Overview
This lecture explains the structure and function of blood vessels in the circulatory system, detailing how blood flows from the heart throughout the body and back.
Blood Vessel Structure & Functions
- The circulatory system is a closed network that starts and ends at the heart.
- Blood vessels are dynamic organs that contract and expand to regulate blood flow and pressure.
- There are three main types of blood vessels: arteries, veins, and capillaries.
- Arterioles are small branches of arteries leading to capillaries; venules gather blood from capillaries to veins.
- Blood vessels are structured in three layers: tunica intima (inner), tunica media (middle), and tunica externa (outer).
Layers of Blood Vessels (Tunics)
- Tunica intima: innermost layer, contains endothelium for smooth blood flow.
- Tunica media: middle layer of smooth muscle and elastin, controls vasoconstriction and vasodilation.
- Tunica externa: outer layer made of collagen fibers, provides protection and support.
Types of Arteries and Their Roles
- Elastic arteries (like the aorta) absorb pressure from heartbeats using more elastin.
- Muscular arteries (like brachial and radial) distribute blood to body parts and have a thick tunica media for regulation.
- Arterioles control blood flow to capillaries and help regulate blood pressure.
Capillaries and Exchange
- Capillaries have thin walls (one epithelial cell layer) for easy exchange of oxygen, nutrients, and wastes.
- Capillary beds regulate blood pressure and temperature via sphincters that control blood flow.
Veins and Blood Return
- Venules collect blood from capillaries and merge into veins that return to the heart.
- Veins have low pressure and use valves (especially in limbs) to prevent backflow.
Blood Flow Pathway Example
- Blood leaves the left ventricle via the aorta, travels through arteries and arterioles to capillaries, then returns via venules and veins.
- The full circuit from the heart to the thumb and back takes about one minute.
Key Terms & Definitions
- Artery — Blood vessel that carries blood away from the heart.
- Vein — Blood vessel that returns blood back to the heart.
- Capillary — Smallest blood vessel where exchange of materials occurs.
- Arteriole — Small artery leading to a capillary.
- Venule — Small vein collecting blood from capillaries.
- Tunica intima — Innermost blood vessel layer with endothelium.
- Tunica media — Middle, muscular, and elastic vessel layer.
- Tunica externa — Outer protective vessel layer of collagen.
- Vasoconstriction — Narrowing of blood vessels.
- Vasodilation — Widening of blood vessels.
- Endothelium — Smooth inner lining of blood vessels.
Action Items / Next Steps
- Review the blood flow pathway and structure of vessel layers.
- Study the role of vasoconstriction and vasodilation in regulating blood pressure.