Blood Vessels: MDCAT Biology notes

Three kinds of vessel

Blood leaves the heart in arteries, which branch into arterioles and then capillaries. Capillaries join into venules, which unite into veins that return blood to the heart. Artery and vein walls have three layers: an inner endothelium (tunica intima), a middle layer of smooth muscle and elastic fibres (tunica media) and an outer connective tissue layer (tunica externa).

Feature	Artery	Capillary	Vein
Wall	Thick, muscular and elastic	One cell thick (endothelium only)	Thin, less muscle
Lumen	Narrow	Very narrow (about one red cell wide)	Wide
Valves	Absent	Absent	Present (semilunar, pocket-like)
Pressure	High, pulsatile	Falling	Lowest
Main job	Carry blood away from heart	Exchange with tissues	Return blood to heart

Pressure and velocity

• Pressure is highest in the aorta, which receives the full force of the left ventricle, and falls steadily to its lowest in the venae cavae.
• Velocity is slowest in capillaries because their total cross-sectional area is the greatest of any vessel type. Slow flow gives time for exchange.
• Flow is driven by the heartbeat and blood pressure and depends on the vessels' cross-sectional area. In veins, return is helped by skeletal muscle contraction squeezing the veins, with valves preventing backflow.

Capillary exchange

Materials pass between blood and body cells through the interstitial (tissue) fluid that bathes the cells. Flow into each capillary bed is controlled by precapillary sphincters, rings of smooth muscle at the entrance to the capillaries, which open or close according to the tissue's needs.

Major human vessels

• The aorta leaves the left ventricle; the descending aorta divides into two common iliac arteries, which continue as the femoral arteries of the legs.
• The iliac veins from the lower limbs unite to form the inferior vena cava.
• The renal vein carries deoxygenated blood from each kidney into the inferior vena cava (the renal artery brings blood to the kidney).
• The mesenteric veins drain the intestines and, with the gastric and splenic veins, form the hepatic portal vein to the liver. The hepatic vein drains the liver into the vena cava.
• The pulmonary veins are the only veins in the adult carrying oxygenated blood; the pulmonary arteries are the only arteries carrying deoxygenated blood.

Blood cells often asked alongside

• Granulocytes: neutrophils (phagocytosis), eosinophils (allergy, parasites), basophils (release heparin, an anticoagulant, and histamine).
• Agranulocytes: monocytes and lymphocytes.
• Platelets are cell fragments of megakaryocytes and have no nucleus.

Common MDCAT traps

• Capillaries have the slowest flow because of their largest total cross-section, not because of two-way flow.
• The aorta divides into iliac arteries, not iliac veins.
• Veins, not arteries, carry valves; veins have wide lumens and thin walls.
• The renal vein, not the renal artery, returns kidney blood to the vena cava.
• Monocytes are agranulocytes; neutrophils and basophils are granulocytes.

Quick revision

• Highest pressure: aorta.
• Slowest velocity: capillaries.
• Precapillary sphincters regulate capillary flow.
• Exchange occurs through interstitial fluid.
• Pulmonary vein carries oxygenated blood.