For the survival of complex multicellular organisms, it is essential to have an efficient system for transporting nutrients, oxygen, hormones, and other vital substances to the cells, while simultaneously removing metabolic wastes. This critical function is performed by the circulatory system, which utilizes specialized body fluids. In humans and most higher animals, the primary fluids for this transport are blood and lymph.

Blood, a specialized fluid connective tissue, is the main circulatory medium. It is composed of two primary components: a fluid matrix called plasma and the formed elements that are suspended within it. Plasma, which constitutes about 55% of the blood’s volume, is a straw-colored fluid that is mostly water but also contains a rich mixture of proteins—such as fibrinogen (for clotting), globulins (for defense), and albumin (for osmotic balance)—along with nutrients, hormones, and electrolytes. The formed elements include erythrocytes (Red Blood Cells), leucocytes (White Blood Cells), and platelets. Red Blood Cells (RBCs) are the most numerous cells and are responsible for oxygen transport, a function facilitated by the iron-containing pigment hemoglobin. White Blood Cells (WBCs) are a crucial part of the immune system, defending the body against pathogens. Platelets are small cell fragments that play a vital role in blood coagulation, the process of forming a clot to prevent excessive blood loss from an injury.

Human blood is categorized into different groups based on the presence or absence of specific antigens on the surface of RBCs. The two most important groupings are the ABO system and the Rh system. The ABO system classifies blood into four types: A, B, AB, and O. This system is critical for ensuring compatibility during blood transfusions. The Rh system classifies blood as either Rh-positive or Rh-negative, which is also a crucial factor in transfusions and can have significant implications during pregnancy (erythroblastosis fetalis).

The hub of the human circulatory system is the heart, a four-chambered muscular organ. It consists of two upper chambers, the atria, and two lower, more muscular chambers, the ventricles. The right side of the heart handles deoxygenated blood, while the left side manages oxygenated blood. A specialized nodal tissue, including the sino-atrial (SA) node and the atrio-ventricular (AV) node, is responsible for initiating and coordinating the rhythmic contractions of the heart. The SA node, often called the “pacemaker,” generates the electrical impulse that triggers each heartbeat.

The sequential events of a single heartbeat constitute the cardiac cycle, which involves the contraction (systole) and relaxation (diastole) of the atria and ventricles. The cycle begins with the atria contracting to push blood into the ventricles, followed by the powerful contraction of the ventricles to pump blood out to the lungs and the rest of the body. The characteristic “lub-dub” sounds of the heartbeat are produced by the closing of the heart valves. The electrical activity of the heart can be graphically recorded using an electrocardiogram (ECG), a valuable diagnostic tool.

Humans possess a double circulatory system, which means the blood passes through the heart twice for each complete circuit of the body. This system consists of two distinct pathways: the pulmonary circulation and the systemic circulation. In the pulmonary circuit, the right ventricle pumps deoxygenated blood to the lungs, where it gets oxygenated and then returns to the left atrium. In the systemic circuit, the left ventricle pumps this newly oxygenated blood through the aorta to the rest of the body’s tissues. Deoxygenated blood from the body then returns to the right atrium, completing the cycle. This double system is highly efficient, ensuring that tissues receive a constant supply of oxygen-rich blood.

The circulatory system also includes lymph, a colorless fluid that is formed when plasma filters out of the capillaries into the spaces between tissue cells. The lymphatic system, a network of vessels, collects this fluid and returns it to the bloodstream. Lymph plays a vital role in the immune system and in the absorption of fats from the digestive tract.