Types of Capillaries
According to structure, capillaries can be classified into continuous, sinusoidal and fenestrated.
The most common type of capillaries is continuous arteries, found mostly in the muscles and skin. They are termed continuous because their endothelial cells provide an uninterrupted lining. The adjacent cells are joined laterally by tight junctions. However, intercellular clefts or gaps within these junctions are usually present. These gaps allow passage of small solutes and fluids.
Pinocytotic vesicles, found in the endothelial cell cytoplasm, are believed to ferry fluids across the capillary wall. Among all the capillaries in the body, the capillaries of the brain are unique. The junctions of the continuous capillaries are complete, extending around the entire perimeter, thus creating what is called the blood-brain barrier.
Fenestrated capillaries are similar to continuous capillaries except that their endothelial cells are riddled with fenestrations or tiny windows. Because of these fenestrations, fenestrated capillaries are much more permeable to small solutes and fluids compared to continuous capillaries. The pores are usually covered by a delicate membrane of condensed basal lamina material. These types of capillaries are found wherever filtrate formation or active capillary absorption occurs.
Fenestrated capillaries in the small intestine function by receiving nutrients from digested foods and capillaries with perpetually open pores are in the kidneys, which help in the filtration of blood.
Sometimes referred to as sinusoidal capillaries, these capillaries are highly modified. They are leaky capillaries found only in the bone marrow, some endocrine organs, liver and lymphoid tissues. Upon closer infection, sinusoids have both characteristics of fenestrated and continuous capillaries. They have lumens that are irregularly shaped and are usually fenestrated. Intercellular clefts are also present but with few tight junctions. These structural adaptations permit the passage of large molecules and even blood cells are allowed to pass between the blood and the surrounding tissues.
A special case happens in the liver, where the endothelium of the sinusoids is discontinuous, which allows Kupffer cells to form as part of its lining. Kupffer cells are large macrophages that remove and destroy any contained bacteria. The blood in the tortuous sinusoid channels flow sluggishly, allowing time for it to be modified in various ways.
Capillaries don’t work alone. To compensate for their small size, they form a network called capillary beds. Microcirculation or the flow of blood from the arteriole to a venule happens in the capillary bed.
The amount of blood that enters a capillary bed is being regulated by local chemical conditions and nerve fibers. Depending on conditions of the body, a capillary bed may be flooded with both or it may be totally bypassed.