Structure and Function of Blood Vessels

Function of blood

Transportation, protection and homeostasis (regulation)
Blood vessels are needed to achieve this

Blood vessels allow blood to perform its function by providing…

Closed system of tubes - carry blood away from the heart and organs and then back to the heart and lungs
Pulmonary circulation - blood to lungs
Systemic circulation - blood to the rest of the body

Three layers of blood vessel walls

Tunica intima (interna)
Tunica media (middle)
Tunica externa (adventitial)

Tunica intima

Innermost layer (endothelium) of blood vessel walls that consists of thin, flattened cells, a basement membrane and internal elastic lamina that is smooth to reduce friction and secrete locally acting chemical mediators

Tunica media

Muscle and connective tissue (elastin) which account for differences in vessels, muscles arranged circularly to regulate diameter, controlled by sympathetic nervous system, external elastic lamina separates middle from outer layer

Tunica externa

Elastic and collagen fibres, nerves and in larger vessels anchors the vessels to surrounding tissues

Anastomosis

Union of branches of two or more arteries - alternative route for blood

Features of arteries

Carry blood away from the heart under high pressure (oxygenated blood apart form the pulmonary artery)
15% of blood volume
Elastic arteries = largest (aorta), large diameter, walls relatively thin
Muscular arteries = tunica media smooth muscle and less elastin, range in sizes, continue to branch as distributing arteries (brachial artery)
Anastomosis

Structure of arteries

Tunica intima, tunica media (large amount of smooth muscle and elastic fibres), tunica externa (thick and contains fibroblasts, collagen and elastic fibres arranged longitudinally

Function of arteries

Distribute blood to major organs
Elastic fibres enable vessel walls to stretch
Muscular arteries can adjust blood flow

Features of arterioles

Deliver blood to capillaries
Approximately 400 million ranging in size from 15um to 300um
Terminal end, distal most muscle cell forms the pre-capillary sphincter

Structure of arterioles

Tunica intima and tunica media (smooth muscle and elastic fibre)

Function of arterioles

Regulate blood flow to tissues and affect blood pressure
Pre-capillary sphincters regulate resistance to blood flow

Features of capillaries

Microcirculation from capillaries to venules
20 billion for large surface area
5% of blood volume
Smallest blood vessel (5-10um diameter)
Found near every cell in the body

Structure of capillaries

Tunica intima but no tunica media or externa
Fenestrated capillaries found in kidneys, small intestine, ventricles of the brain have small pores to allow larger molecules out of the capillaries
Sinusoids found in the liver have larger pores to allow movement of protiens

Function of capillaries

Permit exchange of nutrients and waste between blood and tissues

Features of venules

Collects blood from capillaries to veins
Thin walls
10-50um in diameter

Structure of venules

Tunica intima
Tunica media - little smooth muscle

Functions of venules

Exchange of nutrients and waste

Features of veins

Returns blood to the heart under low pressure
Thin walls
0.5mm for small and 3cm for large such as the vena cava
Distensible (capable of being stretched or dilated)

Structure of veins

Tunica intima thinner than that of arteries
Tunica media thin
Tunica externa thickest layer consisting of collagen and elastic fibres
Valves that are thin folds of the tunica intima

Function of veins

Blood reservior
Unidirectional flow of blood due to valves preventing the back flow of blood

Venous return

Veins - low pressure system that needs assistance to return blood to the heart
Skeletal muscle pump - walking - skeletal muscle contraction assists with the pumping of blood back to the heart
Valves prevent the backflow of blood
Respiratory muscle pump - alternating compression and decompression in the veins, when breathing, the diaphragm moves down, this reduces pressure in the thorax which increases pressure in the abdominal cavity, compressing veins and moving blood towards the heart