Physiology
The capillary wall is a one-layer endothelium that allows gas and lipophilic molecules to pass through without the need for special transport mechanisms. This transport mechanism allows bidirectional diffusion depending on osmotic gradients and is further explained by the Starling equation.
Capillary beds may control their blood flow via autoregulation. This allows an organ to maintain constant flow despite a change in central blood pressure. This is achieved by myogenic response and in the kidney by tubuloglomerular feedback. When blood pressure increases the arterioles that lead to the capillaries bed are stretched and subsequently constrict (a phenomenon known as the Bayliss Effect to counteract the increased tendency for high pressure to increase blood flow. In the lungs special mechanisms have been adapted to meet the needs of increased necessity of blood flow during exercise. When the heart rate increases and more blood must flow through the lungs capillaries are recruited and are also distended to make room for increased blood flow. This allows blood flow to increase while resistance decreases.
Capillary permeability can be increased by the release of certain cytokines, anaphylatoxins, or other mediators (such as leukotrienes, prostaglandins, histamine, bradykinin, etc.) highly influenced by the immune system.
The Starling equation defines the forces across a semipermeable membrane and allows calculation of the net flux:
where:
- is the net driving force,
- is the proportionality constant, and
- is the net fluid movement between compartments.
By convention, outward force is defined as positive, and inward force is defined as negative. The solution to the equation is known as the net filtration or net fluid movement (Jv). If positive, fluid will tend to leave the capillary (filtration). If negative, fluid will tend to enter the capillary (absorption). This equation has a number of important physiologic implications, especially when pathologic processes grossly alter one or more of the variables.
Read more about this topic: Capillary
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