Blood is a fluid tissue with cellular components that play roles in transport of respiratory gases, immune system function, and blood clotting. The properties of the arteries, arterioles, capillaries, venules, and veins reflect their functions. Exchanges between the blood and interstitial fluids occur in the smallest of those vessels, the capillaries. These exchanges are governed by Starling’s forces and the movement of HCO3– into the blood. Lymphatic vessels return interstitial fluid to the blood.
learning outcomes
You should be able to:
Describe the features, life cycle, and function of a red blood cell.
Describe the blood clotting process.
Explain the role of arterioles in the control of blood flow.
Explain the forces that cause water to leave and reenter the capillaries.
Describe the factors that affect the return of blood in veins to the heart.
Why do athletes train at high altitudes?
The partial pressure of O2 is lower at high altitudes; therefore the athletes experience hypoxia and acclimate to the high altitude environment by producing more red blood cells. As a result, when they compete at lower altitudes, they have greater aerobic capacity.
Why is the blood clotting process called a cascade?
Blood clotting is a massive event involving large numbers of cells and signals, yet it can be initiated by a small wound that exposes collagen fibers. Platelets that come into contact with those fibers are activated. They become sticky and stick to other platelets, which also become activated. The activated platelets release chemical signals that trigger a long line of clotting reactions, and with each step, the reaction grows. Thus a small event triggers a larger and larger number of events leading to clotting—
Why are arterioles called resistance vessels and veins called capacitance vessels, and what do these terms imply about function?
Arterioles are called resistance vessels because of the smooth muscles in their walls that can constrict the vessels and increase their resistance. Changing the resistance of different arterioles allows blood flow to be directed to the tissues and organs that need it the most. Veins are called capacitance vessels because their thin walls with many elastic fibers can stretch to hold larger volumes of blood. When the body is at rest, all of the blood does not have to circulate to serve the body’s needs, so a large proportion of it can be held in reserve in capacitance vessels.
In advanced starvation, the body breaks down its blood plasma proteins for energy. At this stage of disease, fluid accumulates in the abdomen and the extremities. What is the connection between these two consequences of starvation?
The blood proteins are responsible for the osmotic pressure that draws interstitial fluid back into the capillaries. If the blood proteins are metabolized, the blood osmotic pressure drops and more plasma that leaves the capillaries due to hydrostatic pressure remains in the interstitial spaces. Thus interstitial fluid accumulates in the abdominal cavity and the extremities.
Every tissue in the body requires an adequate flow of oxygen-