The capillaries

Lecture Objectives
By the end of this lecture, students are expected to:

1. Summarize the structures of muscular arteries and veins differ
2. identify the three layers of the vein:
3. identify the different types of capillaries, and how their structure is related to their function.
4. identify the three layers of the artery
5. Compare the histological features of conducting and distributing arteries
6. Summarize the functional and histological structure of capillary bed

Capillary Beds
Capillaries permit and regulate metabolic exchange between blood and surrounding tissues, these smallest blood vessels always function in groups called capillary beds, whose size and overall shape conforms to that of the structure , the richness of the capillary network is related to the metabolic activity of the tissues, tissues with high metabolic rates, such as the kidney, liver, and cardiac and skeletal muscle, have an abundant capillary network; the opposite is true of tissues with low metabolic rates, such as smooth muscle and dense connective tissue.
Capillary beds are supplied preferentially by one or more terminal arteriole branches called metarterioles, which are continuous with thoroughfare channels connected with the postcapillary venules .true capillaries branch from the metarterioles, which are encircled by scattered smooth muscle cells, and converge into the thoroughfare channels, which lack muscle, at the beginning of each true capillary, muscle fibers act as precapillary sphincters that contract or relax to control the entry of blood, these sphincters contract and relax cyclically, with 5 to 10 cycles per minute, causing blood to pass through capillaries in a pulsatile manner, when the sphincters are closed, blood flows directly from the metarterioles and thoroughfare channels into postcapillary venules.

Capillaries are small, normally around 3-4µm, but some capillaries can be 30-40 µm in diameter. The largest capillaries are found in the liver. Capillaries connect arterioles to venules.

Capillaries have a single layer of flattened endothelial cells, as shown here in the diagram, there are no muscular or adventitial layers, the thinness of the capillaries helps efficient exchange between the lumen of the capillary and the surrounding tissue.