you know the pressure a vessel carries you can very nearly predict its wall.
An artery takes the full force of ventricular systole, so its wall is thick, with a lot of elastic tissue and a layer of smooth muscle, and its lumen is narrow relative to that wall. The elastic tissue does something subtler than resist bursting: it stretches during systole, stores energy, then recoils during diastole and pushes the blood on. That recoil is why arterial pressure never falls to zero between beats and why the flow smooths out rather than arriving in slugs.
An arteriole is the same design shrunk, but with the proportions shifted towards muscle. Contract that muscle (vasoconstriction) and the lumen narrows, resistance rises and less blood reaches the tissue beyond. Relax it (vasodilation) and more does. This is how blood is redistributed during exercise, and it is why the arterioles are where the pressure drop happens.
A capillary is a tube whose wall is a single layer of endothelium, about 0.6 μm thick, with a lumen around 7 μm — narrow enough that red blood cells go through in single file, pressed against the wall. Three consequences follow. The diffusion distance is tiny. The red cells are held right at the exchange surface. And because the total cross-sectional area of all the capillaries is enormous compared with the aorta, the blood slows right down, giving longer for exchange. Gaps between the endothelial cells let water and small solutes through while keeping the plasma proteins in.
Venules collect blood from capillary beds and merge into veins, which carry it back at about 8 mmHg. At that pressure there is no point in a thick wall, so veins have thin walls and very wide lumens, which lowers resistance and holds a lot of blood — around 60 per cent of your blood volume is in your veins right now. With almost no pressure behind it, blood needs help: semilunar valves along the veins stop it flowing backwards, and contraction of skeletal muscle around them squeezes it along. Standing still for an hour on a hot day is unpleasant for exactly this reason.
| Artery | Arteriole | Capillary | Vein | |
|---|---|---|---|---|
| Pressure carried | ≈ 100 mmHg | ≈ 60 mmHg | ≈ 25 mmHg | ≈ 8 mmHg |
| Wall | Thick: elastic + muscle | Muscular, adjustable | One cell thick | Thin |
| Lumen | Narrow | Narrow, variable | ≈ 7 μm | Very wide |
| Valves | No | No | No | Yes |
| Main job | Withstand and smooth the surge | Control where blood goes | Exchange | Return blood cheaply |
Where the pressure goes
Blood leaves the left ventricle at about 120 mmHg and arrives back at the right atrium at close to zero. Following where it is lost tells you why the circulation is built the way it is.
Arterioles are narrow, and narrow tubes resist flow ferociously: halve the radius and the resistance goes up sixteen-fold. Millions of them in parallel take the mean pressure from about 90 mmHg down to about 35 mmHg at the start of a capillary bed. That is not waste, it is protection — a capillary wall one cell thick would not survive 120 mmHg arriving in pulses.
By the time blood enters the capillaries the flow is steady, slow and at low pressure, which is exactly what an exchange surface wants. Slow means time to exchange; steady means no damaging surges; low pressure means a wall thin enough to diffuse through can hold.
Tissue fluid: the hardest thing in this unit
Your cells are not bathed in blood. They are bathed in tissue fluid, which is plasma that has been squeezed out of capillaries, minus most of the plasma proteins and all of the red blood cells, which are too large to leave. Working out where it leaves and where it returns is a matter of two opposing pressures, and the reason people find it hard is that only one of the two changes along the capillary.
- Hydrostatic pressure
- The physical pressure the blood exerts against the capillary wall, pushing fluid out through the gaps between endothelial cells.
- Oncotic pressure
- The inward pull created by plasma proteins, which stay in the capillary and lower the water potential of the blood relative to the tissue fluid. Also called colloid osmotic pressure.
- Tissue fluid
- The fluid surrounding cells, formed by ultrafiltration from capillaries; it has effectively no plasma proteins, no red blood cells and no platelets.
