The given structure is of the capillary.
- A capillary is a small blood vessel from 5 to 10 μm in diameter, and having a wall one endothelial cell thick.
- They are the smallest blood vessels in the body: they convey blood between the arterioles and venules.
- These microvessels are the site of exchange of many substances with the interstitial fluid surrounding them.
- Substances which exit include water (proximal portion), oxygen, and glucose; substances which enter include water (distal portion), carbon dioxide, uric acid, lactic acid, urea and creatinine.
- Lymph capillaries connect with larger lymph vessels to drain lymphatic fluid collected in the microcirculation.
- Blood flows from the heart through arteries, which branch and narrow into arterioles, and then branch further into capillaries where nutrients and wastes are exchanged.
- The capillaries then join and widen to become venules, which in turn widen and converge to become veins, which then return the blood back to the heart through the venae cavae.
- There are three types of blood capillaries:
Continuous capillaries are continuous in the sense that the endothelial cells provide an uninterrupted lining, and they only allow smaller molecules, such as water and ions, to pass through their intercellular clefts. Lipid-soluble molecules can passively diffuse through the endothelial cell membranes along concentration gradients. Continuous capillaries can be further divided into two subtypes:
- Those with numerous transport vesicles, which are found primarily in skeletal muscles, fingers, gonads, and skin.
- Those with few vesicles, which are primarily found in the central nervous system. These capillaries are a constituent of the blood-brain barrier.
Fenestrated capillaries have pores known as fenestrae (Latin for “windows”) in the endothelial cells that are 60–80 nm in diameter. They are spanned by a diaphragm of radially oriented fibrils that allows small molecules and limited amounts of protein to diffuse.
In the renal glomerulus, there are cells with no diaphragms, called podocyte foot processes or pedicels, which have slit pores with a function analogous to the diaphragm of the capillaries. Both of these types of blood vessels have continuous basal laminae and are primarily located in the endocrine glands, intestines, pancreas, and the glomeruli of the kidney.
Sinusoidal capillaries or discontinuous capillaries are a special type of open-pore capillary, also known as a sinusoid, that have wider 30–40 μm diameters, and wider openings in the endothelium.
Fenestrated capillaries have diaphragms that cover the pores whereas sinusoids lack a diaphragm and just have an open pore.
These types of blood vessels allow red and white blood cells (7.5 μm – 25 μm diameter) and various serum proteins to pass, aided by a discontinuous basal lamina.