Breathing is the physical process of taking in oxygen-rich air (inspiration) and giving out carbon dioxide-rich air (expiration). The human respiratory system consists, in sequence, of the external nostrils, nasal cavity, pharynx, larynx, trachea (windpipe), two bronchi, bronchioles and finally the alveoli of the lungs, where the exchange of gases takes place.

Oxygen is transported mainly (about 97 percent) by binding to the haemoglobin of red blood cells, forming oxyhaemoglobin; a small amount is dissolved in the plasma. Carbon dioxide is transported in three ways: mostly (about 70 percent) as bicarbonate ions in the plasma, about 20 to 25 percent bound to haemoglobin as carbaminohaemoglobin, and a small amount dissolved in the plasma.

Tidal volume (TV) is the volume of air breathed in or out during a normal (quiet) breath, about 500 mL. Residual volume (RV) is the volume of air that remains in the lungs even after a forceful expiration, about 1100 to 1200 mL. Vital capacity (VC) is the maximum volume of air that a person can breathe out after the deepest possible inspiration (it equals tidal volume plus inspiratory reserve volume plus expiratory reserve volume).

VC = TV + IRV + ERV

Blood is a fluid connective tissue made of a liquid plasma (about 55 percent) and formed elements (about 45 percent). Plasma carries dissolved nutrients, wastes, hormones and proteins. The formed elements are: red blood cells (erythrocytes), which carry oxygen using haemoglobin; white blood cells (leucocytes), which defend the body against infection (immunity); and platelets (thrombocytes), which help in the clotting of blood.

The ABO blood group system classifies human blood into four groups - A, B, AB and O - based on the presence or absence of two antigens (A and B) on the surface of red blood cells and the corresponding antibodies in the plasma. The Rh factor is another antigen (Rh antigen or D antigen) first found in rhesus monkeys; people who have it on their red cells are Rh-positive and those who lack it are Rh-negative. Blood groups must be matched before transfusion to avoid clumping (agglutination).

(1) Arteries carry blood away from the heart (usually oxygenated, except the pulmonary artery), whereas veins carry blood towards the heart (usually deoxygenated, except the pulmonary vein). (2) Arteries have thick, elastic muscular walls and a narrow lumen and lack valves, whereas veins have thinner walls and a wider lumen and possess valves that prevent the backflow of blood.

These terms describe the main nitrogenous waste excreted by animals. Ammonotelic animals excrete ammonia, which is highly toxic and needs a lot of water (e.g. many bony fishes and aquatic amphibians). Ureotelic animals excrete urea, which is less toxic and needs less water (e.g. mammals including humans, and adult amphibians). Uricotelic animals excrete uric acid, which is least toxic and almost insoluble, needing very little water (e.g. birds, reptiles and insects).

Haemodialysis is an artificial method of removing nitrogenous wastes such as urea from the blood using a machine (an artificial kidney) when a person's kidneys have failed. The patient's blood is passed through tubes made of a semipermeable membrane that are surrounded by a dialysing fluid; wastes diffuse out of the blood into the fluid while useful substances are retained, and the cleaned blood is returned to the body. It is used to keep patients with kidney failure alive.

The two main contractile proteins of a muscle are actin (the thin filament) and myosin (the thick filament). A muscle fibre contains many myofibrils, each made of repeating units called sarcomeres. A sarcomere is the region of a myofibril between two adjacent Z-lines and is the basic functional (contractile) unit of a striated muscle; it contains an arrangement of actin and myosin filaments whose sliding produces contraction.

A joint is the point of contact between two or more bones (or between bone and cartilage) that allows movement or gives support. Three types of freely movable (synovial) joints are: (1) ball-and-socket joint, allowing movement in all directions, e.g. the shoulder and hip joint; (2) hinge joint, allowing movement in one plane, e.g. the elbow and knee; and (3) pivot joint, allowing rotational movement, e.g. between the atlas and axis vertebrae of the neck.

The human brain has three main parts: the forebrain, midbrain and hindbrain. The cerebrum (part of the forebrain) is the centre for thinking, memory, intelligence and voluntary actions. The cerebellum (part of the hindbrain) maintains balance and coordinates precise voluntary movements. The medulla oblongata (part of the hindbrain) controls involuntary vital activities such as heartbeat, breathing, blood pressure, swallowing and vomiting.

The resting potential is the electrical potential difference across the membrane of a neuron when it is not conducting an impulse; the inside is negatively charged relative to the outside (polarised), maintained by the sodium-potassium pump keeping more sodium ions outside and potassium ions inside. The action potential (nerve impulse) is the rapid reversal of this charge when a stimulus is applied: sodium ions rush in and the membrane becomes depolarised (positive inside), and this change travels along the axon as the nerve impulse before the resting state is restored.

A synapse is the junction between the axon terminal of one neuron and the dendrite (or cell body) of the next neuron, with a tiny gap called the synaptic cleft between them. When a nerve impulse reaches the axon terminal, it triggers the release of chemical messengers called neurotransmitters (such as acetylcholine) into the synaptic cleft. These diffuse across the gap and bind to receptors on the next neuron, generating a new impulse in it. Thus transmission across a synapse is chemical and one-directional.

Insulin is a hormone secreted by the beta cells of the pancreas; it lowers the level of glucose in the blood by promoting the uptake of glucose by cells and its conversion into glycogen in the liver. A deficiency of insulin causes the disease diabetes mellitus. Adrenaline (epinephrine) is secreted by the adrenal medulla; it is the emergency or fight-or-flight hormone that prepares the body for stress by increasing heart rate, blood pressure, breathing rate and blood glucose level.