The human body is made of matter organised into multiple biological levels, beginning with atoms and molecules and progressing through organelles, cells, tissues, organs, organ systems and the whole organism. The most abundant elements by mass include oxygen, carbon, hydrogen and nitrogen, with calcium and phosphorus also making major contributions, particularly in the skeleton. These elements combine into water, proteins, lipids, carbohydrates, nucleic acids, minerals and many other compounds.
Water is the most abundant component of the human body by mass in a typical adult, although the proportion varies with age, sex, body composition and tissue type. Atoms do not exist as an undifferentiated mixture. They are assembled into molecules, molecules form cellular structures, and cells organise into specialised tissues. The human body is therefore a highly ordered chemical and biological system rather than simply a collection of substances.
Water is fundamental to human biology because it provides the major solvent and medium for biochemical reactions, transport and temperature regulation. Body water is distributed between intracellular fluid, which is contained within cells, and extracellular fluid, which includes blood plasma and fluid surrounding cells. Electrolytes such as sodium, potassium, calcium, chloride, bicarbonate and phosphate contribute to osmotic balance, membrane potentials, acid base regulation and cellular signalling.
The exact concentration of these substances is tightly regulated because substantial deviations can disrupt cell function. Water also participates directly in chemical reactions, including hydrolysis reactions involved in digestion and metabolism. The proportion of water differs among tissues because fat contains relatively little water compared with many lean tissues. Consequently, total body water varies substantially among individuals and changes with growth, ageing and body composition.
Proteins are major biological macromolecules and perform an enormous range of functions. They are composed of amino acids joined by peptide bonds and folded into specific three dimensional structures. Structural proteins contribute to tissues such as muscle, skin, connective tissue and hair. Enzymes catalyse biochemical reactions, receptors detect signals, transport proteins move substances, antibodies participate in immune defence, and many hormones or signalling molecules influence cellular activity.
Collagen is a major structural protein in connective tissues, while actin and myosin are important for muscle contraction. Haemoglobin transports oxygen in red blood cells. Protein structure and function depend on amino acid sequence, folding, chemical modification and interactions with other molecules. Proteins are continually synthesised and degraded, allowing tissues to adapt to changing physiological requirements and allowing damaged or obsolete molecules to be removed.
Lipids include fats, phospholipids, steroids and other hydrophobic or amphipathic molecules. They serve as energy stores, structural components of cell membranes, signalling molecules and precursors for other biological compounds. Triglycerides provide a concentrated form of stored chemical energy and are accumulated primarily in adipose tissue. Phospholipids form the basic structure of cellular membranes, creating barriers that separate intracellular environments from the external environment and organise membrane proteins.
Cholesterol is an important membrane component and precursor for steroid hormones and other molecules, although its concentration and distribution are physiologically regulated. Lipids also contribute to insulation and protection of organs. Dietary fats provide essential fatty acids that humans cannot synthesise adequately and therefore must obtain from food.
Carbohydrates provide important sources of metabolic energy and participate in cellular structures and recognition processes. Glucose is a central metabolic fuel and can be stored in humans as glycogen, primarily in liver and skeletal muscle. Cells break down glucose through pathways including glycolysis and, under aerobic conditions, mitochondrial oxidative metabolism to generate ATP. ATP is the principal immediate energy currency used by cells for processes such as active transport, biosynthesis and mechanical work.
Carbohydrates also occur in glycoproteins and glycolipids, which participate in cell recognition and communication. Nucleic acids, including DNA and RNA, contain genetic information and participate in its expression and regulation. DNA stores hereditary information, while RNA performs numerous roles in gene expression, protein synthesis and cellular regulation.
Cells are the fundamental living units of the human body. Human cells are eukaryotic and contain a nucleus enclosed by a membrane, along with organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes and peroxisomes. Different cell types specialise in different functions. Neurons transmit information, muscle cells contract, epithelial cells form barriers and surfaces, red blood cells transport oxygen, immune cells defend against threats, and connective tissue cells produce or maintain extracellular materials.
Cells communicate through chemical signals, electrical activity and direct interactions. They also maintain internal conditions through membranes, transport systems, metabolic pathways and regulatory networks. Although cells share fundamental mechanisms, specialised gene expression and cellular architecture allow the human body to contain hundreds of distinct cell types and functional states.
Cells are organised into tissues, tissues into organs, and organs into organ systems. The major tissue categories are epithelial, connective, muscle and nervous tissue. An organ normally contains multiple tissue types arranged together to perform particular functions. The heart, for example, contains muscle tissue, connective tissue, nervous elements, blood vessels and specialised conducting structures. Organ systems coordinate related physiological activities.
The digestive system processes and absorbs nutrients, the respiratory system exchanges gases, the cardiovascular system transports substances, the nervous system coordinates information, the endocrine system regulates processes through hormones, and the renal system regulates fluid composition and removes metabolic waste. These systems are not independent. Blood connects them physically and chemically, while nervous, endocrine and immune signalling coordinates their activities.
The human body also contains minerals, gases, genetic material, microorganisms and many specialised chemical substances. Calcium and phosphorus are important components of bone mineral, while iron is essential for haemoglobin and numerous enzymes. Oxygen and carbon dioxide are transported through the body and participate in respiration and acid base regulation. Trace elements such as zinc, copper, iodine and selenium are required in small quantities for particular biological functions.
The human microbiome consists of communities of microorganisms associated with body surfaces and internal environments, especially the gastrointestinal tract, and these organisms can interact with nutrition, metabolism and immune function. Human tissues are continuously renewed to varying degrees, damaged molecules are repaired or removed, and cells can die through regulated and unregulated processes. The human body is therefore a dynamic system whose composition changes continuously while maintaining organised physiological function.
