Blood vessels, tissue fluid and lymph

Open and closed circulatory systems

Not every animal with a heart keeps its blood inside vessels. Insects, and most other arthropods and molluscs, have an open circulatory system.

In an insect the pump is a dorsal vessel, a muscular tube running along the back of the animal with a row of valved holes in its wall. It contracts from the rear forwards and pushes haemolymph, the insect's equivalent of blood, towards the head. There it leaves the vessel through an open end and empties into the body cavity, the haemocoel, where it washes directly over the organs. Nothing separates the fluid from the tissues, so haemolymph is blood and tissue fluid at once. It drains slowly back along the body and re-enters the dorsal vessel through the valved holes, and the cycle starts again.

In a closed circulatory system, which is what vertebrates have, the blood never leaves the vessels. It goes from heart to artery to arteriole to capillary to venule to vein and back to the heart, and the only thing that leaves is a filtrate pushed through the capillary wall as tissue fluid.

Two consequences follow. The first is delivery rate. In a haemocoel there is nothing to contain the fluid, so the pressure the pump generates is lost almost at once and the haemolymph moves slowly, at a few millimetres a second. In a closed system the vessel walls hold the pressure, so the ventricle can generate 120 mmHg and blood leaves the aorta at something like half a metre a second. Fast flow means a substance is delivered and carried away quickly, which is what an animal with a high metabolic rate needs.

The second consequence is control of distribution. Blood in vessels can be directed. Constricting the arterioles supplying the gut and dilating those supplying the leg muscles redistributes blood to the tissues using it, which is what happens at the onset of exercise. Haemolymph in a body cavity cannot be directed, because there is no vessel whose radius can be changed. An open system delivers to all tissues at the same rate.

Insects are nevertheless active animals, and the reason is that they do not transport oxygen in haemolymph. Air is delivered to the tissues directly through a branching system of tracheae opening at the body surface, so the slow circulation only has to move nutrients, hormones and waste, for which a low-pressure system is adequate. State the conclusion in those terms: an open system is enough for an insect because its haemolymph does not carry the oxygen, while a large, active animal whose blood does carry it needs the pressure and directed flow of a closed system.

Open circulatory system

A circulation in which blood leaves the vessels and bathes the organs directly in a body cavity before draining back to the heart.

Closed circulatory system

A circulation in which blood remains inside vessels at all times and is returned to the heart through them.

Haemocoel

The body cavity of an insect or other arthropod, in which haemolymph collects and washes over the organs.

Haemolymph

The fluid of an open circulatory system, acting as blood and tissue fluid at the same time.

The five vessel types and their walls

Each vessel wall is a compromise between three requirements: withstanding the pressure inside it, offering low resistance to flow, and allowing exchange across the wall. The vessels differ in which of the three predominates, and the wall structure follows from the pressure the vessel carries.

Describe the drawing

Four vessels drawn in cross-section down the left of the figure as rings, with a description of each beside it. The artery has a thick wall around a narrow lumen, the arteriole a smaller version of the same, the capillary a wall one cell thick around a lumen wide enough for a single red blood cell, and the vein a thin wall around a very wide lumen. The pressure each carries falls from about 100 millimetres of mercury in the artery to about 8 in the vein.

The walls of arteries and veins are built in three layers. The inner layer, the tunica intima, is the endothelium: one layer of flattened cells whose smooth surface reduces friction with the blood. The middle layer, the tunica media, holds the elastic tissue and smooth muscle, and it is much thicker in an artery than in a vein. The outer layer, the tunica externa, is mostly collagen, a tough fibrous protein that stops the vessel being overstretched or bursting. A capillary wall is the endothelium alone.

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 more than resist bursting: it stretches during systole, stores energy, then recoils during diastole and drives the blood onwards. That recoil is why arterial pressure does not fall to zero between beats and why the flow becomes continuous rather than intermittent.

That description treats the artery wall as one design, which is sufficient for most questions. Some specifications divide arteries into two kinds, which resolve the same compromise differently at different distances from the heart.

An elastic artery is one of the great vessels near the heart: the aorta, the pulmonary artery, the common carotid and the subclavian. Its middle layer is dominated by thick concentric sheets of elastic tissue, the elastic laminae, with comparatively little smooth muscle between them. These are the vessels that take the full force of ventricular systole, and what they do with it is stretch. The wall balloons outward as blood is ejected, storing energy in the stretched elastic; during diastole it recoils and squeezes the stored blood onwards. That is what turns the intermittent output of a pump into a continuous flow, and it is why arterial pressure is still about 80 mmHg at the moment the ventricle is empty and relaxed.

A muscular artery is one of the named branches further out: the femoral, the brachial, the renal, the splenic. By then the pulse has already been smoothed, so the elastic laminae thin down to little more than a boundary at each face of the middle layer, and smooth muscle takes over the bulk of the wall. These vessels distribute blood between organs. Contracting the muscle constricts the vessel and reduces the supply to the tissue downstream; relaxing it increases the supply. Arterioles make the finer adjustment, described below, while a muscular artery controls the supply to a whole organ.