At the arteriole end the hydrostatic pressure is about 4.6 kPa, pushing outwards. The oncotic pressure pulls inwards at about 3.3 kPa. Hydrostatic wins by 1.3 kPa, so water and dissolved small molecules — glucose, oxygen, amino acids, mineral ions — are forced out through the gaps in the wall. This is ultrafiltration: the wall acts as a filter, and the plasma proteins are what gets filtered out of the fluid leaving.
Along the capillary, hydrostatic pressure falls. Some of it is lost to friction against the wall, and some to the fluid that has already left. By the venule end it is down to about 2.3 kPa. The oncotic pressure, meanwhile, has not budged: it is still 3.3 kPa, because the proteins responsible are still inside the capillary and if anything are now slightly more concentrated. So the balance reverses. Oncotic wins by 1.0 kPa, and water moves back into the capillary down the water potential gradient, bringing dissolved waste including carbon dioxide with it.
Working out the direction of movement at each end
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 is 2.3 kPa and the oncotic pressure is unchanged. Calculate the net pressure at each end and state which way fluid moves.
Arteriole end: 4.6 − 3.3 = 1.3 kPa net outwards. Fluid leaves the capillary and forms tissue fluid.
Venule end: 3.3 − 2.3 = 1.0 kPa net inwards. Fluid returns to the capillary.
Notice that 1.3 is bigger than 1.0. More is pushed out than is pulled back, so around 10 per cent of what left never returns directly, and that surplus has to be dealt with some other way. The examiner's follow-up question is always about the lymphatic system, and it is really a question about that arithmetic.
Two habits will keep you out of trouble. First, say which pressure is greater and by how much, rather than saying 'the pressure is high'. Second, remember that the oncotic pressure is constant along the capillary — writing that 'oncotic pressure increases at the venule end' is the commonest error in this topic, and although the concentration of protein does creep up a little as fluid leaves, the explanation you need is that hydrostatic pressure fell.
Where the leftover fluid goes
Roughly nine tenths of the fluid that left the capillary returns to it at the venule end. The rest is collected by lymph capillaries: blind-ended, very permeable tubes that thread through every tissue. Once inside, the fluid is called lymph.
Lymph has no pump. It is moved by the same things that move venous blood — contraction of surrounding skeletal muscle, and one-way valves that stop it sliding back. It passes through lymph nodes, where lymphocytes and phagocytes intercept bacteria and other material, and eventually drains back into the bloodstream at the subclavian veins near the collarbone. So the fluid does return to the blood, just by a much longer route.
One detail explains a lot of medicine: lymph capillaries are permeable enough to take up proteins that have escaped from the blood, and capillaries are not. Without the lymphatic system, protein would accumulate in the tissue fluid, its oncotic pressure would climb, and less and less water would be drawn back into the blood.
- Lymph
- Tissue fluid that has drained into the lymphatic system; similar to tissue fluid but containing lymphocytes and more fatty acids after a meal.
- Lymph node
- A swelling in a lymph vessel where lymphocytes collect and pathogens in the lymph are removed.
Oedema: three causes, one explanation
Oedema is the swelling caused by too much fluid sitting in a tissue. Any question about it is a question about the same two pressures, and there are only three ways to tip the balance.
Raise the hydrostatic pressure. If the left side of the heart is failing, blood backs up in the pulmonary veins, capillary hydrostatic pressure in the lungs rises, more fluid is forced out than can be drawn back, and fluid collects in the alveoli. If the right side is failing, or if venous return from the legs is obstructed, the same thing happens in the ankles. Sitting still on a long flight does a mild version of this by removing the muscle pump.
Lower the oncotic pressure. A diet severely short of protein means the liver cannot make enough plasma proteins, especially albumin. The inward pull at the venule end weakens, less fluid returns, and it accumulates — most visibly in the abdomen, which is the swollen belly of kwashiorkor. Severe liver or kidney disease can do the same, the liver by failing to make albumin and the kidney by losing it in the urine.
Block the drainage. If lymph vessels are blocked — by the parasitic worms of filariasis, or by lymph nodes removed during cancer surgery — then the tenth of the fluid that relies on that route has nowhere to go, and the affected limb swells.
