Body Fluids and Circulation — Core Principles

Core Principles

Body fluids and circulation form the essential transport system of the human body. Blood, the primary body fluid, is composed of plasma (water, proteins, salts) and formed elements: red blood cells (RBCs) for oxygen transport via hemoglobin, white blood cells (WBCs) for immunity, and platelets for blood clotting.

Blood groups (ABO, Rh) are determined by antigens on RBCs and are critical for safe transfusions. Blood clotting is a complex cascade involving platelets and plasma proteins to prevent blood loss. The lymphatic system, a parallel network, collects interstitial fluid (lymph), returns it to blood, transports fats, and plays a vital immune role via lymph nodes and lymphocytes.

The human circulatory system is closed and double, featuring a four-chambered heart (two atria, two ventricles) that pumps blood. Arteries carry blood away from the heart, veins carry it back, and capillaries facilitate exchange.

The cardiac cycle describes the heart's rhythmic contraction (systole) and relaxation (diastole), generating heart sounds ('lubb-dubb'). Electrical activity is recorded by an ECG (P, QRS, T waves). Double circulation ensures efficient oxygenation, with pulmonary circulation to the lungs and systemic circulation to the rest of the body.

Heart activity is regulated by neural and hormonal mechanisms. Common disorders include hypertension, coronary artery disease, and heart failure.

Important Differences

vs Arteries vs. Veins

Aspect	This Topic	Arteries vs. Veins
Direction of Blood Flow	Carry blood away from the heart.	Carry blood towards the heart.
Oxygenation State (General)	Carry oxygenated blood (except pulmonary artery).	Carry deoxygenated blood (except pulmonary veins).
Wall Thickness	Thick, muscular, and elastic walls.	Thin, less muscular, and less elastic walls.
Lumen Size	Narrow lumen.	Wide lumen.
Blood Pressure	High pressure.	Low pressure.
Valves	Generally absent (except semilunar valves at origin).	Present, especially in limbs, to prevent backflow.
Location	Deeply seated in the body.	Superficial (visible under skin) and deep.

Arteries are robust vessels designed to transport high-pressure blood away from the heart, featuring thick, elastic walls and narrow lumens. They primarily carry oxygenated blood. In contrast, veins are thinner-walled, wider-lumen vessels that return lower-pressure, deoxygenated blood to the heart, often equipped with valves to counteract gravity and prevent backflow. Understanding these structural and functional differences is crucial for comprehending the dynamics of blood circulation and potential cardiovascular pathologies.