It may contain two types of capillary networks.

• Vascular shunts are the connections between arteries and venules directly from all the side of capillary bed.

• True capillaries supplies exchange of material between cells and blood.

TYPES

Following are the types of capillaries in the capillary bed:

• Continuous capillaries present uninterrupted endothelial lining and permit only smaller molecules to pass through.

• Fenestrated capillaries, from Latin (“fenestra: window”) have pores in them, 60-80nm in diameter. These allow relatively larger molecule to pass through.

• Sinusoids capillaries are the capillaries that have larger openings and discontinuous basal lamina, and permit enlarge size molecules to move across.

ANATOMY

Capillaries as cited above are the main sites for gaseous exchange. Blood coming through arteries pass through these sites and provide surrounding structure with gases and nutrients.

Capillaries are 5-10mm in diameter on one side there are arteries coming from heart carrying oxygenated blood and on the other side are veins carrying deoxygenated blood back to heart.

As blood pass through these capillaries exchange of oxygen, water and waste material takes place. And this exchange depends on following factors

• Capillary hydrostatic pressure

• Tissue (interstitial) hydrostatic pressure

• Capillary (plasma) oncotic pressure

• Tissue (interstitial) oncotic pressure.

CLINICAL POINTS

• In macular degeneration there is overgrowth of capillaries and anti-VEGF (vascular endothelial growth factor)is given to treat this.

• Capillary blood is chosen to test blood glucose, hemoglobin, pH and lactic acid in newborn infants.

• Decreased capillary density is associated with increased cardiovascular risks.