Elastic arteries therefore absorb the pressure surge and muscular arteries distribute the flow. Both have a thick wall, a narrow lumen and no valves, so the general description of an artery applies to each.

Elastic artery

A large artery near the heart, such as the aorta, whose wall is dominated by elastic laminae and which stretches and recoils to smooth out the pulse.

Muscular artery

An artery further from the heart, such as the femoral or renal artery, whose wall holds more smooth muscle and which distributes blood to organs by vasoconstriction and vasodilation.

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 large volume of blood. About 60 per cent of the blood volume is in the veins at any moment. With little pressure behind it, venous return depends on two features: semilunar valves along the veins prevent backflow, and contraction of surrounding skeletal muscle drives blood along. Standing still without using those muscles reduces venous return.

The pressure profile around the circulation

Blood leaves the left ventricle at about 120 mmHg and returns to the right atrium at close to zero. Where that pressure is lost accounts for the structure of each part of the circulation.

Describe the drawing

A graph of blood pressure against position round the circulation. A solid line for mean pressure falls from about 100 millimetres of mercury in the arteries to near zero in the great veins, with the steepest fall across the arterioles. Two dashed lines above and below it mark the systolic and diastolic swing, and they close onto the mean line at the arterioles, showing the pulse disappearing before the capillaries.

Arterioles are narrow, and resistance to flow rises steeply as radius falls: halving the radius raises the resistance sixteen-fold. Millions of arterioles in parallel take the mean pressure from about 90 mmHg down to about 35 mmHg at the start of a capillary bed. That pressure drop protects the capillaries, whose walls are one cell thick and would not withstand 120 mmHg arriving in pulses.

Blood therefore enters the capillaries with a flow that is steady, slow and at low pressure, which suits an exchange surface: slow flow allows time for exchange, steady flow avoids damaging surges, and low pressure permits a wall thin enough for rapid diffusion.

Formation of tissue fluid

Cells are surrounded by tissue fluid, not by blood. Tissue fluid is plasma forced out of capillaries, without most of the plasma proteins and without the red blood cells, which are too large to leave. Where fluid leaves and where it returns depends on two opposing pressures, of which only one 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 is very poor in plasma protein, though not literally protein-free, and has no red blood cells and no platelets.

Describe the drawing

A capillary drawn as a horizontal tube running from an arteriole on the left to a venule on the right, with blood flowing left to right. Above the left end the hydrostatic push out of 4.6 kilopascals is set against the oncotic pull in of 3.3, giving a net outward pressure; above the right end the hydrostatic push has fallen to 2.3 while the oncotic pull is unchanged, giving a net inward pressure. Arrows point down out of the capillary at the left end and up back into it at the right end, into and out of tissue fluid surrounding a row of cells, and a further arrow leads down from the tissue into a lymph vessel. A line beneath the drawing labels it as the A-level examination model, in which most of the fluid returns at the venous end.

At the arteriole end the hydrostatic pressure is about 4.6 kPa, pushing outwards. The oncotic pressure pulls inwards at about 3.3 kPa. The net pressure is 1.3 kPa outwards, so water and dissolved small molecules such as glucose, oxygen, amino acids and mineral ions are forced out through the gaps in the wall. This is ultrafiltration: the wall acts as a filter, and almost all the plasma protein is held back from 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. The balance therefore reverses: the net pressure is 1.0 kPa inwards, and water moves back into the capillary down the water potential gradient, carrying dissolved waste including carbon dioxide with it.

State the direction of each pressure and compare their magnitudes. In the classical A-level model, hydrostatic pressure falls along the capillary while the inward oncotic pressure due to plasma proteins is treated as approximately constant.

Two accounts of the same capillary are in use, and they should be labelled rather than blended.

A-level examination model. Hydrostatic pressure favours filtration at the arteriole end; as hydrostatic pressure falls, plasma proteins favour reabsorption towards the venule end; excess fluid enters lymph vessels. This is the classical Starling model, and it is the account the A-level specifications and mark schemes in this topic are written around.

Current physiological model. In most tissues at steady state, sustained venous reabsorption is limited and most net filtered fluid returns through the lymphatic system. This is the revised Starling principle, drawn from measurements made since the 1990s. Check your own specification for the model it requires.

Where the leftover fluid goes

In the examination model, 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 central pump, but it is not entirely passive either. The larger collecting vessels carry smooth muscle in their walls that contracts rhythmically, working alongside the squeeze of surrounding skeletal muscle and the one-way valves that stop the lymph 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.

One detail explains a lot of medicine: lymph capillaries are permeable enough to take up proteins that have escaped from the blood, and blood 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, after a meal, more lipid, carried in chylomicrons.

Lymph node

A swelling in a lymph vessel where lymphocytes collect and pathogens in the lymph are removed.

Oedema and its three causes

Oedema is swelling caused by an accumulation of fluid in a tissue. Explain any case of it using the same two pressures. There are three ways to alter the balance between them.

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, whether 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.