22.8
Blood Flow
Blood is pumped by the heart into the aorta, the largest artery in the body, and then into increasingly smaller arteries, arterioles, and capillaries.…
Blood flow is driven by the heart which must be strong enough to push blood throughout the body into all of the major arteries, arterials and smallest capillary beds.
When blood leaves through the aorta and enters smaller arterials and capillaries, its high velocity and high pressure decrease, as a result of the increased combined diameters of the many blood vessels in comparison to the diameter of the aorta alone.
Such slow rate of movement provides adequate time for gas and nutrient exchange through the walls of the small blood vessels.
In contrast, blood travels faster through the venules, veins and back to the vena cava with the aid of smooth muscle in the vessel walls and compression from neighboring skeletal muscles preventing blood from pooling. Plus one way valves in veins prevent backflow despite the pull of gravity.
Ultimately, the process of blood flow is controlled by the body's needs and can be regulated by neurological signaling and hormones. For example, while exercising, blood is targeted to muscles due to vasodilation and away from the digestive system via vasoconstriction, directing blood where it is needed most.
Thus not all of the capillary beds have blood flowing through at all times.
View the full transcript and gain access to JoVE Core videos
Q1: Why does blood slow down as it enters smaller blood vessels?
Blood slows when entering smaller vessels because it flows from one large vessel into multiple smaller ones with a greater combined cross-sectional area. This increased resistance and larger total diameter cause velocity to decrease, providing adequate time for gas exchange and nutrient transfer through capillary walls.
Q2: How does the heart maintain blood pressure throughout the circulatory system?
The heart pumps blood into the aorta with sufficient force to push it through all major arteries, arterioles, and capillary beds. As blood travels through progressively smaller vessels, pressure decreases due to increased combined vessel diameters, but the heart's initial force drives circulation throughout the body.
Q3: What mechanisms prevent blood from pooling in veins?
Blood returns through veins via smooth muscle contractions in vessel walls and compression from neighboring skeletal muscles. One-way valves in veins prevent backflow despite gravity's pull, ensuring continuous return of blood to the heart without pooling or reversal of flow.
Q4: How does the body redirect blood flow during exercise?
During exercise, vasodilation increases blood flow to muscles while vasoconstriction decreases flow to the digestive system. Chemical signals direct blood where it is needed most, ensuring muscles receive more oxygen and nutrients during physical activity. Not all capillary beds receive blood simultaneously.
Q5: What is the relationship between blood vessel diameter and blood velocity?
Blood velocity decreases as vessel diameter decreases because blood enters multiple smaller vessels with greater combined cross-sectional area. This counterintuitive relationship creates increased resistance, slowing blood movement and allowing sufficient time for nutrient and gas exchange at the capillary level.
Q6: How do hormones and neurological signals regulate blood flow?
The body controls blood flow through neurological signaling and hormonal regulation based on physiological needs. These signals trigger vasodilation to increase blood flow or vasoconstriction to decrease it, allowing selective distribution of oxygen and nutrients to organs requiring them most.
Q7: Why is slow blood movement in capillaries important for body function?
Slow blood movement through capillaries provides adequate time for gas and nutrient exchange across vessel walls. This reduced velocity, caused by increased combined cross-sectional area of smaller vessels, ensures efficient transfer of oxygen and nutrients to tissues and removal of metabolic waste products.