Revision NotesA2

homeostasis — The maintenance of a relatively constant internal environment for the cells within the body.

This ensures that cells can function efficiently by keeping physiological factors like temperature, pH, and solute concentrations within optimum ranges, preventing damage to enzymes and metabolic processes. Like a thermostat in a house, homeostasis keeps the body's internal conditions stable despite external changes, ensuring all 'rooms' (cells) are comfortable for work.

set point — The ideal value of a physiological factor that the body controls in homeostasis.

The set point is the target value around which a physiological factor is maintained. Homeostatic mechanisms work to keep the actual value fluctuating within a narrow range around this ideal. The set temperature on a thermostat is the set point for the room's temperature.

stimulus — A change in the external or internal environment that is detected by a receptor and which may cause a response.

Stimuli are the triggers that initiate homeostatic responses. They can be internal, such as a change in blood glucose, or external, such as a change in ambient temperature. A sudden loud noise is a stimulus that might cause you to jump (a response).

receptor — A cell or tissue that is sensitive to a specific stimulus and communicates with a control centre by generating nerve impulses or sending a chemical messenger.

Receptors detect changes in internal or external conditions, initiating a response to maintain homeostasis. They are the 'sensors' of the body's control systems. Like a smoke detector in a building, a receptor detects a specific change (smoke/stimulus) and sends a signal to a central alarm system (control centre).

effector — A tissue or organ that carries out an action in response to a stimulus; muscles and glands are effectors.

Effectors are the 'responders' in a homeostatic loop, performing corrective actions to restore the physiological factor to its set point. Their actions are coordinated by the nervous or endocrine system. If a thermostat detects the room is too cold, the effector (furnace) turns on to heat the room, correcting the temperature.

corrective action — A response or series of responses that return a physiological factor to the set point so maintaining a constant environment for the cells within the body.

These actions are the output of the homeostatic control system, designed to reverse the initial deviation from the set point and restore balance. If your body temperature rises, sweating and vasodilation are corrective actions to cool you down.

negative feedback — A process in which a change in some parameter (e.g. blood glucose concentration) brings about processes which return it towards normal.

This mechanism minimises the difference between the actual value of a factor and its ideal set point, ensuring stability. If a factor increases, negative feedback causes it to decrease, and vice versa. Imagine a car's cruise control: if the car speeds up (change), the system reduces engine power (response) to bring it back to the set speed (normal).

positive feedback — A process in which a change in some parameter such as a physiological factor brings about processes that move its level further in the direction of the initial change.

Unlike negative feedback, positive feedback amplifies the initial change, moving the system further away from the set point. It is rare in homeostatic control but important in processes like childbirth. A microphone feedback loop where the sound gets progressively louder and louder, amplifying the initial noise.

hormone — A substance secreted by an endocrine gland that is carried in blood plasma to another part of the body where it has an effect.

Hormones are chemical messengers of the endocrine system, enabling long-distance cell signalling to coordinate physiological responses, often slower but longer-lasting than nervous responses. Like a message in a bottle sent across an ocean, a hormone travels through the bloodstream to deliver its specific message to distant target cells.

excretion — The removal of toxic or waste products of metabolism from the body.

Excretion is vital for maintaining a healthy internal environment by eliminating substances that could be harmful if allowed to accumulate, such as carbon dioxide and urea. Like taking out the trash from your house, excretion removes unwanted metabolic byproducts from the body.

deamination — The breakdown of excess amino acids in the liver, by the removal of the amine group; ammonia and, eventually, urea are formed from the amine group.

This process allows the body to utilise the energy content of excess amino acids by converting the remaining keto acid into glucose or fat, while safely disposing of the nitrogenous waste. Like disassembling a toy to reuse its parts: the amine group is removed for disposal, and the remaining carbon skeleton is repurposed for energy or storage.

urea — A nitrogenous excretory product produced in the liver from the deamination of amino acids.

Urea is less toxic than ammonia, which is initially formed during deamination, making it a safer form for transport in the blood to the kidneys for excretion. Like converting highly toxic industrial waste into a less harmful, transportable form before disposal, the body converts ammonia to urea.

nephron — The structural and functional unit of the kidney composed of Bowman’s capsule and a tubule divided into three regions: proximal convoluted tubule, loop of Henle and distal convoluted tubule.

Each kidney contains thousands of nephrons, which are responsible for filtering blood, reabsorbing useful substances, and forming urine to regulate body fluid composition. Like a miniature water treatment plant, each nephron filters, purifies, and concentrates waste from the blood.

afferent arteriole — Arteriole leading to glomerular capillaries.

It has a wider diameter than the efferent arteriole, contributing to the high hydrostatic pressure within the glomerulus, which is essential for ultrafiltration. Like a wide pipe feeding water into a narrower hose, it creates pressure in the glomerulus.

efferent arteriole — Arteriole leading away from glomerular capillaries.

