Overview
This lecture covers the structure and function of blood vessels, how blood circulates through the body, and the adaptations that support circulation.
Blood Vessel Types and Structure
- The circulatory system includes arteries, veins, and capillaries, plus smaller arterioles (mini-arteries) and venules (mini-veins).
- Blood vessels are dynamic organs that actively contract and expand to regulate blood flow and pressure.
- All blood vessels (except capillaries) have three layers: tunica intima (inner), tunica media (middle), and tunica externa (outer).
- The tunica intima contains endothelium, a smooth lining that reduces blood friction.
- The tunica media is made of smooth muscle and elastin, controlling vessel diameter via vasoconstriction and vasodilation.
- The tunica externa is mostly collagen fiber and provides protection and reinforcement.
Differences Among Vessel Types
- Arteries carry blood away from the heart; veins return it to the heart; capillaries allow exchange between blood and tissues.
- Elastic arteries (like the aorta) absorb pressure surges from the heart and recoil to dampen fluctuations.
- Muscular arteries (like the brachial and radial) distribute blood to specific regions and have the thickest tunica media.
- Arterioles branch from arteries and feed capillary beds.
- Capillaries consist of a single epithelial layer (tunica intima) to maximize exchange of oxygen, nutrients, and wastes.
- Venules collect blood from capillaries and merge into veins.
The Circulatory Circuit and Adaptations
- Blood moves in a closed loop, starting and ending at the heart; the system contains about five liters of blood.
- Venous valves, especially in limbs, prevent backflow when returning blood to the heart under low pressure.
- Damage to vessels can cause visible bleeding or bruising (internal bleeding).
- Blood pressure and flow are regulated by vessel contraction; form matches function in vessel design.
Capillary Beds and Thermoregulation
- Capillary beds are networks that regulate blood flow and blood pressure, and play a role in body temperature.
- Sphincters around capillaries can restrict or allow blood flow to conserve or release heat.
Key Terms & Definitions
- Artery — blood vessel carrying blood away from the heart.
- Vein — blood vessel returning blood to the heart.
- Capillary — tiny vessel for exchange of gases and nutrients with tissues.
- Tunica intima — innermost vessel layer, in direct contact with blood.
- Tunica media — middle vessel layer of smooth muscle and elastin.
- Tunica externa — outer vessel layer, mainly collagen fibers.
- Vasoconstriction — narrowing of blood vessels by muscle contraction.
- Vasodilation — widening of blood vessels by muscle relaxation.
- Endothelium — smooth epithelial lining inside blood vessels.
Action Items / Next Steps
- Review the three-layer structure of blood vessels and their functions.
- Study the pathway of blood from the heart through arteries, capillaries, and veins.
- Understand how vessel adaptations help control blood flow and pressure.