Arteries, Veins, Capillaries

Feature	Arteries	Veins	Capillaries
Physiology	Thick, muscular walls Diameter of lumen is smaller than vein Smooth muscle layer and connective tissue layer is thicker than vein. (not important to compare the ephithelial tissue between arteries and veins.	Muscular walls but thinner than arteries Diameter of lumen is larger than arteries. However smooth muscle layer and connective tissue layer is thinner than arteries.	Walls are made of epithelial/endothelial tissue that is 1-cell thick, absence of smooth muscle. This enables rapid diffusion of oxygen and carbon dioxide between red blood cells and the surrounding tissue. Lumen of capillaries are so small that red blood cells travel in a single file, slow enough for gaseous exchange to occur.
Functionality	Carry blood away from heart Carry oxygenated blood (except pulmonary artery)	Carry blood to the heart Carry de-oxygenated blood (except pulmonary vein)	Allow exchange of gases between red blood cells in the blood and the surrounding tissue and cells.
Presence of valves	Absence of valves (except pulmonary artery and aorta)	Presence of valves	Absence of valves.
		Skeletal muscle contractions squeeze blood in veins to allow it to flow through the veins at low pressure and velocity.
Pressure	High	Low	Lowest
Elasticity	More elastic	Less elastic

Components of blood
Plasma	Red blood cells	White blood cells	Platelets
Almost transparent liquid.	Circular biconcave shape (no nucleus) More flexible to squeeze through walls of capillaries and no nucleus = more haemoglobin to bind with oxygen = more efficient transport.	Irregular shaped nucleus (bilobed, trilobed etc.) Largest cells in blood	Smallest cells in blood.
90% water +blood electrolytes etc. help maintain osmotic balance and buffer at pH 7.4	Contains haemoglobin which binds reversibly to oxygen	Defends the body against invading pathogens. (produces antibodies,
A medium of transport.	Each haemoglobin binds with 4 molecules of oxygen so 1 red blood cell can transport up to a billion molecules	When present in large quantities, white blood cells will start destroying the body’s own cells etc. (leukemia)
	Short live span Destroyed in the liver/spleen after 100? days because the body cannot break down iron properly.