Homeostasis

Overview

Homeostasis represents one of the most fundamental principles in biology: the ability of organisms to maintain stable internal conditions despite external fluctuations. The term derives from the Greek words 'homeo' (similar) and 'stasis' (standing still), and it underpins survival across all living systems. In the OCR A-Level specification, homeostasis focuses on three critical regulatory systems: blood glucose control (glucoregulation), body temperature regulation (thermoregulation), and water balance (osmoregulation). Each system operates through negative feedback loops, where deviations from an optimum set point trigger corrective responses that restore equilibrium. Examiners assess this topic heavily through extended response questions worth 4-8 marks, requiring candidates to demonstrate not only factual recall but also the ability to apply the stimulus-receptor-coordinator-effector-response model to novel physiological scenarios. Understanding the precise molecular mechanisms—such as how insulin binds to glycoprotein receptors or how ADH increases aquaporin insertion—is essential for accessing the highest mark bands. This topic also provides synoptic links to cell signalling, enzyme action, and evolutionary adaptations, making it a cornerstone of biological literacy at A-Level.

Key Concepts

Concept 1: Negative Feedback Mechanisms

Negative feedback is the self-regulating process by which biological systems maintain homeostasis. When a physiological variable deviates from its set point (the optimal value), receptors detect this change and initiate a response that counteracts the deviation, returning the system to its normal state. Crucially, negative feedback does not 'stop' a process—it reverses the direction of change. This distinction is vital for exam success, as many candidates lose marks by stating that negative feedback 'switches off' a system rather than explaining that it opposes the initial stimulus.

The general sequence follows a predictable pathway: Stimulus → Receptor → Coordinator → Effector → Response → Return to Normal. For instance, if body temperature rises above 37°C (the set point), thermoreceptors in the hypothalamus detect the increase. The hypothalamus (acting as the coordinator) then activates effectors such as sweat glands and arterioles in the skin. Sweating increases evaporative cooling, while vasodilation increases heat loss via radiation. As temperature falls back to 37°C, the corrective mechanisms are reduced in intensity. This creates a dynamic equilibrium, with the variable oscillating slightly around the set point rather than remaining perfectly static.

Example: In thermoregulation, if core body temperature drops to 36°C, peripheral thermoreceptors and the hypothalamus detect the fall. The hypothalamus stimulates skeletal muscles to shiver (generating heat through increased respiration) and causes vasoconstriction of skin arterioles (reducing heat loss). Once temperature returns to 37°C, these responses diminish.

Concept 2: Glucoregulation – Control of Blood Glucose

Blood glucose concentration must be maintained within a narrow range (approximately 80-100 mg per 100 cm³ of blood) to ensure cells receive a constant supply of glucose for respiration, while preventing damage from hyperglycaemia. The pancreas plays the central role, specifically the Islets of Langerhans, which contain two key cell types: alpha cells (secreting glucagon) and beta cells (secreting insulin).

When blood glucose rises—such as after consuming a carbohydrate-rich meal—beta cells detect the increase and secrete insulin into the bloodstream. Insulin is a globular protein hormone that binds to specific glycoprotein receptors on the surface of target cells, primarily hepatocytes (liver cells) and muscle cells. This binding triggers a cascade of intracellular events. Firstly, it activates the enzyme adenyl cyclase, which converts ATP to cyclic AMP (cAMP), a second messenger. The cAMP activates protein kinases that phosphorylate enzymes, leading to two major effects: (1) increased insertion of glucose transporter proteins (GLUT4) into the cell surface membrane, enhancing glucose uptake, and (2) activation of the enzyme glycogen synthase, which catalyses the conversion of glucose to glycogen for storage—a process called glycogenesis. Insulin also stimulates increased respiration of glucose and the conversion of glucose to fat in adipose tissue. The net result is a fall in blood glucose concentration back to the set point.

