Double circulation, control and disorders

Arteries and veins have walls of three layers: an inner lining of flat cells (tunica intima), a middle layer of smooth muscle and elastic fibres (tunica media), which is thinner in veins, and an outer layer of fibrous connective tissue (tunica externa).

Blood makes two circuits: this is double circulation.

• Pulmonary circulation (to the lungs): right ventricle → pulmonary artery → lungs (where blood takes in oxygen) → pulmonary veins → left atrium.
• Systemic circulation (to the body): left ventricle → aorta → arteries → arterioles → capillaries in the tissues → venules → veins → vena cava → right atrium.
• A special vein, the hepatic portal vein, takes blood from the intestine to the liver first, before it returns to the heart. The heart muscle has its own blood supply: the coronary system.

The heart is myogenic: its beat starts in its own nodal tissue. But a centre in the medulla oblongata can adjust it through the autonomic nervous system. Sympathetic nerves raise the heart rate, the strength of each beat and the cardiac output. Parasympathetic nerves lower the rate, slow the conduction, and reduce output. Hormones from the adrenal medulla also raise cardiac output.

Some disorders:

• Hypertension (high blood pressure): normal pressure is 120/80 mm Hg (systolic/diastolic). Readings of 140/90 or higher, again and again, mean hypertension. It harms the heart, brain and kidneys.
• Coronary artery disease (atherosclerosis): deposits of calcium, fat, cholesterol and fibrous tissue narrow the arteries supplying the heart muscle.
• Angina pectoris: sudden chest pain when not enough oxygen reaches the heart muscle.
• Heart failure (congestive heart failure): the heart cannot pump enough blood for the body's needs. It is not the same as cardiac arrest (the heart stops beating) or a heart attack (heart muscle is suddenly damaged by a poor blood supply).