Human Physiology

The One Thing

Human physiology depends on the coordinated functioning of organ systems to obtain and distribute materials, release energy, remove wastes, produce movement, and regulate the internal environment. Homeostasis is maintained mainly through negative feedback, with nervous and endocrine systems coordinating corrective responses.

Reactions, Processes and Experiments

Key Terms

• Digestion: The mechanical and chemical breakdown of complex food into simple, soluble substances that can be absorbed and used by cells.
• Alimentary canal: A continuous muscular tube consisting mainly of the mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum, and anus.
• Peristalsis: Wave-like muscular contractions that move food through the alimentary canal.
• Digestive enzymes: Biological catalysts that break carbohydrates, proteins, and fats into smaller molecules.
• Absorption: The movement of digested nutrients from the intestine into the blood or lymph.
• Villi: Finger-like projections in the small intestine that increase surface area for absorption and contain blood capillaries and a lymph vessel.
• Assimilation: The use of absorbed nutrients by body cells for energy, growth, repair, and storage.
• Blood: A connective tissue made of plasma, red blood cells, white blood cells, and platelets.
• Plasma: The liquid part of blood that transports nutrients, hormones, gases, proteins, and wastes.
• Haemoglobin: An iron-containing pigment in red blood cells that carries oxygen and contributes to the transport of carbon dioxide.
• Cardiac cycle: The complete sequence of contraction and relaxation of the heart chambers during one heartbeat.
• Double circulation: A circulation pattern in which blood passes through the heart twice during one complete journey: once through pulmonary circulation and once through systemic circulation.
• Cardiac output: The volume of blood pumped by one ventricle per minute; cardiac output = heart rate × stroke volume.
• Blood pressure: The force exerted by circulating blood against the walls of blood vessels, commonly expressed as systolic pressure over diastolic pressure.
• Respiration: The process by which cells release energy from organic molecules, usually using oxygen in aerobic respiration.
• Ventilation: The movement of air into and out of the lungs through inspiration and expiration.
• Alveoli: Tiny air sacs in the lungs where oxygen and carbon dioxide are exchanged between air and blood.
• Gas exchange: The diffusion of oxygen into the blood and carbon dioxide out of the blood across respiratory surfaces.
• Tidal volume: The volume of air inhaled or exhaled during a normal quiet breath.
• Excretion: The removal of metabolic waste products and excess substances from the body.
• Nephron: The structural and functional unit of the kidney that filters blood and forms urine.
• Ultrafiltration: The pressure-driven filtration of water and small solutes from glomerular blood into Bowman's capsule.
• Selective reabsorption: The recovery of useful substances such as glucose, amino acids, ions, and much of the water from the filtrate into the blood.
• Tubular secretion: The movement of selected wastes and excess ions from blood into the nephron tubule.
• Osmoregulation: The maintenance of the correct water and dissolved-salt concentration in body fluids.
• Locomotion: Movement of the whole body from one place to another.
• Skeleton: The internal framework that supports the body, protects organs, stores minerals, and provides attachment for muscles.
• Joint: A place where two or more bones meet; joints may be immovable, slightly movable, or freely movable.
• Skeletal muscle: A voluntary, striated muscle attached to bones and responsible for body movement.
• Sliding filament mechanism: The process in which actin filaments slide over myosin filaments, shortening muscle fibres during contraction.
• Neuron: A specialized nerve cell that receives, processes, and transmits electrical signals.
• Synapse: A junction between neurons or between a neuron and an effector where signals are transmitted, usually by neurotransmitters.
• Reflex action: A rapid, automatic response to a stimulus that commonly follows a receptor-sensory neuron-relay neuron-motor neuron-effector pathway.
• Central nervous system: The brain and spinal cord, which process information and coordinate responses.
• Peripheral nervous system: All nerves outside the brain and spinal cord that connect the central nervous system with receptors and effectors.
• Endocrine gland: A ductless gland that releases hormones directly into the blood.
• Hormone: A chemical messenger produced in small amounts by endocrine tissues and transported through blood to specific target cells.
• Negative feedback: A control mechanism in which a change in a condition produces responses that oppose the original change and restore balance.
• Homeostasis: The maintenance of relatively constant internal conditions despite changes inside or outside the body.
• Insulin: A pancreatic hormone that lowers blood glucose by promoting glucose uptake and glycogen formation.
• Glucagon: A pancreatic hormone that raises blood glucose mainly by stimulating glycogen breakdown in the liver.
• Thyroxine: A thyroid hormone that helps regulate metabolic rate, growth, and development; its production depends on iodine.
• Adrenaline: An adrenal hormone that prepares the body for emergency action by increasing heart rate, breathing rate, and blood flow to skeletal muscles.
• ADH: Antidiuretic hormone, released from the posterior pituitary, which increases water reabsorption by the kidney collecting ducts.