Conversely, when blood glucose falls—during fasting or exercise—alpha cells detect the decrease and secrete glucagon. Glucagon binds to receptors on hepatocytes, activating a similar cAMP-mediated cascade. This activates the enzyme glycogen phosphorylase, which breaks down stored glycogen into glucose (glycogenolysis). Glucagon also promotes gluconeogenesis, the synthesis of glucose from non-carbohydrate sources such as amino acids and glycerol. These processes raise blood glucose back to normal. Adrenaline, secreted during stress or exercise, also stimulates glycogenolysis and has a similar effect to glucagon.

Example: A candidate consumes a meal containing 50g of carbohydrate. Within 30 minutes, blood glucose rises from 90 mg/100 cm³ to 140 mg/100 cm³. Beta cells in the pancreas detect this rise and secrete insulin. Insulin binds to receptors on liver and muscle cells, increasing their permeability to glucose and activating glycogen synthase. Glucose is converted to glycogen and stored. After 90 minutes, blood glucose returns to 90 mg/100 cm³, and insulin secretion decreases.

Concept 3: Osmoregulation – Control of Water Potential

Osmoregulation maintains the water potential of blood plasma within narrow limits, ensuring cells neither shrink (in hypertonic conditions) nor swell and burst (in hypotonic conditions). The key hormone is ADH (Anti-Diuretic Hormone), also known as vasopressin, which regulates water reabsorption in the kidneys.

The process begins with osmoreceptors in the hypothalamus, which detect changes in the water potential of blood. If water potential decreases (e.g., due to dehydration, sweating, or consuming salty food), the osmoreceptors shrink slightly as water leaves by osmosis. This triggers the hypothalamus to send nerve impulses to the posterior pituitary gland, which secretes ADH into the bloodstream. ADH travels to the kidneys, where it targets the cells of the distal convoluted tubule (DCT) and collecting duct.

ADH binds to receptors on the basolateral (blood-facing) membrane of these cells, activating a G-protein-coupled receptor pathway. This triggers the enzyme adenyl cyclase, producing cAMP, which activates protein kinase A. The kinase phosphorylates vesicles containing aquaporin-2 water channel proteins, causing them to fuse with the apical (tubule-facing) membrane. The insertion of aquaporins dramatically increases the permeability of the collecting duct to water. Water moves out of the filtrate by osmosis, down the water potential gradient created by the high solute concentration in the medulla (maintained by the loop of Henle). This water is reabsorbed into the blood capillaries surrounding the nephron. The result is a smaller volume of more concentrated urine, and blood water potential returns to normal.

If water potential increases (e.g., after drinking large volumes of water), osmoreceptors swell, inhibiting ADH secretion. The collecting duct becomes less permeable to water, so less water is reabsorbed. A large volume of dilute urine is produced, and blood water potential falls back to the set point.

Example: A runner completes a marathon on a hot day, losing 2 litres of water through sweat. Blood water potential decreases from -3.3 kPa to -3.6 kPa. Osmoreceptors in the hypothalamus detect this change and stimulate ADH release from the posterior pituitary. ADH increases aquaporin insertion in the collecting duct, leading to greater water reabsorption. Urine volume decreases to 500 cm³ per day (from a normal 1500 cm³), and urine becomes darker and more concentrated. Blood water potential returns to -3.3 kPa.

Concept 4: Thermoregulation – Control of Body Temperature

Maintaining a constant core body temperature (approximately 37°C in humans) is essential for optimal enzyme activity and metabolic function. Thermoregulation involves both physiological and behavioural mechanisms, and differs significantly between endotherms (organisms that generate heat metabolically, such as mammals and birds) and ectotherms (organisms that rely on external heat sources, such as reptiles and amphibians).

In endotherms, temperature is monitored by thermoreceptors in the skin (peripheral receptors detecting external temperature) and in the hypothalamus (central receptors detecting blood temperature). The hypothalamus acts as the thermoregulatory centre, coordinating responses via the autonomic nervous system and endocrine system.

