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Cardiovascular system
- 5. Anatomy of a capillary bed
- 12. Major arteries and veins of the systemic circuit
- 15. Cross-sectional area as it relates to blood pressure and velocity
Editor's Notes
- Blood vessels require oxygen and nutrients, so larger ones have blood vessels in their walls.
- The walls of arteries and veins have three layers. The inner layer is composed largely of endothelium, with a basement membrane that has elastic fibers; the middle layer is smooth muscle tissue; the outer layer is connective tissue (largely collagen fibers). Arteries (on left) have a thicker wall than veins because they have a larger middle layer than veins. Capillary walls (center) are one-cell-thick endothelium. Veins (on right) are larger in diameter than arteries, so that collectively veins have a larger holding capacity than arteries.
- The inner layer of an artery wall is a simple squamous epithelium called endothelium with a connective tissue basement membrane with elastic fibers. The outer layer is fibrous connective tissue near the middle layer, but it becomes loose connective tissue at its periphery.
- Capillaries have one-cell-thick walls composed only of endothelium with a basement membrane. Capillaries form vast networks with a total surface area of 6,000 square meters in humans.
- A capillary bed forms a maze of capillary vessels that lies between an arteriole and a venule. When sphincter muscles are relaxed, the capillary bed is open, and blood flows through the capillaries. When sphincter muscles are contracted, blood flows through a shunt that carries blood directly from an arteriole to a venule. As blood passes through a capillary in the tissues, it gives up its oxygen (O 2 ). Therefore, blood goes from being O 2 -rich in the arteriole (red color) to being O 2 -poor in the vein (blue color).
- The coronary arteries and cardiac veins pervade cardiac muscle. The coronary arteries bring oxygen and nutrients to cardiac cells, which derive no benefit from blood coursing through the heart.
- The blue-colored vessels carry O 2 -poor blood, and the red-colored vessels carry O 2 -rich blood; the arrows indicate the flow of blood. Capillaries are present in all parts of the body, so no cell is located far from a capillary.
- A realistic representation of the major blood vessels of the systemic circuit shows how the systemic arteries and veins are arranged in the body.
- Although the blood pressure in the brachial artery is typically about 120/80, blood pressure actually varies throughout the body. Blood pressure is highest in the aorta and lowest in the venae cavae.
- Blood pressure and blood velocity drop off in capillaries because capillaries have a greater cross-sectional area than arteries.
- Osmotic pressure in the blood is created by the presence of salts and the plasma proteins.
- At the arterial end of a capillary, the blood pressure is higher than the osmotic pressure; therefore, water tends to leave the bloodstream. In the midsection, oxygen and carbon dioxide follow their concentration gradients. At the venous end of a capillary, the osmotic pressure is higher than the blood pressure; therefore, water tends to enter the bloodstream.
- Thromboembolism is a clot that has moved and is now stationary in a new blood vessel where it can cause damage.