Textbook Question
Why do capillaries permit the diffusion of materials, whereas arteries and veins do not?
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19. The Blood Vessels
Capillaries
1:57 minutesProblem 9
Textbook Question
Blood flow in the capillaries is steady despite the rhythmic pumping of the heart because of the:
a. Elasticity of the large arteries
b. Small diameter of capillaries
c. Thin walls of the veins
d. Venous valves
Verified step by step guidance1
Understand the physiological context: The heart pumps blood in a rhythmic, pulsatile manner, but blood flow in capillaries is steady and continuous rather than pulsatile.
Recall the role of large arteries: Large arteries, such as the aorta, have elastic walls that stretch when the heart pumps blood and then recoil to maintain pressure and smooth out the flow between heartbeats.
Analyze the options: Consider how each option might contribute to steady blood flow. The small diameter of capillaries affects resistance but not the smoothing of flow; thin walls of veins relate to permeability, not flow steadiness; venous valves prevent backflow but do not smooth arterial pulsations.
Identify the correct mechanism: The elasticity of large arteries acts as a pressure reservoir, absorbing the pulsatile output of the heart and releasing it steadily, which results in continuous capillary blood flow.
Summarize the concept: The Windkessel effect, due to the elastic recoil of large arteries, is responsible for converting the intermittent pumping of the heart into a steady flow in the capillaries.
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Video duration:
1mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Elasticity of Large Arteries
Large arteries, such as the aorta, have elastic walls that stretch during the heart's pumping phase and recoil during relaxation. This elasticity smooths out the pulsatile blood flow from the heart, converting it into a more steady flow in smaller vessels like capillaries.
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Capillary Structure and Function
Capillaries are tiny blood vessels with thin walls that facilitate exchange of gases and nutrients. Their small diameter slows blood flow, but the steadiness of flow is mainly influenced by upstream factors rather than their size alone.
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Role of Venous Valves
Venous valves prevent backflow of blood in veins, ensuring unidirectional flow toward the heart. While important for venous return, they do not directly affect the steadiness of blood flow in capillaries.
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