The cardiovascular system is an intricate network responsible for transporting substances throughout the body. It is composed of the heart, a vast network of blood vessels, and the blood flowing within them. Its role is to deliver oxygen and nutrients to every cell while removing waste products. This continuous circulation sustains cellular function and maintains the body’s internal balance. The system operates as a closed circuit, with blood pumped by the heart through vessels to all parts of the body and back again.
The Heart as the Central Pump
The heart is a muscular organ, roughly the size of a fist, that functions as the pump for the circulatory system. It is divided into four chambers: two upper atria and two lower ventricles. The right atrium receives deoxygenated blood returning from the body, while the left atrium receives oxygenated blood from the lungs. From the atria, blood moves into the ventricles below; the right ventricle pumps deoxygenated blood to the lungs, and the left ventricle pumps oxygenated blood to the rest of the body.
To ensure that blood flows in a single direction, the heart has four valves. The tricuspid valve controls blood flow from the right atrium to the right ventricle, and the mitral valve manages flow between the left atrium and left ventricle. When the ventricles contract, the pulmonary valve opens to allow blood into the pulmonary artery, and the aortic valve opens to let blood into the aorta. These valves act like one-way doors, shutting to prevent the backward flow of blood.
The rhythmic pumping action of the heart is the cardiac cycle, which consists of two main phases: systole and diastole. Systole is the period of contraction, where the ventricles pump blood out of the heart. Diastole is the relaxation phase, during which the chambers refill with blood. This sequence constitutes a single heartbeat.
Blood Vessels and Blood Composition
The circulatory system relies on a network of blood vessels that act as conduits. These vessels are categorized into three main types: arteries, veins, and capillaries. Arteries carry blood away from the heart and have thick, muscular walls to withstand the high pressure of blood being pumped from the ventricles. As arteries extend from the heart, they branch into smaller arterioles, which lead to the capillaries.
Capillaries are the smallest blood vessels, forming networks that permeate body tissues. Their thin walls facilitate the exchange of gases, nutrients, and waste products between the blood and the surrounding cells. After this exchange, blood is collected into small venules, which merge to form larger veins.
Veins carry blood back toward the heart. Their walls are thinner than arteries because the blood they carry is under lower pressure. To prevent the backflow of blood, many veins contain one-way valves.
The fluid circulating within these vessels is blood, composed of several components. About 55% of blood volume is plasma, a yellowish liquid that transports blood cells, nutrients, hormones, and waste products. The remaining 45% of blood consists of red blood cells, white blood cells, and platelets. Red blood cells, or erythrocytes, contain hemoglobin, a protein that binds to and transports oxygen. White blood cells, or leukocytes, are a component of the immune system that defends the body against infection. Platelets, or thrombocytes, are small cell fragments that play a part in blood clotting.
Circulatory Pathways
The cardiovascular system is structured around two interconnected circulatory pathways: the pulmonary circuit and the systemic circuit. The entire journey through both circuits is a continuous loop, with the heart acting as the central switching station.
The pulmonary circuit is dedicated to gas exchange in the lungs. This pathway begins as the right ventricle pumps deoxygenated blood into the pulmonary artery, which carries it to the lungs. In the lungs, blood flows through capillaries where carbon dioxide is released and oxygen is absorbed. The now oxygen-rich blood returns to the heart’s left atrium through the pulmonary veins.
The systemic circuit delivers oxygen and nutrients to all other tissues. This process starts as the left ventricle pumps oxygenated blood into the aorta. From the aorta, blood is distributed through a branching network of arteries to every part of the body.
After delivering oxygen and picking up waste products, the deoxygenated blood begins its return trip. It travels through veins of increasing size, eventually entering the heart’s right atrium via the superior and inferior vena cava. The arrival of this blood marks the end of the systemic circuit, and it is then ready to be sent through the pulmonary circuit.
Systemic Functions and Transport
The cardiovascular system performs a range of transport functions necessary for cellular survival. Its most recognized role is delivering oxygen to the tissues and removing carbon dioxide, a waste product of cellular metabolism. This gas exchange is fundamental for cellular respiration, the process by which cells generate energy.
Beyond gas transport, the circulatory system is the body’s main distribution channel for nutrients. Substances absorbed from digested food enter the bloodstream and are carried to cells for energy, growth, and repair. The system also collects metabolic waste products, like urea, and transports them to the kidneys and liver for removal.
The cardiovascular system also serves as a communication network by transporting hormones. Endocrine glands release these chemical messengers into the bloodstream, which carries them to target organs to regulate bodily processes.
Finally, the system helps maintain a constant internal body temperature. Blood absorbs heat from active tissues, such as muscles, and distributes it throughout the body. To cool down, blood flow can be increased to the skin’s surface to release excess heat, or to conserve heat, blood flow to the periphery can be reduced.