The primary purpose of the cardiovascular system is to circulate gases, nutrients, wastes, and other substances to and from the cells of the body. small molecules, such as gases, lipids, and lipid-soluble molecules, can diffuse directly across the endothelial cell membranes of the capillary wall. glucose, amino acids, and ions (including sodium, potassium, calcium, and chloride) use transporters to move through specific channels in the membrane by facilitated diffusion. glucose, ions, and larger molecules can also exit the blood through intercellular clefts. larger molecules can pass through the pores of the fenestrated capillaries, and even large plasma proteins can pass through the large spaces in the sinusoids. Some large proteins in blood plasma can enter and exit endothelial cells packed within vesicles by endocytosis and exocytosis. water moves by osmosis.
bulk stream
The bulk movement of fluids in and out of capillary beds requires a much more efficient transport mechanism than mere diffusion. This movement, often referred to as bulk flow, involves two pressure-driven mechanisms: Fluid volumes move from an area of higher pressure in a capillary bed to an area of lower pressure in tissues through the filtration. conversely, the movement of fluid from an area of higher pressure in the tissues to an area of lower pressure in the capillaries is reabsorption. Two types of pressure interact to drive each of these movements: hydrostatic pressure and osmotic pressure.
hydrostatic pressure
The primary force driving fluid transport between capillaries and tissues is hydrostatic pressure, which can be defined as the pressure of any fluid enclosed in a space. Hydrostatic blood pressure is the force exerted by blood confined within the blood vessels or chambers of the heart. more specifically, the pressure exerted by blood against the wall of a capillary is called capillary hydrostatic pressure (chp), and is the same as capillary arterial pressure. chp is the force that drives fluid out of the capillaries into the tissues.
As fluid leaves a capillary and moves into the tissues, the hydrostatic pressure in the interstitial fluid increases correspondingly. this opposite hydrostatic pressure is called interstitial fluid hydrostatic pressure (ifhp). In general, the chp originating from the arterial pathways is considerably higher than the ifhp, because the lymphatic vessels continually absorb excess fluid from the tissues. therefore, the fluid usually leaves the capillary and enters the interstitial fluid. this process is called filtration.
osmotic pressure
The net pressure that drives reabsorption (the movement of fluid from the interstitial fluid into the capillaries) is called the osmotic pressure (sometimes called the oncotic pressure). while hydrostatic pressure forces fluid out of the capillary, osmotic pressure draws fluid in. Osmotic pressure is determined by osmotic concentration gradients, that is, the difference in solute to water concentrations in blood and tissue fluid. a region with higher solute concentration (and lower water concentration) draws water across a semi-permeable membrane from a region with higher water concentration (and lower solute concentration).
As we discuss osmotic pressure in blood and tissue fluids, it is important to recognize that the formed elements of blood do not contribute to osmotic concentration gradients. rather, it is the plasma proteins that play the key role. solutes also move across the capillary wall down their concentration gradient, but in general, the concentrations should be similar and not have a significant impact on osmosis. Due to their large size and chemical structure, plasma proteins are not really solutes, that is, they do not dissolve but are dispersed or suspended in their fluid medium, forming a colloid instead of a solution.
The pressure created by the concentration of colloidal proteins in the blood is called blood colloid osmotic pressure (bcop). its effect on capillary exchange explains the reabsorption of water. Plasma proteins suspended in the blood cannot move through the semipermeable capillary cell membrane, so they remain in the plasma. as a result, blood has a higher colloidal concentration and a lower water concentration than tissue fluid. therefore, it attracts water. We can also say that the bcop is higher than the interstitial fluid colloid osmotic pressure (ifcop), which is always very low because the interstitial fluid contains few proteins. therefore, water is drawn from the tissue fluid back into the capillary, dragging dissolved molecules with it. this difference in colloid osmotic pressure explains the reabsorption.
