Closer to the heart, arteries would be expected to have a higher percentage of __. a. endothelium b. smooth muscle fibers c. elastic fibers d. collagenous fibers
The correct answer and explanation is:
The correct answer is: c. elastic fibers
Explanation:
Arteries closer to the heart, such as the aorta and other large elastic arteries, have a higher percentage of elastic fibers in their walls. This is a key structural adaptation that helps them perform their vital function in the circulatory system.
Why elastic fibers?
- Function of elastic arteries:
Arteries near the heart, called elastic arteries, must withstand and accommodate the high pressure generated by the heart’s contractions. When the heart pumps blood during systole (the contraction phase), blood is ejected forcefully into these arteries, causing them to stretch. - Elastic recoil:
The abundant elastic fibers allow these arteries to stretch to absorb the surge of blood and then recoil during diastole (the relaxation phase). This recoil helps maintain a continuous flow of blood through the circulatory system even when the heart is resting between beats. This function is known as the Windkessel effect. - Smooth muscle fibers and other fibers:
- Smooth muscle fibers are more abundant in muscular arteries that are further from the heart. These arteries regulate blood flow to different organs by constricting or dilating.
- Endothelium lines all blood vessels but is not the dominant structural fiber in arteries.
- Collagenous fibers provide strength and limit overstretching but are not the primary fiber for elasticity.
- Structural layers of arteries:
The arterial wall consists of three layers:- Tunica intima (innermost) – contains endothelium.
- Tunica media (middle) – rich in elastic fibers and smooth muscle cells, especially in elastic arteries.
- Tunica externa (adventitia) – mainly collagen and connective tissue for support.
Summary:
Arteries close to the heart have a greater proportion of elastic fibers to accommodate the high-pressure pulses of blood, stretch during systole, and recoil during diastole to maintain smooth blood flow. This elastic property is crucial for protecting smaller downstream vessels and ensuring continuous circulation. As arteries branch further from the heart, the proportion of smooth muscle fibers increases to help regulate blood distribution.