Its narrower diameter, compared to the afferent arteriole, helps maintain the high pressure in the glomerulus, facilitating ultrafiltration. Like a narrower pipe restricting water flow out of a system, it helps build up pressure upstream.

glomerulus — A group of capillaries within the ‘cup’ of a Bowman’s capsule in the cortex of the kidney.

The high pressure within these capillaries, due to the afferent arteriole being wider than the efferent, drives the process of ultrafiltration, forcing fluid and small solutes into Bowman's capsule. Like a high-pressure sprinkler head, the glomerulus forces water (blood plasma) through tiny holes (capillary pores) into the surrounding collection basin (Bowman's capsule).

Bowman’s capsule — The cup-shaped part of a nephron that surrounds a glomerulus and collects filtrate from the blood.

This is where ultrafiltration occurs, as blood plasma (minus large proteins and cells) is forced out of the glomerulus and into the capsule to form glomerular filtrate. Like a coffee filter and funnel, the Bowman's capsule collects the liquid (filtrate) that passes through the filter (glomerulus).

podocyte — One of the cells that makes up the lining of Bowman’s capsule surrounding the glomerular capillaries.

These cells have finger-like projections with gaps (slit pores) that form part of the filtration barrier, allowing filtrate to pass through while preventing the passage of larger molecules. Like a hand with fingers spread out, the podocyte's projections create spaces for fluid to pass through, but still act as a barrier.

ultrafiltration — Filtration on a molecular scale separating small molecules from larger molecules, such as proteins (e.g. the filtration that occurs as blood flows through capillaries, especially those in glomeruli in the kidney).

This process occurs in the Bowman's capsule, driven by high blood pressure, forcing water and small solutes out of the blood while retaining large proteins and blood cells. Like a very fine sieve, ultrafiltration allows small particles (water, ions, glucose) to pass through but blocks larger ones (proteins, blood cells).

selective reabsorption — Movement of certain substances from the filtrate in nephrons back into the blood.

This process ensures that useful substances like glucose, amino acids, and most water are reclaimed from the filtrate and returned to the bloodstream, preventing their loss in urine. Like a quality control check, the nephron selectively reclaims valuable items from the filtered fluid, sending them back to circulation.

proximal convoluted tubule — Part of the nephron that leads from Bowman’s capsule to the loop of Henle.

This region is responsible for the majority of selective reabsorption, taking back essential substances like glucose, amino acids, and most water from the filtrate into the blood. Like a diligent recycling plant, the proximal convoluted tubule reclaims almost all the valuable materials from the initial waste stream.

loop of Henle — The part of the nephron between the proximal and distal convoluted tubules.

Its primary function is to create a high concentration of sodium and chloride ions in the tissue fluid of the medulla, establishing a water potential gradient crucial for water reabsorption from the collecting duct. Like a counter-current heat exchanger, the loop of Henle uses opposing flows to build up a concentration gradient in the surrounding tissue.

distal convoluted tubule — Part of the nephron that leads from the loop of Henle to the collecting duct.

This region allows for fine-tuning of ion and water reabsorption, with its permeability to water being regulated by ADH, contributing to the final concentration of urine. Like the final adjustment knob on a machine, the distal convoluted tubule makes precise changes to the filtrate composition before it becomes urine.

collecting duct — Tube in the medulla of the kidney that carries urine from the distal convoluted tubules of many nephrons to the renal pelvis.

The collecting duct's permeability to water is regulated by ADH, allowing variable amounts of water to be reabsorbed by osmosis into the highly concentrated medullary tissue fluid, thus controlling urine concentration. Like a water tap that can be opened or closed, the collecting duct's permeability is controlled by ADH to adjust how much water is retained or lost.

osmoregulation — The control of the water potential of blood and tissue fluid by controlling the water content and/or the concentration of ions, particularly sodium ions.

This homeostatic process maintains the appropriate water balance in the body, preventing cells from swelling or shrinking due to osmotic imbalances, which would disrupt metabolic functions. Like a gardener carefully watering plants to keep the soil moisture just right, osmoregulation maintains the perfect water balance in the body's internal environment.

osmoreceptor — Type of receptor that detects changes in the water potential of blood.

Located in the hypothalamus, these specialised sensory neurones monitor blood water potential and initiate the release of ADH when a decrease is detected, triggering water reabsorption. Like a humidity sensor, an osmoreceptor detects changes in the 'wetness' (water potential) of the blood and signals for adjustment.

antidiuretic hormone (ADH) — Hormone secreted from the posterior pituitary gland that increases water reabsorption in the kidneys and therefore reduces water loss in urine.

