Overview
This lecture covers the basic anatomy and function of blood vessels, the types of blood vessels, their structural layers, and how blood circulates through the vascular system.
Types of Blood Vessels and Blood Flow
- Blood vessels carry blood to and from the heart, starting and ending at the heart.
- Five main types: arteries, arterioles, capillaries, venules, and veins.
- Arteries carry blood away from the heart and branch into smaller arterioles.
- Arterioles further branch into capillaries where gas and nutrient exchange occurs.
- Capillaries merge into venules, which combine to become veins that return blood to the heart.
Vessel Wall Structure
- Blood vessel walls have three layers (tunics): Tunica interna (intima), Tunica media, and Tunica externa (adventitia).
- Tunica interna: innermost, composed of endothelium (simple squamous lining).
- Tunica media: middle layer, mostly smooth muscle; responsible for vasoconstriction (narrowing) and vasodilation (widening).
- Tunica externa: outermost, mainly loose collagen fibers for protection and anchoring.
- Large vessels may have vasa vasorum ("vessels of the vessels") in the Tunica externa for supplying nutrients.
Arteries and Their Types
- Arteries have thicker tunica media and more elastic fibers to handle higher pressure.
- Elastic (conducting) arteries modulate blood pressure by expanding and recoiling.
- Muscular arteries control blood distribution by contracting or relaxing smooth muscle.
- Arterioles ("resistance vessels") regulate blood flow and pressure before reaching capillaries.
Capillaries and Capillary Beds
- Capillaries are the smallest vessels, composed only of endothelium for exchange.
- Three capillary types:
- Continuous (least leaky, most common),
- Fenestrated (have pores for greater permeability, found in kidneys, endocrine organs),
- Sinusoidal (most leaky, found in liver, spleen, bone marrow).
- Capillary beds are networks where exchange occurs; most have pre-capillary sphincters to regulate blood flow.
- Vascular shunts direct minimal flow when beds are closed.
- Arterial anastomosis provides alternate pathways to ensure blood supply.
Venules and Veins
- Venules collect blood from capillaries and combine to form veins.
- Veins have larger lumens and thinner walls than arteries, with valves to prevent backflow.
- Blood return is aided by skeletal muscle pump, respiratory pump, and arterial pulsation.
- Valve failure can lead to varicose veins and descending valvular incompetence.
- Treatments for varicose veins include compression therapies and surgical interventions.
Key Terms & Definitions
- Artery — vessel carrying blood away from the heart.
- Vein — vessel carrying blood toward the heart.
- Arteriole — small branch of an artery leading to capillaries.
- Venule — small vein collecting blood from capillaries.
- Capillary — tiny vessel where exchange of gases/nutrients/wastes occurs.
- Tunica intima (interna) — innermost vessel layer of endothelium.
- Tunica media — middle smooth muscle layer of vessel wall.
- Tunica externa (adventitia) — outer connective tissue layer.
- Vasoconstriction — narrowing of blood vessels.
- Vasodilation — widening of blood vessels.
- Vasa vasorum — small vessels that supply walls of large vessels.
- Precapillary sphincter — muscle ring controlling blood flow into capillaries.
- Anastomosis — connection between two vessels for alternate blood flow.
- Varicose veins — swollen, twisted veins from valve failure.
Action Items / Next Steps
- Review specific names and locations of major blood vessels in upcoming lab sessions.
- Study diagrams of vessel wall structure, capillary types, and capillary beds for identification.