6 The Cardiovascular System
Learn how the heart, blood, and blood vessels work together to circulate blood, exchange materials, and maintain the body’s internal balance.
The cardiovascular system
The cardiovascular system consists of the , blood, and blood vessels. Together, they move oxygen, nutrients, hormones, heat, and waste products through the body. Blood carries oxygen from the lungs to tissues and returns carbon dioxide to the lungs; it also helps regulate body temperature and maintain internal balance.
structure and pumping
The is a muscular pump in the chest between the lungs. The separates its right and left sides. Each side has an upper , which receives blood, and a lower , which pumps blood out.
Four valves help keep blood moving in one direction:
The tricuspid valve lies between the right and right .
The pulmonary valve lies between the right and pulmonary trunk.
The mitral valve lies between the left and left .
The aortic valve lies between the left and aorta.
The ’s own electrical signals coordinate its contractions. Each heartbeat includes filling as the chambers relax and ejection as the ventricles contract. The left has a thick muscular wall because it must generate enough pressure to propel blood through the body.
Blood vessels and their roles
Blood travels through a branching network of vessels. Arteries carry blood away from the , and their relatively thick, elastic walls withstand the pressure generated by the . Arterioles adjust their diameter, helping control blood flow into tissues and influencing .
Capillaries have very thin walls and are the main sites where oxygen, nutrients, and other substances move between blood and tissues. Carbon dioxide and some wastes enter the blood there. Venules collect blood from capillaries and carry it toward veins. Veins return blood to the ; many, especially in the limbs, have valves that help prevent backward flow.
Arteries and veins are named by the direction blood travels relative to the , not by how much oxygen it carries.
The pulmonary and systemic circuits
Blood circulates through two connected routes. In the , it travels from the to the lungs and back. In the , it travels from the to the body’s tissues and returns.
The complete route begins at the body tissues. Blood returns through the venae cavae to the right , passes through the tricuspid valve into the right , and leaves through the pulmonary valve into the pulmonary trunk and pulmonary arteries. It reaches the lung capillaries, then returns through the pulmonary veins to the left . From there, it passes through the mitral valve into the left , leaves through the aortic valve into the aorta, and travels back to the body tissues.
In lung capillaries, blood releases carbon dioxide and takes up oxygen. In body capillaries, it delivers oxygen and nutrients and collects carbon dioxide and other wastes. Pulmonary arteries are an exception to the usual pattern because they carry oxygen-poor blood; pulmonary veins are another exception because they carry oxygen-rich blood.
Blood components and functions
Blood consists of and . is the liquid portion and carries water, dissolved substances, and proteins.
The have different roles:
Red blood cells contain hemoglobin and transport most of the blood’s oxygen.
White blood cells help defend the body against infection and other threats.
Platelets are cell fragments that help form clots when a blood vessel is damaged.
Clotting, also called , limits blood loss after injury. Blood also transports nutrients, hormones, and wastes, contributes to immune defense and temperature regulation, and helps maintain a stable internal environment.
Blood flow, pressure, and
The ’s pumping creates pressure that drives blood through vessels. is generally highest in the systemic arteries and falls as blood moves through the circulation. Vessel diameter affects resistance: narrowing an increases resistance and can reduce flow through that vessel, while widening it can increase flow.
is the volume of blood one pumps in a minute. It depends on rate and :
For example, a rate of and a of produce of , or about . The body adjusts rate and vessel diameter to help match blood delivery to changing needs, such as during exercise.