Easily Confused

• Digestion and absorption: Digestion breaks food into small soluble molecules; absorption moves those molecules from the intestine into the blood or lymph.
• Absorption and assimilation: Absorption transfers nutrients into transport systems; assimilation is their use by body cells.
• Bile and digestive enzymes: Bile emulsifies fats and helps provide an alkaline medium; it is not an enzyme.
• Pulmonary and systemic circulation: Pulmonary circulation connects the heart with the lungs; systemic circulation connects the heart with the body tissues.
• Ventilation and gas exchange: Ventilation moves air into and out of the lungs; gas exchange is diffusion of oxygen and carbon dioxide between alveoli and blood.
• Respiration and ventilation: Respiration releases energy in cells; ventilation moves air through the respiratory system.
• Ultrafiltration and selective reabsorption: Ultrafiltration forms filtrate from blood in the glomerulus; selective reabsorption returns useful substances from filtrate to blood.
• Excretion and egestion: Excretion removes metabolic wastes; egestion removes undigested food from the alimentary canal.
• Nervous and endocrine control: Nervous control is generally rapid and short-lasting; endocrine control is chemical, generally slower, and often longer lasting.
• Sympathetic and parasympathetic divisions: The sympathetic division generally prepares the body for action; the parasympathetic division supports rest and digestion.
• Insulin and glucagon: Insulin lowers blood glucose; glucagon raises blood glucose.
• Parathyroid hormone and calcitonin: Parathyroid hormone increases blood calcium concentration; calcitonin generally lowers it.
• Locomotion and skeletal movement: Locomotion is movement of the whole body; skeletal movement is produced by muscles acting on bones and joints.

What Gets Asked

• Describe the stages of nutrition: Questions may require ingestion, digestion, absorption, assimilation, and egestion in the correct sequence. Marks are lost by omitting egestion or confusing absorption with assimilation.
• Explain enzyme and bile functions: Questions may ask about salivary amylase, pepsin, and bile. The key distinction is that bile emulsifies fats and helps provide an alkaline medium but is not an enzyme.
• Trace circulatory pathways: Questions may require the pulmonary or systemic pathway. Marks are lost by reversing the pulmonary artery and pulmonary veins or by confusing the aorta with the venae cavae.
• Calculate or interpret cardiac output: Use cardiac output = heart rate × stroke volume. Stroke volume is the amount pumped by one ventricle in one contraction, not the total blood volume in the heart.
• Explain ventilation and gas exchange: Questions may require the changes during inspiration and quiet expiration or the alveolar adaptations. Marks are lost by confusing thoracic volume with lung pressure or by omitting the large surface area, thin moist walls, rich blood supply, and concentration gradient.
• Explain kidney regulation and homeostasis: Questions may test ultrafiltration, selective reabsorption, tubular secretion, ADH, or negative feedback. Marks are lost by stating that all filtrate is reabsorbed or by confusing ADH-mediated water conservation with the removal of nitrogenous wastes.