Histology of Arteries and Veins | Starling’s Law of the Capillary
Histology of Arteries and Veins | Starling’s Law of the Capillary
Objective 5
Relate the histology of blood vessels with their location and function. Explain how the structure of capillaries and the pressure differences within the capillaries results in the delivery of nutrients on the arteriole side and absorption of wastes on the venule side.
Histology of the Arteries and Veins: The Tunics
In keeping with their discrete functions — arteries carry blood away from the heart, veins carry blood towards the heart — arteries and veins have different structural features.
Arteries carry blood under extremely high pressures, as high as 200 mm Hg or more under certain circumstances. Accordingly, they have a thick smooth muscle layer (tunica media) which allows them to resist pressure. The largest arteries (elastic arteries) have, as the name tells us, fibers made of the protein elastin in the tunica media. The innermost layer of an artery is called the tunica intima (from the same root as the word “initimate”). The external, outermost layer of an artery is called the tunica externa.
Arteries can change their diameter when responding to changing conditions. In a way, we knew this already, because the sympathetic nervous system directs blood flow to the heart, lungs, and skeletal muscles while the parasympathetic nervous system directs blood flow to the digestive organs. Even at the very end of the arterial tree, in the small arterioles, there are mechanisms for directing blood flow where it is needed.
Regulating Capillary Blood Flow
Arterioles meet venules (the smallest part of the venous network) at capillaries. Capillaries are the site of oxygen and glucose diffusion into tissue cells, and the diffusion of waste products into the blood to be carries to the liver, kidneys, and other organs that detoxify the blood.
This capillary exchange is regulated by controlling blood flow into the capillaries. Controlling blood flow is the job of smooth muscle collars called precapillary sphincters, which respond to chemicals released by increased metabolism. When the tissue “gets busy”, that means it needs more oxygen and glucose, and it makes more waste. The precapillary sphincters open up to allow more blood flow and more nutrient and waste exchange.
Histology of the Capillaries
There are at least three kinds of capillaries which differ in the structure of the tunica intima (endothelial cell layer).
Water, glucose, and gases can also pass through these tiny openings. These are found in the brain, skin, and lungs. As we have seen already, in the brain, there is a tight regulation of what can go in and out of the capillary. Recall that the continuous capillaries and astrocyte end-feet form the blood-brain barrier which doesn’t even let most molecules through. Only gases, glucose, neurotransmitter precursors, and amino acids are allowed through the blood-brain barrier. In the skin, white blood cells have to force their way through the clefts to get into the connect