Introduction to Physiology & Homeostasis

Physiology: The Science of Function

Physiology is the branch of biology concerned with the normal functions of living organisms and their parts. Human physiology integrates knowledge at the molecular, cellular, tissue, organ, and system levels to explain how the body works as a coordinated whole. The guiding principle unifying all of physiology is homeostasis — the maintenance of a stable internal environment despite continuous changes in the external world.

Levels of Structural Organisation

The human body is organised into a hierarchy of increasing complexity. Atoms combine to form molecules (e.g., proteins, lipids, nucleic acids). Molecules assemble into organelles within cells. Cells — the smallest units capable of independent life — are the fundamental structural and functional units of the body. Similar cells and their surrounding extracellular matrix group together to form tissues. The four primary tissue types are epithelial, connective, muscular, and nervous tissue. Two or more tissue types combine to form organs (e.g., the kidney, heart, lung), and groups of organs working toward a common function constitute organ systems.

The Internal Environment and Body Fluid Compartments

Claude Bernard first articulated the concept of the milieu intérieur — the fluid environment that surrounds cells. Walter Cannon later coined the term "homeostasis" to describe the dynamic steady state maintained within this environment.

Total body water (TBW) constitutes approximately 60% of body weight in an average adult male (50% in females, due to higher adipose content). TBW is divided into two major compartments: intracellular fluid (ICF) contains about two-thirds of TBW, and extracellular fluid (ECF) contains one-third. The ECF is further subdivided into the interstitial fluid (ISF, ~75% of ECF) and plasma (~25% of ECF). A small transcellular compartment (CSF, synovial fluid, aqueous humour) accounts for the remainder.

The ionic composition of each compartment is tightly regulated. ICF is rich in K⁺ and phosphate; ECF is rich in Na⁺, Cl⁻, and HCO₃⁻. These concentration gradients are maintained by membrane transport proteins, particularly the Na⁺/K⁺-ATPase pump, which expels three Na⁺ and imports two K⁺ per ATP hydrolysed.

Homeostatic Mechanisms

Homeostasis is maintained by control systems comprising three essential components: a sensor (receptor) that detects a change in a variable; a control centre (integrating centre, usually the brain or endocrine gland) that processes the signal and generates a response; and an effector that carries out the corrective action.

Negative feedback is the predominant homeostatic mechanism. When a variable deviates from its set point, the response opposes the deviation and returns the variable toward the set point. Examples include thermoregulation (fever triggers sweating and vasodilation to cool the body), blood glucose regulation (rising glucose triggers insulin release, which promotes cellular uptake), and arterial pressure control (a rise in pressure triggers baroreceptor firing that reduces cardiac output and peripheral resistance).

Positive feedback amplifies a deviation, driving the system further from the set point. This mechanism is less common and is typically self-limiting. Examples include the LH surge triggering ovulation, uterine contractions during labour (Ferguson reflex), and platelet aggregation during haemostasis.

Feedforward control (anticipatory regulation) initiates corrective action in advance of a disturbance. For example, the cephalic phase of digestion begins gastric acid secretion in anticipation of a meal before food enters the stomach.

Set Points and Variability

Each regulated variable has a set point (the target value) and a normal range. Set points are not absolutely fixed — they shift with circadian rhythms, physiological state (exercise, pregnancy), and pathological processes. For instance, body temperature peaks in late afternoon and falls during sleep; arterial pressure rises during exercise.

Integration Across Systems

No organ system operates in isolation. The cardiovascular system delivers oxygen and nutrients; the respiratory system provides gas exchange; the renal system regulates fluid volume and pH; the endocrine system coordinates long-range chemical signalling; and the nervous system provides rapid electrochemical communication. HUBS192 examines each of these systems in turn before exploring their integration.