One of the primary functions of capillary is to provide exchanges, so that oxygen and nutrients could be provided to tissues while carbon dioxide and wastes could be taken away from tissues. Other functions that capillary plays include innate and adaptive immune where leukocytes and other cells could recruit to the site of infection/inflammation, hemostasis where platelet and coagulants could reach the site of bleeding, fluids regulation of endocrine that various hormone could play their actions on distal target tissues, and so on. In this thread we only discuss the transcapillary transport, including transcapillary solute diffusion and transcapillary fluid movement.
Transcapillary Solute Diffusion
Capillaries act as efficient exchange sites where most substances cross the capillary walls simply by passively diffusing from regions of high concentration to regions of low concentration. As in any diffusion situation, there are four factors that determine the diffusion rate of a substance between the blood and the interstitial fluid: 1.the concentration difference; 2.the surface area for exchange (being maximized); 3.the diffusion distance (being minimized); and 4.the permeability of the capillary wall to the diffusing substance. The ease with which a particular solute crosses the capillary wall is expressed in a parameter called its capillary permeability. Permeability takes into account all the factors (diffusion coefficient, diffusion distance, and surface area)- except concentration difference-that affect the rate at which a solute crosses the capillary wall.
PS: Pick’s first law of diffusion, Xd = D*A*(Δ[X]/ΔL), whereas,
D: diffusion coefficient
A: surface area
Δ[X]: concentration difference
ΔL: diffusion distance
Permeability: A + D + 1/ΔL
Capillary beds allow huge amounts of materials to enter and leave blood because they maximize the are across which exchange can occur while minimizing the distance over which the diffusing substances must travel. Capillaries are extremely fine vessels with a lumen (inside) diameter of approximately 5 μm, a wall thickness of approximately 1 μm, and an average length of perhaps 0.5 mm.
Capillaries are distributed in incredible numbers in organs and communicate intimately with all regions of the interstitial space. It is estimated that there are approximately 1010 capillaries in the systemic organs with a collective surface area of approximately 100 m2. That is roughly the area of one player’s side of a single tennis court.
Diffusion is a tremendously powerful mechanism for material exchange when operating over such a short distance and through such a large area. We are far from being able to duplicate – in an artificial lung or kidney, for example – the favorable geometry for diffusional exchange that exist in our own tissue.
As illustrated in Figure 6-1, the capillary wall itself consists of only a single thickness of endothelial cells joined to form a tube. The ease with which a particular solute crosses the capillary wall is expressed in a parameter called its capillary permeability. Permeability takes into account all the factors (diffusion coefficient, diffusion distance, and surface area) – except concentration difference – that affect the rate at which a solute crosses the capillary wall.
Careful experimental studies on how rapidly different substances cross capillary walls indicate that two fundamentally distinct pathways exist for transcapillary exchange. Lipid-soluble substances, such as the gases oxygen and carbon dioxide, cross the capillary wall easily. Because the lipid endothelial cell plasma membranes are not a significant diffusion barrier for lipid-soluble substances, transcapillary movement of these substances can occur through the entire capillary surface area.
The capillary permeability to small polar particles such as sodium and potassium ions is approximately 10,000-fold less than that for oxygen. Nevertheless, the capillary permeability to small ions is several orders of magnitude higher than the permeability that would be expected if the ions were forced to move through the lipid plasma member. It is therefore postulated that capillaries are somehow perforated at intervals with water-filled channels or pores.
Calculation from diffusion data indicate that the collective cross-sectional area of the pores relative to the total capillary surface area varies greatly between capillaries in different organs. Brain capillaries appear to be very tight (have few pores), whereas capillaries in the kidney and fluid-producing glands are much more leaky. On an average, however, pores constitute only a very small fraction of total capillary surface area – perhaps 0.01%. This area is, nevertheless, sufficient to allow very rapid equilibration of small water-soluble substances between the plasma and interstitial fluids of most organs. Thus, the concentrations of inorganic ions measured in a plasma sample can be taken to indicate their concentrations throughout the entire extracellular space.
