Double circulation

Blood travels by a fixed route through arteries and veins. Each has three layers: tunica intima, an inner lining of squamous endothelium; tunica media, a middle layer of smooth muscle and elastic fibres; tunica externa, an outer coat of fibrous connective tissue rich in collagen. The middle coat is comparatively thin in veins.

The right ventricle pumps blood into the pulmonary artery; the left ventricle pumps into the aorta.

Pulmonary circulation: deoxygenated blood goes from the right ventricle through the pulmonary artery to the lungs, and oxygenated blood returns through the pulmonary veins to the left atrium.

Systemic circulation: oxygenated blood leaves the left ventricle through the aorta and passes through arteries, arterioles and capillaries to the tissues; deoxygenated blood is collected by venules, veins and the vena cava and returned to the right atrium.

The systemic circulation supplies nutrients, O₂ and other essentials to the tissues and removes CO₂ and other harmful substances for elimination.

Hepatic portal system: a special vascular link between the digestive tract and the liver. The hepatic portal vein carries blood from the intestine to the liver before it enters the systemic circulation.

The coronary system is a special set of vessels that carries blood exclusively to and from the heart muscle.

Humans have complete double circulation: two separate pathways, pulmonary and systemic.