Human body

The human body is the complete physical structure of a human being, composed of numerous specialised cells organised into tissues, organs, and organ systems. Externally, it comprises the head, neck, torso, limbs, and external genitalia, while internally it contains complex networks of bones, muscles, organs, blood vessels, and other structures essential for life. The study of the human body encompasses anatomy, physiology, histology, and embryology, disciplines that collectively describe its form, function, and development. Many biological mechanisms operate in tandem to maintain homeostasis, ensuring the stable regulation of nutrients, gases, electrolytes, and metabolic by-products. The body is studied by clinicians, health professionals, physiologists, anatomists, and artists alike.

Composition

The human body is primarily composed of a small number of chemical elements—most notably oxygen, hydrogen, carbon, calcium, and phosphorus. These elements form billions of molecules across trillions of cells and extracellular substances. In an adult male, roughly 60 per cent of body mass is water, distributed between intracellular fluid and extracellular compartments, including blood plasma and interstitial fluid. Water composition, pH, and ionic concentration are tightly regulated. Sodium and chloride are the predominant ions outside cells, whereas potassium and phosphate dominate within.
Cells are the fundamental building blocks of the body, numbering approximately 30 trillion in a mature human. Alongside them reside roughly 38 trillion bacteria, particularly on the skin and within the gastrointestinal tract, forming a diverse microbiota. Not every structure is cellular; extracellular matrix made of proteins such as collagen contributes to structural integrity. Human cells experience tens of thousands of DNA lesions daily; efficient DNA repair is therefore vital for preventing mutations that threaten cell viability.
Within each nucleated cell, DNA resides in the nucleus, where gene transcription produces RNA. This RNA is translated into proteins, which carry out cellular functions and regulate gene expression. Some cells, such as mature red blood cells, lose their nuclei during maturation and no longer contain DNA.

Tissues

Tissues are groups of cells specialised to perform particular functions. Histology—the microscopic study of tissues—recognises four principal types:

• Epithelial tissue, which lines external surfaces and internal cavities, forming protective barriers and regulating molecular passage. It occurs in many forms, from flat squamous cells to ciliated cells in the respiratory tract and columnar cells in the gastrointestinal tract.
• Connective tissue, providing structural support; examples include bone, cartilage, fat, and blood.
• Muscle tissue, responsible for movement and force generation, including skeletal, cardiac, and smooth muscle.
• Nervous tissue, composed of neurons and supporting cells, transmitting electrical impulses throughout the body.

Organs

Organs are collections of tissues arranged to perform specific physiological functions. Aside from the skin, most organs are contained within the body’s internal cavities. Examples include the heart, lungs, kidneys, liver, and spleen. Major body cavities include the pleural cavities surrounding the lungs and the abdominopelvic cavity containing digestive and associated organs.

Heart

The heart is a hollow muscular organ situated in the thoracic cavity, slightly left of the midline, and enclosed by the pericardium. It drives the circulatory system by transporting oxygen, nutrients, hormones, immune cells, and waste products. Structurally, the heart contains two atria and two ventricles. The atria facilitate continuous venous return and increase cardiac output, while the ventricles pump blood to the lungs and systemic circulation.
The cardiac conduction system, beginning at the sinoatrial node, coordinates contraction. Electrical impulses travel through the atria, pause at the atrioventricular node to allow ventricular filling, then stimulate ventricular contraction. Coronary artery disease—often caused by atherosclerotic plaque—remains the leading cause of death worldwide, with risk factors including obesity, tobacco use, high cholesterol, hypertension, inactivity, and diabetes. Primary cardiac cancer is rare, as cardiac muscle cells rely on hypertrophy rather than active division.

Gallbladder

The gallbladder is a small pear-shaped organ beneath the liver. It stores and concentrates bile, releasing it into the small intestine via the common bile duct to aid lipid digestion. Blood supply is provided by the cystic artery, commonly branching from the hepatic artery proper. Gallstones are frequent, often asymptomatic, but obstruction of the biliary tract may cause biliary colic, nausea, and vomiting. Treatment typically involves cholecystectomy. Gallstones also increase the risk of gallbladder carcinoma, which is uncommon but often fatal if diagnosed late.

Systems

Organ systems work in coordination to sustain life. Two major systems include the circulatory and digestive systems.

Circulatory system

The circulatory system comprises the heart, arteries, veins, capillaries, and blood. It transports oxygen, metabolic fuel, waste products, hormones, and immune cells. Circulation is divided into:

• Pulmonary circulation, carrying blood from the heart to the lungs for gas exchange.
• Systemic circulation, distributing oxygenated blood throughout the body.

Blood contains various cell types—including red and white blood cells—and continually exchanges substances with tissues.

Digestive system

The digestive system includes the mouth, teeth, tongue, oesophagus, stomach, small and large intestines, liver, pancreas, gallbladder, and salivary glands. Its function is to break down food into absorbable nutrients such as amino acids, fatty acids, vitamins, and minerals. Following swallowing, food travels through the gastrointestinal tract, undergoing mechanical and chemical digestion before nutrients enter the bloodstream.

Integration and homeostasis

The human body relies on interconnected regulatory systems to maintain homeostasis. Hormonal, neural, and chemical feedback loops adjust parameters such as temperature, pH, blood glucose, and oxygen levels. Organ systems rarely act alone; for instance, the respiratory and circulatory systems work together to oxygenate tissues, while the renal and endocrine systems regulate fluid and electrolyte balance.