Overview
This lecture introduces the structure and function of blood vessels, including arteries, veins, and capillaries, and explains how blood circulates and exchanges materials in the body.
Types and Structure of Blood Vessels
- Arteries carry blood away from the heart; veins carry blood towards the heart; capillaries allow exchange of gases, nutrients, and wastes.
- Arteries branch into smaller vessels and become capillaries, which are thin and numerous.
- The walls of arteries and veins have three layers: tunica intima (inner), tunica media (middle), and tunica adventitia/externa (outer).
- Tunica intima is made of a single layer of simple squamous epithelium lining the vessel.
- Tunica media contains smooth muscle, allowing vasoconstriction (narrowing) and vasodilation (widening).
- Tunica adventitia consists of dense fibrous connective tissue for support.
- Large vessels have their own blood supply (vasa vasorum) within the tunica externa.
Differences Between Arteries and Veins
- Arteries have thicker walls and more smooth muscle than veins due to higher blood pressure.
- Veins have larger lumens and thinner tunica media; they hold more blood (about 65–70% of total volume).
- Arteries contain elastic fibers and membranes for stretch and recoil.
- Veins have valves made from the tunica intima to prevent backflow.
- Blood pressure is much higher in arteries and decreases toward the veins.
Types of Arteries and Capillaries
- Elastic arteries (e.g., aorta) are large and stretchable due to more elastic fibers.
- Muscular arteries have more smooth muscle and regulate blood flow and pressure.
- Arterioles are smallest arteries and regulate flow to capillary beds.
- Capillary beds are networks for exchange; pre-capillary sphincters regulate local blood flow.
- Collateral arteries can supply the same area, forming arterial anastomoses for backup circulation.
Types of Capillaries and Anastomoses
- Continuous capillaries: tight junctions, found in the brain (blood-brain barrier).
- Fenestrated capillaries: have pores for rapid exchange, found in kidneys and choroid plexus.
- Sinusoid capillaries: large gaps, found in red bone marrow, liver, and spleen for blood cell passage.
- Anastomoses: arterial, venous, and arteriovenous (connections between arteries, veins, or both).
Venous Return and Blood Flow
- Veins rely on skeletal muscle and respiratory pumps to help return blood to the heart.
- Blood always flows from high to low pressure.
- Veins act as blood reservoirs; most blood volume is in veins.
- Veins do not normally have a pulse; pulse is present in arteries due to systolic and diastolic pressure.
Key Terms & Definitions
- Tunica intima — innermost layer of a vessel, made of simple squamous endothelium.
- Tunica media — middle layer, composed of smooth muscle, controls vessel diameter.
- Tunica adventitia/externa — outer layer, consists of connective tissue.
- Vasa vasorum — small blood vessels supplying the walls of larger vessels.
- Vasoconstriction — narrowing of blood vessels by smooth muscle contraction.
- Vasodilation — widening of blood vessels by smooth muscle relaxation.
- Anastomosis — interconnection between blood vessels.
- Precapillary sphincter — band of smooth muscle regulating flow into capillaries.
- Skeletal muscle pump — skeletal muscle contractions aiding venous return.
- Respiratory pump — thoracic pressure changes during breathing assisting venous return.
Action Items / Next Steps
- Review diagrams of vessel structure and types.
- Study the functions of different capillaries.
- Learn key differences between arteries, veins, and capillaries.