Capillary Anatomy

Capillaries

Capillaries are the smallest of the vessels.  These vessels operate in networks or beds, as illustrated in the diagram on the left.  They are thin-walled and fragile, with only a tunica interna and no tunica mediaThe middle layer of a blood vessel, composed of smooth muscle and elastic tissue, responsible for va or externa. This thinness allows them to permit nutrient and gas exchange.  Capillaries are just wide enough to accommodate correctly-shaped erythrocytes(RBCs): Red blood cells responsible for oxygen transport..  Sometimes, you can see a capillary with red blood cellsThe basic structural and functional units of life. lined up. They appear as if they are waiting to get on a bus.  The pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. in capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. is low and flow through capillary beds is usually pulsatile. 

Capillary Beds

This diagram shows a river flowing fast, hitting a delta with tributaries fanning out. Many of us instinctively know that when the fast-flowing river hits the delta, the waterThe universal solvent essential for life. loses speed.  This is the same situation in the capillary beds.   This happens because capillaries, despite their small individual diameters, collectively have a large combined surface area in a bed. Blood flow slows in capillaries due to this increased collective surface area.

Capillary beds are fed by an arteriole and drained by a venule. Connecting the arteriole and venule is the thoroughfare channel.  When blood enters this thoroughfare channel, it slows down as it diffuses out into the capillary bed.  Controlling blood flow to the capillaries are pre capillary sphincters. These are like open-close valves and are only on the arteriole side of the capillary bed.  When the precapillary sphinctersRings of smooth muscle at the entrance of capillary beds that regulate blood flow based on tissue ne constrict blood shunts through the thoroughfare channel and bypasses the capillary bed.  When you’re exercising, blood is just shunting through the capillary beds in your digestive organs.  You’re not going to digest while running a marathon.  But the skeletal muscles will have open precapillary sphincters allowing blood to fill the capillary bed.

The flow through capillaries is so far from the heart that it is a pulsatile flow called vasomotion. Erythrocytes lurch along in beg steps as the heart pumps and relaxes, pumps and relaxes. This allows oxygen and carbon dioxide to be exchanged. Think about it as a train making stops along the way, not just speeding through all the stations. 

Pressure in Capillary Beds

We consider the surface area of our blood vessels by looking at the cumulative surface area. We focus on one type of vessel at a time.  Theoretically, we’d take all the vessels in your body of one type. This includes all your muscular arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr or all your venulesSmall veins that collect blood from capillaries and transport it to larger veins.. We add together the diameter of each one of those vessels.  Arterioles would have the smallest cumulative surface area and capillaries would have the largest. 

By the time the blood reaches the peripheral tissues, the blood pressure has decreased to thirty-five millimeters of mercury. Once it exits the capillary beds, the blood pressure is down to fifteen millimeters of mercury. This low pressure is beneficial. If the pressure were too high, the nutrients carried in circulation could not diffuse out into the tissues. If blood flow were too fast, the nutrients carried in circulation could not diffuse out into the tissues. Cells would die. High blood pressure can easily cause a blow out of the capillary wall.

Capillary Types

There are three types of capillaries. Continuous capillaries are the most common and least permeable. They have squamous cells tightly packed with small pathways for movementA fundamental property of life involving motion of the body or its parts. in and out. Fenestrated capillaries, seen in kidneys, have larger holes between cells called fenestrae (meaning windows). Sinusoidal capillaries, also called discontinuous in some texts, have large clefts or sinusesAir-filled spaces in the skull that lighten it and help humidify air. between cells. These are leaky, allowing even white blood cells to pass through. Found in places like bone marrow, they help large cells leave the bloodstream. Large cells, such as white blood cells, can also enter the bloodstream.