TRY IT — Explaining a clinical observation
A person with a long-term shortage of protein in their diet develops a swollen abdomen. Explain, in terms of the pressures acting at a capillary bed, why this happens.
Check your answer
Too little dietary protein means too few amino acids for the liver to build plasma proteins, so the concentration of protein in the blood plasma falls.
Plasma proteins are what generate the oncotic pressure, so that inward pull is reduced — perhaps from 3.3 kPa to well under 2 kPa. The hydrostatic pressure is unchanged, so at the arteriole end even more fluid is forced out than usual, and at the venule end the reduced oncotic pressure can no longer overcome the hydrostatic pressure, so far less returns.
Fluid therefore accumulates in the tissues faster than the lymphatic system can drain it, and the tissue swells. In the abdomen this is visible as distension.
Look at what the answer did not do: it did not mention osmosis in the vague sense, or say the blood was 'weaker'. It named the pressure that changed, said which way, and followed the consequence to both ends of the capillary.
In the exam
- Quote pressures with a direction. 'Hydrostatic pressure of 4.6 kPa outwards exceeds oncotic pressure of 3.3 kPa inwards, so fluid leaves' is a complete answer; 'the pressure is higher so fluid moves' is not.
- Oncotic pressure does not change along the capillary. What changes is the hydrostatic pressure, which falls. Saying otherwise costs marks in almost every mark scheme for this topic.
- Tissue fluid contains no red blood cells, no platelets and effectively no plasma proteins, because those are too large to pass through the gaps. Everything small enough goes.
- If asked to relate a vessel's structure to its function, use the structure-then-consequence form: 'elastic tissue in the wall, so it stretches and recoils, so pressure is maintained between beats'.
- Oedema questions are pressure questions. Name whether hydrostatic went up or oncotic went down, then say what that does at each end of the capillary.
- Do not say arteries carry oxygenated blood. The definition is direction of flow: away from the heart.
Check yourself
Explain why a capillary wall being one cell thick is useful, and why the same feature makes it essential that arterioles reduce blood pressure before blood arrives.
Answer
A wall one endothelial cell thick, roughly 0.6 μm, gives a very short diffusion pathway. Oxygen, glucose and other small solutes cross quickly, and the rate of diffusion is inversely proportional to the distance travelled, so a thin wall means fast exchange. The gaps between the endothelial cells also allow the bulk flow of fluid out of the capillary that forms tissue fluid.
The cost of that thinness is fragility, and low resistance to being stretched. Blood leaving the left ventricle at 120 mmHg in pulses would rupture such a wall, or at the very least drive far more fluid out of it than could ever be drawn back, producing oedema.
Arterioles are narrow and heavily muscled, and resistance to flow rises steeply as radius falls. Millions of them in parallel drop the mean pressure from around 90 mmHg to around 35 mmHg and damp out the difference between systolic and diastolic entirely, so what reaches the capillary bed is a slow, steady, low-pressure flow.
The two features are a matched pair: the capillary can only afford to be that thin because the arteriole upstream has already taken the pressure out, and the arteriole's expensive muscle is only worth having because the capillary downstream is that thin.
Questions
Question 14 marks
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
Question 24 marks
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
Question 34 marks
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
Question 43 marks
Explain why blocking the lymph vessels that drain a limb makes that limb swell.
Mark scheme
- B1 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 53 marks
Standing still for an hour on a hot day often makes a person's ankles swell. Suggest why.
Mark scheme
- 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
- 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 62 marks
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.
Mark scheme
- A1 it is a vein, or a venule merging into one
- A1 roughly 8 mmHg, which is a very low pressure
Worth remembering
- Wall thickness follows the pressure a vessel carries; lumen width follows what is left over.
- The arterioles are where mean pressure collapses and where the pulse disappears.
- Tissue fluid forms where hydrostatic pressure exceeds oncotic pressure, at the arteriole end, and returns where the balance reverses, at the venule end.
- Only the hydrostatic pressure changes along a capillary. The oncotic pressure stays put because the proteins do.
- About a tenth of the fluid leaving never returns directly; the lymphatic system takes it back the long way.