Capillaries - Digital Signal

Capillaries

The sum of the diameters of all capillaries is significantly larger than that of the aorta (by about three orders of magnitude), which results in decreases in blood pressure and flow rate. Also, capillaries are very small vessels. Their diameter ranges from 4-15 µm. The wall of a segment of capillary may be formed by a single endothelial cell. This results in a very large surface to volume ratio. The low rate of blood flow and large surface area facilitate the functions of capillaries in

• providing nutrients and oxygen to the surrounding tissue, in
• the absorption of nutrients, waste products and carbon dioxide, and in
• the excretion of waste products from the body.

These functions are also facilitated by a very simple organisation of the wall of capillaries. Only the tunica intima is present, which typically only consists of the endothelium, its basal lamina and an incomplete layer of cells surrounding the capillary, the pericytes. Pericytes have contractile properties and can regulate blood flow in capillaries. In the course of vascular remodelling and repair, they can also differentiate into endothelial and smooth muscle cells.

Three types of capillaries can be distinguished based on features of ethe endothelium.

Continuous capillaries

are formed by "continuous" endothelial cells and basal lamina. The endothelial cell and the basal lamina do not form openings, which would allow substances to pass the capillary wall without passing through both the endothelial cell and the basal lamina. Both endothelial cells and the basal lamina can act as selective filters in continuous capillaries.

Fenestrated capillaries

The endothelial cell body forms small openings called fenestrations, which allow components of the blood and interstitial fluid to bypass the endothelial cells on their way to or from the tissue surrounding the capillary. The fenestrations may represent or arise from pinocytotic vesicles which open onto both the luminal and basal surfaces of the cell. The extent of the fenestration may depend on the physiological state of the surrounding tissue, i.e. fenestration may increase or decrease as a function of the need to absorb or secrete. The endothelial cells are surrounded by a continuous basal lamina, which can act as a selective filter.

Discontinuous capillaries

are formed by fenestrated endothelial cells, which may not even form a complete layer of cells. The basal lamina is also incomplete. Discontinuous capillaries form large irregularly shaped vessels, sinusoids or sinusoid capillaries. They are found where a very free exchange of substances or even cells between bloodstream and organ is advantageous (e.g. in the liver, spleen, and red bone marrow).

 

Suitable Slides

Sections of any well preserved tissue - H&E, Whipf's polychrome
cardiac and skeletal muscle, glands or the papillary layer of the skin contain dense capillary beds.

Cardiac Muscle, sheep - Whipf's polychrome Large numbers of capillaries are present in almost all tissues. At least a few dozen cross sections are present in every sqr. mm of section of poorly vascularised tissues. There may be thousands in highly vascularised tissues. However, a "good" capillary is not that easy to find because of their small size and because the capillary walls are very thin, which often leads to the collapse of the capillary during tissue preparation. Cardiac muscle is highly vascularised. Each muscle cell is surrounded by one or more capillaries. The capillaries roughly follow the course of the muscle cells. To find capillaries in transverse and longitudinal sections it is easiest to first find areas in which the muscle cells have been cut in these planes. Only one or two red blood cells fit side by side in the capillary. A single endothelial cell forms the wall around the entire circumference of a segment of the capillary. Endothelial cell nuclei are therefore not always visible, and some red blood cells are only surrounded by a fine line representing the capillary wall. Identify and draw a few capillaries. Include some of the surrounding tissue features - maybe a cardiac muscle cell, a venule or arteriole - as a scale.