1 Circulatory System Overview

The system’s role

The Circulatory system, also called the cardiovascular system, is a closed network that moves blood around the body. Its three main components—the heart, blood, and blood vessels—work together to deliver materials to tissues and carry wastes away.

Circulation delivers oxygen and nutrients to cells, carries carbon dioxide and other wastes away for removal, transports immune cells, supports blood clotting, and helps regulate body temperature and maintain internal balance.

The heart’s chambers and blood flow

The heart is a muscular pump with four chambers. The Atria are the upper chambers that receive blood, while the Ventricles are the lower chambers that pump blood out. Valves help keep blood moving in one direction.

Blood returning from the body enters the right atrium through the superior and inferior vena cava. It passes into the right ventricle, which pumps it through the pulmonary arteries to the lungs. In the lungs, blood picks up oxygen and releases carbon dioxide. It then returns through the pulmonary veins to the left atrium, moves into the left ventricle, and is pumped through the aorta to the body.

Two connected circuits

The heart and lungs are connected by Pulmonary circulation: blood travels from the right side of the heart to the lungs and returns to the left side. The lungs are where blood picks up oxygen and releases carbon dioxide.

Systemic circulation carries blood from the left side of the heart to body tissues and returns it to the right side. For example, oxygen picked up in the lungs travels through the pulmonary veins to the left side of the heart, then through systemic arteries to working muscles.

Blood vessels and their roles

Blood follows a continuous pathway: heart → arteries → arterioles → capillaries → venules → veins → heart.

• Arteries carry blood away from the heart. Their walls are generally thick and elastic to withstand higher pressure.

• Arterioles are small arteries that direct blood into capillary networks.

• Capillaries have very thin walls. Oxygen, nutrients, and wastes move between blood and surrounding tissues there.

• Venules collect blood leaving capillaries.

• Veins return blood to the heart. Many veins, particularly in the limbs, contain valves that help prevent backward flow.

Arteries and veins are named by the direction blood travels, not by how much oxygen it carries. For example, pulmonary arteries carry oxygen-poor blood to the lungs, while pulmonary veins carry oxygen-rich blood back to the heart.

Blood and the coordinated circulation

Blood consists of Plasma and formed elements. Plasma is the liquid portion and carries water, salts, proteins, nutrients, hormones, and dissolved wastes.

The formed elements include:

• Red blood cells carry most oxygen from the lungs to tissues and help transport carbon dioxide back toward the lungs.

• White blood cells help defend the body against infection.

• Platelets help form clots that limit blood loss when a vessel is damaged.

Each heartbeat creates pressure that moves blood through the vessels. At capillaries, exchange supplies tissues with needed substances and collects wastes. Veins return blood to the heart, which sends blood either to the lungs for gas exchange or back to the body. Together, the heart pumps blood, vessels guide it, and blood transports gases, nutrients, immune cells, and wastes.