The Cardiovascular System: An Overview

In an average day, your heart beats around 100,000 times. Each beat helps keep blood moving through the lungs and the rest of the body, using the heart, the blood vessels and the blood itself as one connected transport system.

Your heart repeatedly contracts and relaxes to keep blood moving through an enormous network of vessels. In an average adult, the combined blood vessel length is often estimated at tens of thousands of miles. Circulation is not one simple loop. It contains two linked circuits, and the heart's right and left sides drive different parts of the journey. Three linked elements The heart provides the pumping force. It lies behind the breastbone in the middle of the chest, tilted slightly left, and beats around 100,000 times each day. The blood vessels form the transport network. Arteries carry blood away from the heart, veins return it and capillaries allow exchange with surrounding tissues. Blood vessels are not passive pipes. Their walls can narrow or widen to help control resistance, meaning opposition to blood flow, and how much blood reaches different tissues. The blood is the circulating tissue. It transports oxygen, nutrients, hormones and waste products. It also contains white blood cells involved in defence and platelets that help limit bleeding. The two circuits The pulmonary circuit, also called pulmonary circulation , moves blood from the right side of the heart to the lungs and back to the left side. In the lungs, the blood releases carbon dioxide and takes up oxygen. The systemic circuit, also called systemic circulation , moves blood from the left side of the heart through the body and returns it to the right side. Within the pulmonary circuit, the pulmonary artery carries blood low in oxygen from the right side of the heart to the lungs. The pulmonary veins return oxygenated blood to the left side. Capillary exchange Arteries branch into smaller arteries and arterioles, which lead to microscopic capillary networks. Capillary walls are thin enough for oxygen, nutrients, water and waste products to move between the blood and surrounding tissue fluid. The exchanged substances then move between the tissue fluid and nearby cells. Capillaries supply most tissues, but they do not directly touch every cell. How the system works together Circulation depends on all three elements working together. The heart generates pressure, vessels direct and regulate flow, and blood carries substances between organs and tissues. A change in one part can affect the others. Reduced pumping can reduce blood flow to tissues, vessel narrowing can increase resistance, and changes in blood volume or oxygen carrying capacity can alter the demands placed on the circulation. What clinicians can assess Pulse and blood pressure provide different information about circulation. Listening to the heart and checking peripheral pulses or swelling can add further clues about the heart, vessels and tissue circulation. These findings describe parts of the system rather than providing one complete measure of cardiovascular function. When this system is affected Cardiovascular problems can contribute to breathlessness, fatigue, chest discomfort, palpitations (an awareness of the heartbeat), dizziness or ankle swelling. Those symptoms can also arise from other systems. Later symptom lessons examine chest pain, palpitations and feeling short of breath in their own clinical context.

Circulation uses two linked circuits: the right heart pumps through the lungs, the left heart pumps through the body, vessels regulate the routes and blood carries the transported substances.