Organism level systems Revision Notes

Responding to High Body Temperature

When the body becomes too warm, the skin initiates several responses to cool down. Sweat glands produce sweat, which evaporates from the skin's surface. This evaporation is an endothermic process that absorbs heat energy from the body, thus cooling it. Simultaneously, blood vessels (arterioles) close to the skin surface dilate (vasodilation). This widening of the blood vessels increases the volume of blood flowing near the surface, allowing more heat to be lost to the surroundings through radiation and convection. These actions help to dissipate excess heat efficiently.

Responding to Low Body Temperature

If the body's temperature drops too low, the skin’s mechanisms work to conserve heat. Blood vessels near the skin surface constrict (vasoconstriction), which reduces the amount of blood flowing close to the surface. This decreases heat loss by radiation and convection. Additionally, tiny hair erector muscles attached to hair follicles contract, causing hairs to stand upright. This creates a layer of trapped air close to the skin. Trapped air is a poor conductor of heat, acting as an insulating layer that reduces heat loss from the body. Shivering, though not directly a skin function, also occurs, generating heat through muscle contractions.

Key Structures in the Skin for Thermoregulation

The skin contains several specialized structures essential for controlling body temperature. Sweat glands are responsible for producing and secreting sweat. Arterioles, small blood vessels, are crucial for regulating blood flow to the skin's surface through dilation and constriction. Hair follicles, with their associated arrector pili (hair erector) muscles, play a role in trapping insulating air. Nerve endings in the skin detect changes in external temperature and transmit this information to the brain, which then coordinates the appropriate responses.

Thyroxine: Regulating Metabolic Rate (Higher Tier)

Thyroxine is a hormone produced by the thyroid gland, located in the neck. Its primary function is to control the body's basal metabolic rate. This refers to the rate at which energy is used by the body to maintain essential functions at rest. Thyroxine influences the speed of chemical reactions, including those involved in growth, development, and cellular respiration. A higher thyroxine level generally leads to a faster metabolic rate, while lower levels slow it down. The regulation of thyroxine involves a negative feedback loop: the hypothalamus releases TRH (thyrotropin-releasing hormone), which stimulates the pituitary gland to release TSH (thyroid-stimulating hormone). TSH then acts on the thyroid gland to produce thyroxine. High levels of thyroxine inhibit the release of TRH and TSH, thus maintaining a stable concentration. Imbalances can cause conditions like hypothyroidism (low thyroxine, leading to fatigue, weight gain) and hyperthyroidism (high thyroxine, leading to weight loss, anxiety).

Adrenaline: The 'Fight or Flight' Hormone (Higher Tier)

Adrenaline, also known as epinephrine, is a hormone released by the adrenal glands, which sit on top of the kidneys. It is deployed in response to stressful, dangerous, or exciting situations, triggering the body's 'fight or flight' response. The effects of adrenaline are rapid and widespread, preparing the body for immediate action. Key physiological changes include an increase in heart rate and heart contraction force, leading to higher blood pressure. Blood is diverted from the digestive system to the muscles and brain, enhancing physical and mental alertness. Adrenaline also causes the liver to break down glycogen into glucose, releasing this energy source into the bloodstream for use by muscles. Breathing rate and depth also increase, improving oxygen uptake. In essence, adrenaline mobilises the body's resources for a quick and powerful response to perceived threats, enabling either confrontation or evasion.

Differences in Action and Control (Higher Tier)

Although both thyroxine and adrenaline are hormones, they differ significantly in their roles, speed of action, and control mechanisms. Thyroxine provides a long-term, sustained regulation of metabolism crucial for overall growth and development, with its effects taking hours to days to become fully apparent. Its release is controlled by a complex negative feedback system involving the hypothalamus and pituitary gland, ensuring metabolic stability. Adrenaline, conversely, is responsible for immediate, short-term responses to acute stressors. Its effects are almost instantaneous, preparing the body for rapid physical exertion. Its release is triggered by nerve impulses from the brain to the adrenal glands, reflecting a direct response to perceived threats, without the slower feedback loops that govern thyroxine. Thyroxine is vital for maintaining homeostasis over time, while adrenaline is essential for survival in emergencies.