When core temperature rises, the hypothalamus activates several cooling mechanisms: (1) Vasodilation of arterioles in the skin increases blood flow to the surface, enhancing heat loss by radiation and convection. (2) Sweating increases; evaporation of water from the skin surface requires latent heat, cooling the body. (3) Pilorelaxation (flattening of body hairs) reduces the insulating air layer, though this is less significant in humans. (4) Behavioural responses such as seeking shade or removing clothing.

When core temperature falls, the hypothalamus activates warming mechanisms: (1) Vasoconstriction of skin arterioles reduces blood flow to the surface, minimising heat loss. (2) Shivering (rapid involuntary muscle contractions) generates heat through increased cellular respiration. (3) Piloerection (raising of body hairs) traps an insulating layer of air, though again, this is minimal in humans. (4) Increased metabolic rate, stimulated by adrenaline and thyroxine secretion. (5) Behavioural responses such as seeking warmth or adding clothing.

In ectotherms, thermoregulation is primarily behavioural. Reptiles bask in sunlight to absorb heat, orient their bodies perpendicular to the sun's rays to maximise surface area, or seek shade and burrow underground to cool down. Some ectotherms also exhibit physiological changes, such as altering blood flow to the skin or changing skin colour to absorb or reflect more heat.

Example: A lizard (ectotherm) has a body temperature of 15°C in the early morning. It emerges from its burrow and basks on a rock in direct sunlight, orienting its body to maximise heat absorption. After 30 minutes, its body temperature rises to 30°C, approaching its optimal range for enzyme activity. As midday approaches and temperature exceeds 35°C, the lizard retreats to the shade to prevent overheating.

Mathematical/Scientific Relationships

While homeostasis is primarily a conceptual topic, certain quantitative relationships are important:

1. Water Potential (Ψ): Ψ = Ψ_s + Ψ_p, where Ψ_s is solute potential (always negative or zero) and Ψ_p is pressure potential (positive in turgid cells, zero in flaccid cells). In blood plasma, Ψ is typically around -3.3 kPa. A decrease in water potential (becoming more negative) indicates higher solute concentration or dehydration.

2. Glucose Concentration: Normal fasting blood glucose is 80-100 mg/100 cm³ (or 4.4-5.6 mmol/dm³). Values above 126 mg/100 cm³ indicate hyperglycaemia; below 70 mg/100 cm³ indicate hypoglycaemia.

3. Core Body Temperature: Normal human core temperature is 36.5-37.5°C. Hypothermia occurs below 35°C; hyperthermia above 38°C. Enzyme denaturation begins around 40-42°C.

These values are not typically given on formula sheets and must be memorised. Candidates should be able to interpret data showing changes in these variables and explain the homeostatic responses triggered.

Practical Applications

Diabetes Mellitus: This disease results from the failure of blood glucose regulation. Type 1 diabetes is an autoimmune condition where the immune system destroys beta cells in the pancreas, preventing insulin production. Candidates cannot produce insulin and must inject it to control blood glucose. Type 2 diabetes involves insulin resistance, where target cells become less responsive to insulin, often due to obesity and poor diet. It is managed through diet, exercise, and medications that increase insulin sensitivity or stimulate insulin secretion.

Dehydration and Rehydration Therapy: Understanding osmoregulation is critical in treating dehydration, particularly in developing countries where diarrhoeal diseases are common. Oral rehydration solutions contain glucose and salts to facilitate water absorption in the intestine via co-transport mechanisms.

Hypothermia and Hyperthermia: Knowledge of thermoregulation is applied in emergency medicine. Hypothermia is treated by gradual rewarming; rapid rewarming can cause dangerous shifts in blood flow. Hyperthermia (heatstroke) requires rapid cooling and rehydration.

Required Practical: While homeostasis itself is not typically a required practical, candidates may investigate the effect of temperature on enzyme activity (linking to thermoregulation) or osmosis in plant tissues (linking to water potential). Examiners frequently test practical skills by presenting data from homeostatic investigations and asking candidates to analyse trends, identify variables, and suggest improvements.