6 The Cardiovascular System
Learn how the heart, blood vessels, and blood work together to move blood, exchange materials, and support the body.
The system’s role
The cardiovascular system consists of the heart, blood vessels, and blood. Together, these structures circulate blood to deliver oxygen and nutrients to tissues, carry wastes away, help regulate the body’s internal conditions, and support defense against infection and blood loss.
Heart structure and valves
The heart is a muscular pump in the chest. A wall called the septum separates its right and left sides. Each side has two chambers: the upper chambers, called atria, receive blood entering the heart, while the lower chambers, called ventricles, pump blood out.
Four valves help keep blood moving forward. The tricuspid valve is between the right atrium and right ventricle, and the pulmonary valve is between the right ventricle and pulmonary artery. On the left, the mitral valve separates the left atrium and ventricle, while the aortic valve lies between the left ventricle and aorta.
The is the heart’s contracting muscle. The endocardium lines the chambers, and the pericardium is the surrounding protective sac. Coronary arteries branch from the aorta and supply the heart muscle with oxygen-rich blood. The heart’s electrical system coordinates the rhythm of chamber contractions.
Blood vessels and their roles
Blood vessels form a branching network throughout the body. Their names describe the direction blood travels relative to the heart, not necessarily its oxygen content.
Arteries carry blood away from the heart. Their walls are generally thick and elastic to withstand pressure.
Arterioles are smaller arteries. Their smooth muscle can narrow or widen them, helping regulate blood flow to tissues.
are tiny vessels with very thin walls. They are the main sites where gases, nutrients, and wastes pass between blood and tissues.
Venules collect blood from , and veins return blood to the heart. Many veins have valves that help limit backward flow.
Pulmonary arteries carry oxygen-poor blood, while pulmonary veins carry oxygen-rich blood. Thus, arteries carry blood away from the heart and veins carry it toward the heart, regardless of oxygen content.
The two circuits of circulation
Blood travels through two connected circuits.
Oxygen-poor blood enters the right atrium from the body through the superior and inferior vena cava. It passes through the tricuspid valve into the right ventricle, then through the pulmonary valve and pulmonary arteries to the lungs. There, it releases carbon dioxide and takes up oxygen. Pulmonary veins return the blood to the heart, where it enters the left atrium.
Blood moves from the left atrium through the mitral valve into the left ventricle. The left ventricle pumps it through the aortic valve into the aorta, then through arteries to body tissues. After exchange in , blood returns through veins to the right atrium.
In order, the full route is: body; venae cavae; right atrium; tricuspid valve; right ventricle; pulmonary valve; pulmonary arteries; lungs; pulmonary veins; left atrium; mitral valve; left ventricle; aortic valve; aorta; body.
Blood and its components
Blood is a fluid connective tissue made of and formed elements. is the liquid portion, composed mostly of water, with dissolved proteins and other substances. It transports blood cells and carries substances such as nutrients, hormones, proteins, and wastes.
The formed elements include:
, which contain hemoglobin. Hemoglobin carries most oxygen from the lungs to tissues and helps transport some carbon dioxide back toward the lungs.
White blood cells (leukocytes), which help defend the body against infection and other threats.
Platelets (thrombocytes), which are cell fragments that help form a plug and support clotting when a blood vessel is injured.
Pumping and
The heart generates the pressure that moves blood through vessels. Each heartbeat includes coordinated filling and pumping: the atria move blood into the ventricles, then the ventricles contract to send blood to the lungs and body. The valves close to help prevent blood from flowing backward.
is the amount of blood a ventricle pumps each minute. It depends on heart rate and stroke volume, the amount pumped with each beat:
For example, during exercise, a faster heartbeat and more blood pumped per beat raise to help deliver more oxygen to active muscles. Overall, the cardiovascular system transports materials, supports tissue exchange, helps maintain internal balance, and contributes to immune defense and clotting.