A closed circulatory system is a circulatory system for blood where the blood is enclosed in individual vessels kept under pressure to force it to flow into all corners of the body. Humans and other vertebrates have circulation like this, and some other animals do as well. By contrast, in an open circulatory system, a fluid called hemolymph moves freely throughout the body, bathing the organs to deliver nutrients and carry wastes away. Closed circulatory systems tend to be more efficient and allow animals to reach a larger size.
The closed circulatory system includes a heart, acting as the pump for circulation, and a network of arteries and veins of various sizes, spreading in an extensive web throughout the body. Nutrients and oxygen cross the walls of the blood vessels for delivery to individual cells, while cellular wastes are taken back up for removal from the body. A separate fluid, lymph, circulates through the body to provide immune functions, among other things. Lymph floats freely in the interstitial fluid between cells, rather than being directed in specific vessels like blood.
The closed circulatory system can vary slightly between different animals. Some animals have two or three chambers in their hearts instead of four, and may have a simpler network of vessels. In all cases, the key feature is the enclosed nature, with blood not flowing freely throughout the body. The system allows for rapid and efficient transport of oxygen, nutrients, and wastes. The pressure created in the blood vessels forces the blood upwards, into the head, as well as making sure it reaches all the way to the ends of the extremities, providing the fingers and toes with a blood supply.
Some organs require more or less blood than others and the closed circulatory system allows blood to be directed precisely to where it needs to go. The liver, for instance, needs a steady supply of blood and a way to eliminate wastes, and has its own dedicated vein and artery to meet these needs. A rich network of capillaries spreads out across this organ to make sure it is amply served by the circulatory system.
Problems with the closed circulatory system can include a loss of blood pressure leading to tissue death, obstructions in the vessels that limit circulation, or injuries to major vessels, causing rapid blood loss and death if they are not addressed. Cardiovascular health is an important medical concern and patients are usually evaluated at every doctor's visit to check for unusual heart sounds, bluing extremities, and other signs that the circulatory system is not functioning properly.