B3.2.3 Adaptations of arteries for the transport of blood away from the heart Notes

Structure and Function of Artery Walls

1. Arteries are specialized blood vessels that transport blood from the heart to various tissues of the body.
2. Their walls are designed to handle high pressures generated during the cardiac cycle.

Layers of the Artery Wall

Artery walls consist of three distinct layers, each with specific components and roles:

1. Tunica externa:
   1. Composition: Tough outer layer of connective tissue containing collagen fibers.
   2. Function: Provides strength to withstand the high pressures inside arteries.
2. Tunica media:
   1. Composition: Thick layer of smooth muscle and elastic fibers made of the protein elastin.
   2. Function:
      1. The smooth muscle regulates the diameter of the artery through contraction and relaxation.
      2. The elastic fibers allow arteries to stretch and recoil, accommodating pressure fluctuations.
3. Tunica intima:
   1. Composition: A smooth endothelial lining that minimizes friction for blood flow. In some arteries, this layer also includes elastic fibers.
   2. Function: Provides a low-resistance surface for efficient blood flow.

How Arteries Handle High Pressure

Thick Walls with Elastic Fibers and Collagen

1. Elastic fibers can stretch during systole (when the heart pumps blood) and recoil during diastole (when the heart relaxes). This stretch-recoil mechanism ensures that blood pressure is maintained and blood flows smoothly.
2. Collagen fibers provide tensile strength to prevent ruptures or aneurysms under high pressure.

Narrow Lumen

1. Arteries have relatively narrow lumens, which maintain high pressure and promote fast blood flow.

Elastic Fibers and Energy Conservation

1. Elastic fibers account for up to 50% of the dry mass of artery walls.
2. During systole, they stretch to absorb the energy from high-pressure blood.
3. During diastole, they recoil, pushing blood forward and maintaining diastolic pressure.
4. This energy conservation minimizes the work required by the heart to pump blood.

Role in Blood Flow Regulation

Smooth Muscle Cells

1. Artery walls contain circular smooth muscle cells, especially in smaller branches called arterioles.
2. These muscles contract and relax to regulate the diameter of the lumen:
   1. Vasoconstriction: Smooth muscle contraction narrows the lumen, reducing blood flow.
   2. Vasodilation: Smooth muscle relaxation widens the lumen, increasing blood flow.
3. This regulation ensures that blood is distributed to tissues based on their oxygen and nutrient needs.

Preventing Backflow

1. When elastic fibers recoil, they push blood forward in the arteries.
2. At the same time, semilunar valves at the exit of the heart prevent blood from flowing backward into the ventricles.
3. This ensures efficient one-way blood flow to organs.