Transcapillary Fluid Movement
In addition to providing a diffusion pathway for small charged molecules, the water-filled channels that traverse capillary walls permit fluid flow through the capillary wall. Net shifts of fluid between the blood and the interstitial compartments are important for a host of physiological functions, including the maintenance of circulating blood volume, intestinal fluid absorption, tissue edema formation, and saliva, sweat, and urine production. Net fluid movement out of capillaries is referred to as filtration, and fluid movement into capillaries is called reabsorption.
Fluid movement = Kf[(Pc – Pi) – (πc – πi)] [Equation 1]
- Pc is about 25 mm Hg
- Pi is about 0 mm Hg or negative values
- πc is about 25 mm Hg
- πi is nearly 0 mm Hg
PS: Actually, Rate of filtration = hydraulic permeability × surface area × NFP. As it is difficult to estimate the surface area of a capillary bed, a parameter called the filtration coefficient (Kf) is used to denote the product of the hydraulic permeability and surface area.
NFP: Net filtration pressure
Fluid flows through transcapillary channels in response to pressure differences between the interstitial and intracapillary fluids according to the basic flow equation. However, both hydrostatic and osmotic pressures influence transcapillary fluid movement. The intravascular hydrostatic pressure provides the driving force for causing blood flow along vessels. For example, the hydrostatic pressure inside capillaries, Pc, is approximately 25 mm Hg and is the driving force that causes blood to return to the right side of the heart from the capillaries of systemic organs. In addition, however, the 25-mm Hg hydrostatic intracapillary pressure tends to cause fluid to flow through the transcapillary pores into the interstitium where the hydrostatic pressure (Pi) is near or below 0 mm Hg. Thus, there is normally a large hydrostatic pressure difference favoring fluid filtration across the capillary wall. Our entire plasma volume would soon be in the interstitial if there were not some counteracting force tending to draw fluid into the capillaries. The balancing force is an osmotic pressure that arises from the fact that plasma has a higher protein concentration than does interstitial fluid.
Plasma has a total osmotic pressure of approximately 5000 mm Hg – nearly all of which is attributable to dissolved mineral salts such as NaCl and KCl. As discussed in the section of transcapillary solute diffusion, the capillary permeability to small ions is very high. Their concentrations in plasma and interstitial fluid are very nearly equal and, consequently, they do not affect transcapillary fluid movement.
There is however a small but important difference in the osmotic pressures of plasma and interstitial fluid that is due to the presence of albumin and other large proteins in the plasma, which are normally absent from the interstitial fluid. A special term, oncotic pressure (or colloid osmotic pressure), is used to denote the portion of a solution’s total osmotic pressure that is due to particles that do not move freely across capillaries. Because of the plasma proteins, the oncotic pressure of plasma (πc) is approximately 25 mm Hg. Because of the absence of protein, the oncotic pressure of the interstitial fluid (πi) is nearly 0 mm Hg. Thus, there is normally a large osmotic force for fluid reabsorption into capillaries that counteracts the tendency for intracapillary hydrostatic pressure to drive fluid out of capillaries.
So what is the true fact for the movement of fluid between capillary and interstitum? Fluid balance within a tissue (the absence of net transcapillary water movement) occurs when the bracketed term in [Equation 1] is zero. This equilibrium may be upset by alterations in any of the four pressure terms. The pressure imbalances that cause capillary filtration and reabsorption are indicated on the right side of Figure 6-2.
In most tissues, rapid net filtration of fluid is abnormal and cause tissue swelling as a result of excess fluid in the interstitial space (edema). For example, a substance called histamine (in inflammation) is often released in damaged tissue. One of the actions of histamine is to increase capillary permeability to the extent that protein leak into the interstitum. Net filtration and edema accompany histamine release, in part, because the oncotic pressure difference (πc – πi) is reduced below normal.
Transcapillary fluid filration is not always detrimental. Indeed, fluid-producing organs such as salivary glands and kidneys utilize high intracapillary hydrostatic pressure to produce continual net filtration. Moreover, in certain abnormal situations, such as severe loss of blood volume through hemorrhage, the net fluid reabsorption accompanying diminished intracapillary hydrostatic pressure helps restore the volume of circulating fluid.