capillary

Blood flows from the heart to arteries, which branch and narrow into arterioles, and then narrow further still into capillaries. After the tissue has been perfused, capillaries branch and widen to become venules and then widen more and connect to become veins, which return blood to the heart.

Capillaries are the smallest of a body's blood vessels and are part of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste chemical substances between blood and surrounding tissues.^[1]

Anatomy

Blood flows from the heart to arteries, which branch and narrow into arterioles, and then branch further still into the capillaries. After the tissue has been perfused, capillaries join and widen to become venules and then widen more to become veins, which return blood to the heart.

The "capillary bed" is the network of capillaries supplying an organ. The more metabolically active the cells, the more capillaries it will require to supply nutrients and carry away waste products.

Metarterioles provide direct communication between arterioles and venules and are important in bypassing the bloodflow through the capillaries. True capillaries branch mainly from metarterioles and provide exchange between cells and the circulation. The internal diameter of 8 μm forces the red blood cells to partially fold into bullet-like shapes and to go into single file in order for them to pass through.

Precapillary sphincters are rings of smooth muscles at the origin of true capillaries that regulate blood flow into true capillaries and thus control blood flow through a tissue.

Types

Capillaries come in 3 types:

• Continuous - Continuous capillaries have a sealed endothelium and only allow small molecules, like water and ions to diffuse. Continuous capillaries have tight junctions and can be further divided into two subtypes:

1. Those with numerous transport vesicles that are primarily found in skeletal muscles, lungs, gonads, and skin.
2. Those with few vesicles that are primarily found in the central nervous system. These capillaries are a constituent of the blood-brain-barrier.

• Fenestrated - Fenestrated capillaries (derived from "fenestra," the Latin word for "window") have pores in the endothelial cells (60-80 nm in diameter) that are spanned by a diaphragm of radially oriented fibrils and allow small molecules ^[2][3] and limited amounts of protein to diffuse. In the renal glomerulus there are larger fenestrae which have no diaphragms (although there are pedicels (podocyte foot processes) that have slit pores with an analogous function to the diaphragm of the capillaries). Both types of fenestrated blood vessels have continuous basal lamina and are primarily located in the endocrine glands, intestines, pancreas, and glomeruli of kidney.
• Sinusoidal - Sinusoidal or discontinuous capillaries are special fenestrated capillaries that have larger openings (30-40 μm in diameter) in the endothelium to allow red and white blood cells (7.5μm - 25μm diameter) and various serum proteins to pass, a process that is aided by a discontinuous basal lamina. These capillaries lack pinocytotic vesicles and gaps may be present in cell junctions permitting leakage between endothelial cells. Sinusoid blood vessels are primarily located in the liver, spleen, bone marrow, lymph nodes, and adrenal gland.

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 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:

• ([P_c − P_i] − σ[π_c − π_i]) is the net driving force,
• K_f is the proportionality constant, and
• J_v 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 (J_v). 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.

The variables

According to Starling's equation, the movement of fluid depends on six variables:

1. Capillary hydrostatic pressure ( P_c )
2. Interstitial hydrostatic pressure ( P_i )
3. Capillary oncotic pressure ( π_c )
4. Interstitial oncotic pressure ( π_i )
5. Filtration coefficient ( K_f )
6. Reflection coefficient ( σ )

• Note that oncotic pressure is not illustrated in the image.

History

Ibn al-Nafis theorized a "premonition of the capillary circulation in his assertion that the pulmonary vein receives what comes out of the pulmonary artery, this being the reason for the existence of perceptible passages between the two."^[4]

Marcello Malpighi was the first to observe and correctly describe capillaries when he discovered them in a frog's lung in 1661.^[5]

See also

• Alveolar-capillary barrier
• Blood brain barrier
• Capillary action
• Hagen-Poiseuille equation

References

External links

Look up capillary in Wiktionary, the free dictionary.

• Histology at BU 00903loa
• {http://microcirc.org Microcirculatory Society, Inc}