As the vascular cross-sectional area increases (for example, moving from small arteries to capillaries) the velocity of flow falls, exchange of substrates and waste takes place in the capillaries, and velocity increases again as blood returns toward the heart.
The velocity of flow is lowest in the capillaries, highest in the aorta (120 cm/sec during systole).
Fluid moves through a tube in response to a pressure gradient. There are frictional forces within the fluid and between the fluid molecules and the vessel wall that tend to oppose the flow. As fluid moves through a segment of tube, there is a fall in pressure due to the loss of energy attributable to these frictional forces.
Fluid and solute exchange between blood and the interstitium occurs almost exclusively in capillaries, so it is at this level that the cardiovascular system fulfils its primary function: the support of cellular metabolism.
Arterioles branch into smaller metarterioles, each of which supplies a number of capillary vessels. The opening to each capillary is surrounded by circular smooth muscle forming a precapillary sphincter. These sphincters contract and relax spontaneously, and this activity leads to continuous changes in blood flow through a single capillary. Mean flow and pressure within an entire capillary bed remain fairly constant. Shunt vessels can also be opened, diverting blood away from adjacent capillaries .
The capillary wall contains no smooth muscle and consists of a single layer of endothelial cells surrounded by a basement membrane. There are potential spaces (intercellular clefts) between adjacent cells. In some tissues these gaps are effectively closed, reducing the permeability of the capillary wall to plasma solutes. This is seen in the brain (blood-brain barrier). In the kidney, there are holes, or fenestrations, through the endothelial cells themselves. Fenestrated capillaries offer much less resistance to fluid exchange across the capillary wall than normal.
The direction and rate of diffusion for any molecule or ion depends on:
1. Transcapillary concentration gradients provide the driving force for diffusion. Glucose and O 2 are more concentrated in plasma than in the interstitium, and they diffuse out of the capillary, while waste products like CO 2 , which tend to accumulate around metabolizing cells, diffuse in.
2. Capillary permeability dictates the rate of diffusion under any given concentration conditions. Lipid-soluble substances can rapidly diffuse across the endothelial cells themselves (e.g. O 2 and CO 2) . Polar substances, such as ions and glucose, are lipid insoluble but they also cross the capillary wall through the intercellular clefts (large polar molecules, like the plasma proteins, cannot easily escape from the capillary).
Pressure gradients across the capillary wall lead to bulk flow of fluid, in which water and the small ions and molecules dissolved in it are driven across the capillary wall. The following factors are important in determining the resulting rates of capillary filtration or absorption.
The hydrostatic pressure gradient is the difference between capillary pressure and interstitial fluid pressure. The hydrostatic gradient acts out of the capillary, favouring filtration.
The osmotic pressure gradient is generated because the capillary wall acts as a semipermeable membrane with respect to plasma proteins, i.e. water can cross the capillary but proteins cannot. Protein concentration, which determines the osmotic effect, is higher in plasma than in interstitial fluid, so the osmotic gradient favours absorption of fluid into the capillary. The osmotic pressure generated by the plasma proteins is called the oncotic pressure of plasma.
The net filtration pressure gradient determines the direction of fluid transfer at any given point along the capillary. As blood travel from the arteriolar to the venular end, capillary pressure falls from about 30 mmHg to about 10 mmHg. This decline represents the pressure gradient necessary to drive blood flow through the capillary resistance.
a) The normal situation. Filtration and absorption are balance.
b) Result of dilating the arterioles. PA increases and the tissue space becomes engorged with interstitial fluid.
c) Result of constricting the arterioles. PA decreases and interstitial fluid is withdrawn from the tissue space.
d) Result of a lowered concentration of protein in the blood (such as occurs during prolonged malnutrition). Because of the reduced osmotic pressure (lower horizontal line), fluid accumulates in the tissue spaces resulting in edema.
OEDEMA is swelling caused by an accumulation of interstitial fluid and is a common finding in a range of clinical conditions. It is best understood in terms of the factors which control transcapillary fluid exchange
1. Increases in the hydrostatic gradient increase the rate of capillary filtration. This occurs whenever capillary pressure increases, e.g. because of a rise in venous pressure. This may result from prolonged standing, heart failure or venous obstruction by a clot or tumour. Reductions in interstitial fluid pressure also increase filtration; this can occur during prolonged air flights since cabin pressure is less than atmospheric pressure at sea level. Swelling of the feet often results.
2. Decreases in the osmotic gradient reduce absorption, increasing net filtration. This commonly results from a low plasma protein concentration, e.g. because of liver failure, renal disease. If the capillary permeability to protein rises, this also reduces the effective osmotic gradient and this helps account for the oedema seen in inflammatory responses to infection or trauma.
3. Lymphatic obstruction prevents both fluid and protein from being cleared from the tissues. The resulting swelling is referred to as lymphoedema.