Higher: ADH and Blood Water Potential

Antidiuretic hormone (ADH) is a hormone vital for osmoregulation, the process of maintaining a stable water potential within the body. When the blood's water potential falls (e.g., due to dehydration), specialised osmoreceptors in the hypothalamus detect this change. This triggers a response that increases ADH secretion from the posterior pituitary gland into the bloodstream. Higher levels of ADH indicate the body needs to conserve water.

Higher: ADH's Effect on Kidney Tubules

Once released, ADH travels to the kidneys and primarily acts upon the collecting ducts and the distal convoluted tubules of the nephrons. Its key action is to increase the permeability of the walls of these tubules to water. This increased permeability allows more water to be reabsorbed from the filtrate back into the bloodstream via osmosis. This mechanism is crucial for conserving body water when needed.

Higher: Molecular Mechanism of ADH Action

The increase in permeability is mediated by ADH stimulating the insertion of water channel proteins, called aquaporins, into the cell membranes of the tubule cells. More aquaporins mean more pathways for water to move out of the tubule and back into the capillaries surrounding the nephron. This effectively concentrates the urine and prevents excessive water loss from the body.

Higher: Regulation via Negative Feedback

The release of ADH is regulated by a negative feedback loop. If blood water potential is too high (blood is too dilute), osmoreceptors detect this, leading to a decrease in ADH secretion. Lower ADH levels result in fewer aquaporins in the tubule membranes, reducing water reabsorption. Consequently, more water is excreted in the urine, which helps to lower the blood's water potential back to the set point. This ensures precise control over water balance.

The Importance of Blood Glucose Regulation

Maintaining stable blood glucose directly affects cellular function, particularly in the brain, which relies almost exclusively on glucose for energy. Disruptions, such as uncontrolled high blood sugar (hyperglycemia) or low blood sugar (hypoglycemia), can lead to severe health issues. Hyperglycemia can damage blood vessels and nerves over time, leading to complications like heart disease, kidney disease, and nerve damage. Hypoglycemia can cause confusion, seizures, and even coma, as brain cells are starved of energy. Therefore, precise regulation via hormonal control is vital for survival and overall health.

Role of the Pancreas

The pancreas, an endocrine gland, contains clusters of cells called the islets of Langerhans. These islets house two primary types of cells: alpha cells and beta cells. Beta cells are responsible for producing and secreting insulin, while alpha cells produce and secrete glucagon. The pancreas constantly monitors blood glucose levels and adjusts the release of these hormones accordingly. This dual-hormone system allows for fine-tuned control over the absorption, storage, and release of glucose throughout the body, ensuring metabolic balance.

Insulin's Mechanism of Action

When blood glucose levels are high, beta cells in the pancreas release insulin. Insulin travels through the bloodstream to target cells, primarily in the liver, muscles, and adipose (fat) tissue. It binds to specific receptors on these cells, signalling them to increase their uptake of glucose from the blood. In the liver and muscle cells, insulin promotes the conversion of excess glucose into glycogen for storage (glycogenesis). In fat cells, it encourages the uptake of glucose and its conversion into fat for long-term energy storage. These actions collectively reduce blood glucose levels.

Glucagon's Mechanism of Action (Higher)

When blood glucose levels are low, alpha cells in the pancreas release glucagon. Glucagon's main target organ is the liver. Upon binding to liver cell receptors, glucagon stimulates the breakdown of stored glycogen into glucose (glycogenolysis), releasing glucose into the bloodstream. It also promotes gluconeogenesis, the synthesis of glucose from non-carbohydrate precursors like amino acids and glycerol, further contributing to blood glucose elevation. This crucial counter-regulatory action ensures that vital organs, particularly the brain, receive a continuous supply of glucose even when dietary intake is absent.