ADH acts on the collecting ducts and distal convoluted tubules, increasing their permeability to water by inserting aquaporins, allowing more water to be reabsorbed into the blood when the body is dehydrated. Like a 'water-saving' signal, ADH tells the kidneys to open more water channels, allowing the body to conserve water.

islet of Langerhans — A group of cells in the pancreas which secrete glucagon and insulin.

These endocrine clusters within the pancreas contain alpha cells (secreting glucagon) and beta cells (secreting insulin), which are crucial for the homeostatic control of blood glucose concentration. Like tiny islands of specialized factories within a larger organ, the islets produce the specific hormones needed to regulate blood sugar.

insulin — A small peptide hormone secreted by the β cells in the islets of Langerhans in the pancreas to bring about a decrease in the concentration of glucose in the blood.

Insulin stimulates liver, muscle, and adipose tissue cells to increase glucose uptake from the blood, convert glucose to glycogen (glycogenesis), and increase glucose use in respiration, thereby lowering blood glucose. Like a 'key' that unlocks cells to let glucose in, insulin helps remove excess glucose from the bloodstream.

glycogenesis — Synthesis of glycogen by addition of glucose monomers.

This process is stimulated by insulin when blood glucose levels are high, converting excess glucose into glycogen for storage primarily in the liver and muscle cells. It is a key mechanism for lowering blood glucose concentration.

glucagon — A small peptide hormone secreted by the α cells in the islets of Langerhans in the pancreas to bring about an increase in the concentration of glucose in the blood.

Glucagon primarily acts on liver cells, stimulating glycogenolysis (breakdown of glycogen) and gluconeogenesis (formation of new glucose) to release glucose into the bloodstream when blood glucose levels are low. Like an 'emergency release' button for stored energy, glucagon signals the liver to release glucose when blood sugar drops.

glycogenolysis — The breakdown of glycogen by removal of glucose monomers.

This process is stimulated by glucagon when blood glucose levels are low, releasing stored glucose from glycogen in the liver into the bloodstream. It is a crucial mechanism for increasing blood glucose concentration.

gluconeogenesis — The formation of glucose in the liver from non-carbohydrate sources such as amino acids, pyruvate, lactate, fatty acids and glycerol.

This process is stimulated by glucagon when blood glucose levels are low and glycogen stores are depleted, providing an alternative source of glucose to maintain blood sugar. It is vital for long-term glucose regulation.

adenylyl cyclase — Enzyme that catalyses formation of the second messenger cyclic AMP.

Adenylyl cyclase is a key enzyme in cell signalling pathways, particularly in the response to hormones like glucagon. Its activation leads to the production of cAMP, which then amplifies the hormonal signal within the cell.

cyclic AMP (c-AMP) — A second messenger in cell-signalling pathways.

Cyclic AMP acts as an intracellular signal, relaying and amplifying the message from a hormone (first messenger) that binds to a receptor on the cell surface. It triggers a cascade of events inside the cell, leading to the final physiological response.

protein kinase A — Enzyme that is activated by c-AMP and once activated adds phosphate groups to other proteins, including phosphorylase kinase, to activate them.

Protein kinase A is a crucial enzyme in many cell signalling pathways. Its activation by cAMP initiates a phosphorylation cascade, where it modifies other proteins by adding phosphate groups, thereby altering their activity and leading to a cellular response.

phosphorylase kinase — An enzyme that is part of the enzyme cascade that acts in response to glucagon; the enzyme activates glycogen phosphorylase by adding a phosphate group.

Phosphorylase kinase plays a specific role in the glucagon signalling pathway. Upon activation by protein kinase A, it phosphorylates and activates glycogen phosphorylase, which is the enzyme directly responsible for breaking down glycogen into glucose.

biosensor — A device that uses a biological material such as an enzyme to measure the concentration of a chemical compound.

Biosensors are used for measuring blood and urine glucose, often employing enzymes like glucose oxidase and peroxidase. They provide a rapid and accurate way to monitor physiological parameters. Their core function relies on a biological component, typically an enzyme, to detect the target substance.

guard cell — A kidney-shaped epidermal cell found with another, in a pair surrounding a stoma and controlling its opening or closure.

Guard cells regulate the stomatal aperture, balancing carbon dioxide entry for photosynthesis with water loss by transpiration. Their turgor changes, influenced by ion movement and water potential, dictate whether the stoma is open or closed.

electrochemical gradient — A gradient across a cell surface membrane that involves both a difference in concentrations of ions and a potential difference.

This gradient is crucial for the movement of ions, such as potassium ions, into and out of guard cells, which in turn affects their turgor and the opening or closing of stomata. It represents the combined force driving ion movement across a membrane.

abscisic acid (ABA) — An inhibitory plant growth regulator that causes closure of stomata in dry conditions.

Abscisic acid plays a vital role in plant water conservation. During water shortage, ABA signals guard cells to close stomata, reducing transpiration and preventing excessive water loss, thus helping the plant cope with drought stress.