interaction of hydrostatic and osmotic pressure
The normal unit used to express pressures within the cardiovascular system is the millimeter of mercury (mm Hg). When blood leaving an arteriole first enters a capillary bed, the Chp is quite high, about 35 mm Hg. Gradually, this initial Chp decreases as the blood moves through the capillary, so that by the time the blood reaches the venous end, the Chp has dropped to about 18 mm Hg. by comparison, plasma proteins remain suspended in the blood, so the bcop remains fairly constant at about 25 mm Hg along the capillary and considerably below the osmotic pressure in the interstitial fluid.
net filtration pressure (nfp) represents the interaction of hydrostatic and osmotic pressures, forcing fluid out of the capillary. is equal to the difference between the chp and the bcop. Since filtration is, by definition, the movement of fluid out of the capillary, when reabsorption occurs, the nfp is a negative number.
nfp changes at different points in a capillary bed. Near the arterial end of the capillary, it is about 10 mm Hg, because the chp of 35 mm Hg minus the bcop of 25 mm Hg equals 10 mm Hg. remember that the hydrostatic and osmotic pressures of the interstitial fluid are essentially negligible. thus, the 10-mm Hg nfp drives a net movement of fluid out of the capillary at the arterial end. At about the middle of the capillary, the chp is about the same as the bcop of 25 mm Hg, so the nfp drops to zero. at this point, there is no net change in volume: the fluid leaves the capillary at the same rate as it enters the capillary. near the venous end of the capillary, the chp has dropped to about 18 mm Hg due to fluid loss. Because the bcop remains stable at 25 mm Hg, water enters the capillary, that is, reabsorption occurs. another way of expressing this is to say that at the venous end of the capillary there is a pnf of −7 mm Hg.
the role of the lymphatic capillaries
Since the overall chp is higher than the bcop, it is inevitable that more net fluid leaves the capillary through filtration at the arterial end than enters through reabsorption at the venous end. Considering all the capillaries over the course of a day, this can be quite a significant amount of fluid: approximately 24 liters per day are filtered, while 20.4 liters are reabsorbed. This excess fluid is collected by the capillaries of the lymphatic system. these extremely thin-walled vessels have a large number of valves that ensure one-way flow through ever-larger lymphatic vessels that eventually drain into the subclavian veins in the neck. An important function of the lymphatic system is to return fluid (lymph) to the blood. lymph can be considered as recycled blood plasma. (Look for additional content for more details on the lymphatic system.)
chapter review
Small molecules can enter and leave capillaries by simple or facilitated diffusion. some large molecules can cross in vesicles or through slits, fenestrations, or spaces between cells in capillary walls. however, most capillary and tissue fluid flow occurs by filtration and reabsorption. filtration, the movement of fluid out of the capillaries, is driven by the chp. reabsorption, the entry of tissue fluid into the capillaries, is driven by the bcop. filtration predominates at the arterial end of the capillary; in the midsection, the opposing pressures are nearly identical, so there is no net exchange, while reabsorption predominates at the venule end of the capillary. hydrostatic and colloid osmotic pressures in the interstitial fluid are negligible under healthy circumstances.
self-testing
Answer the following questions to see how well you understand the topics covered in the previous section.
glossary
blood colloid osmotic pressure (bcop): pressure exerted by suspended colloids in the blood within a vessel; a main determinant is the presence of plasmatic proteins
hydrostatic pressure of the blood: force exerted by blood against the walls of a blood vessel or chamber of the heart
capillary hydrostatic pressure (chp): force exerted by blood against a capillary
filtration: in the cardiovascular system, the movement of material from a capillary into the interstitial fluid, passing from an area of higher pressure to one of lower pressure
interstitial fluid colloid osmotic pressure (ifcop): pressure exerted by colloids within the interstitial fluid
interstitial fluid hydrostatic pressure (ifhp): force exerted by fluid on tissue spaces
net filtration pressure (nfp): force that expels the liquid from the capillary towards the tissue spaces; equal to the difference of the capillary hydrostatic pressure and the colloidal osmotic pressure of the blood
reabsorption: in the cardiovascular system, the movement of material from the interstitial fluid into the capillaries
