The Cardiovascular System: An Overview

The cardiovascular system moves blood through the lungs and the rest of the body. It depends on three linked elements: the heart, the blood vessels and the blood travelling within them.

Your heart repeatedly contracts and relaxes to keep blood moving through an enormous network of vessels. In an average adult, the combined 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. Vessels are not passive pipes. Their walls can narrow or widen to help control pressure, resistance 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 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 moves blood from the left side of the heart through the body and returns it to the right side. Arteries and veins are named by direction, not oxygen level. The pulmonary artery carries blood low in oxygen away from the heart, while pulmonary veins return oxygenated blood towards it. 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. Why 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 problem in one element may affect the others. Reduced pumping can mean some tissues do not receive enough blood and oxygen, vessel narrowing can increase resistance, and changes in blood volume or oxygen carrying capacity can change the demands placed on the heart. Cardiovascular assessment A clinician may begin with the pulse, checking its rate, rhythm, strength and symmetry. Blood pressure measures the force of blood against artery walls and reflects several influences, including heart output, vessel resistance and blood volume. Listening to the chest may reveal normal heart sounds, extra sounds or murmurs produced by turbulent blood flow. Examination may also include peripheral pulses, signs of fluid accumulation and possible evidence of reduced oxygenation or blood flow. None of these findings provides a complete answer alone. They are interpreted with symptoms, previous measurements and other examinations or tests. Symptoms Cardiovascular problems can contribute to breathlessness, fatigue, chest discomfort, palpitations, dizziness or ankle swelling. These symptoms are not specific to the cardiovascular system. Similar symptoms may arise from the lungs, blood, medicines or several other causes, so the pattern and clinical assessment matter.

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.