The three main types of blood circulation are systemic circulation, pulmonary circulation, and portal circulation.

Systemic circulation carries oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium. It involves the aorta, systemic arteries, arterioles, systemic capillaries, venules, and systemic veins.

In systemic tissues, systemic capillaries are where the exchange of nutrients and gases occurs. Blood unloads oxygen (O2) and picks up carbon dioxide (CO2).

Pulmonary circulation brings deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium. It involves the pulmonary trunk, pulmonary arteries, capillaries around the alveoli, venules, and pulmonary veins.

Pulmonary veins are the only veins that carry oxygenated blood.

Portal circulation involves venous blood passing from capillary beds in the digestive system, spleen, and pancreas, which is collected by the portal vein and delivered to the liver.

A portal vein is a vein that carries blood from one capillary network to another.

Venous blood from the digestive system, spleen, and pancreas enters the liver via the portal vein. The liver also receives oxygenated blood from the hepatic artery. This mixes in the sinusoids, and blood eventually leaves through the hepatic veins.

In portal circulation, blood with a high concentration of nutrients absorbed from the stomach and intestines goes to the liver first for processing.