- A person moves from a warm room at 25°C to a cold environment at 5°C. To maintain core body temperature, a negative feedback loop is initiated. Which statement correctly describes a key part of this homeostatic response?
Answer: Receptors in the skin detect the cold, signaling the hypothalamus to initiate shivering, a process that generates heat via muscle contraction.
Correct: Receptors detect the external cold, and the hypothalamus (the coordinating centre) initiates shivering. Shivering involves rapid involuntary muscle contractions, which generate heat through an increased rate of cell respiration. This is a primary mechanism for heat production in response to cold.
A is incorrect because vasodilation increases blood flow to the skin, which would increase heat loss. Vasoconstriction is the correct response to cold.
C is incorrect because sweating is a cooling mechanism. Increasing sweat production would exacerbate heat loss through evaporation.
D is incorrect because a decrease in metabolic rate would reduce heat production. In response to prolonged cold, the metabolic rate may increase, not decrease, to generate more heat.
- An individual has a rare genetic disorder that prevents their pancreatic alpha cells from secreting glucagon, but their beta cells function normally. What is a likely consequence of this condition?
Answer: Profound hypoglycemia (low blood sugar) between meals or during exercise.
Correct: Glucagon's primary role is to raise blood glucose levels when they fall, primarily by stimulating the liver to break down glycogen (glycogenolysis) and synthesize glucose (gluconeogenesis). Without glucagon, the body's ability to counteract falling blood glucose is severely impaired. This would lead to dangerously low blood sugar levels, especially during periods of fasting or exercise.
A is incorrect because the beta cells are normal and produce insulin, so glucose can be converted to glycogen after a meal. The problem is reversing this process.
B is incorrect because the beta cells, which produce insulin, are stated to be functioning normally.
D is incorrect because while muscle protein can be broken down into amino acids for gluconeogenesis in the liver, muscle cells cannot release free glucose directly into the blood.
- In this homeostatic control system, what is the role of the osteoclasts?
Answer: The effector, which carries out the response to raise blood calcium.
Correct: In a feedback loop, the effector is the cell or organ that carries out the final response. Here, the response is the release of calcium into the blood. The osteoclasts are the cells that perform this action by breaking down bone, thus they are the effectors.
A is incorrect because the sensor is the parathyroid gland itself, which detects low calcium levels.
B is incorrect because the control centre is also the parathyroid gland, which compares the detected calcium level to the set point and decides to release PTH.
D is incorrect because the stimulus is the low level of blood calcium, not the cells that respond to the hormone.
- A mountain climber ascends to a high altitude where atmospheric oxygen is low. In response, the body increases the production of red blood cells to enhance oxygen-carrying capacity. This process takes several days to complete. How can this long-term acclimatization be described in the context of homeostasis?
Answer: It is a negative feedback mechanism where the effector (bone marrow) produces a response (more red blood cells) that counteracts the initial stimulus (hypoxia).
Correct: This is a classic example of a negative feedback loop. The stimulus is low oxygen levels in the blood (hypoxia). The sensor (cells in the kidney) detects this and releases the hormone erythropoietin (EPO). The effector (bone marrow) responds to EPO by increasing red blood cell production. The resulting increase in oxygen-carrying capacity counteracts the initial stimulus, restoring tissue oxygen levels closer to the norm.
A is incorrect because the response counteracts, rather than amplifies, the stimulus. An amplification would lead to dangerously high red blood cell counts (polycythemia).
B is incorrect because homeostasis includes both rapid and slow-acting mechanisms. Acclimatization is a successful, albeit slow, homeostatic adjustment.
D is incorrect because while the number of red blood cells changes, the goal is to maintain the regulated variable—tissue oxygenation—at its set point. This is the essence of homeostasis.
- A tumour in the posterior pituitary gland causes it to continuously secrete excessive amounts of ADH, a condition known as SIADH (Syndrome of Inappropriate Antidiuretic Hormone secretion). What are the likely consequences for urine output and blood composition? (HL only)
Answer: Low volume of concentrated urine, and hyponatremia (low blood sodium).
Correct: Continuously high levels of ADH cause the collecting ducts of the kidneys to be constantly permeable to water. This leads to excessive reabsorption of water from the filtrate back into the blood, regardless of the body's actual hydration status. This results in a very low volume of highly concentrated urine. Simultaneously, the retention of excess water dilutes the blood, leading to hyponatremia (low concentration of sodium in the blood).
A is incorrect because high ADH leads to low urine volume, not high, and causes hyponatremia, not hypernatremia.
B is incorrect because high ADH leads to low urine volume.
D is incorrect because the urine would be concentrated, not dilute, and the blood would be hyponatremic, not hypernatremic.
- A patient with untreated diabetes mellitus often exhibits glycosuria (glucose in the urine). What is the physiological explanation for this symptom? (HL only)
Answer: The concentration of glucose in the glomerular filtrate exceeds the transport capacity of the proximal tubule.
Correct: In the kidney nephron, virtually all filtered glucose is reabsorbed back into the blood in the proximal convoluted tubule via membrane transport proteins. However, these transporters have a maximum capacity (transport maximum, or Tm). In untreated diabetes, blood glucose levels are so high that the filtered load of glucose exceeds this capacity. The remaining glucose cannot be reabsorbed and is therefore excreted in the urine.
A is incorrect because the collecting ducts' permeability to water is regulated by ADH; they are not significantly permeable to glucose.
B is incorrect because ADH affects water reabsorption, not glucose excretion. Dehydration associated with diabetes might increase ADH, but this would conserve water, not excrete glucose.
D is incorrect because glucagon acts on the liver, not the kidneys, to regulate glucose, and there is no mechanism for active secretion of glucose into the urine.
- Following a meal rich in carbohydrates, what would be the expected physiological state of this patient if left untreated?
Answer: Persistently high blood glucose levels because muscle and liver cells cannot effectively take up glucose from the blood.
Correct: The condition described is Type 1 diabetes, where the lack of insulin means that glucose transporters (like GLUT4) in muscle and adipose tissue are not signaled to move to the cell membrane. The liver also does not receive the signal to convert glucose to glycogen. This results in prolonged high blood glucose levels (hyperglycemia).
A is incorrect because glycogenesis (glucose to glycogen) is stimulated by insulin. Without insulin, this process is inhibited, preventing hypoglycemia.
B is incorrect because high blood glucose levels would normally inhibit glucagon release from alpha cells, although this regulation can be impaired in diabetes.
D is incorrect because adipose tissue stores fatty acids; it does not convert them to glucose. The liver can perform gluconeogenesis, but this process is inhibited by insulin, not accelerated by its absence in this context.
- The mechanism of ADH action on the kidney collecting duct involves intracellular signalling. Which statement accurately describes this molecular process? (HL only)
Answer: ADH binds to basolateral receptors, triggering a cAMP cascade that causes vesicles containing aquaporins to fuse with the apical membrane.
Correct: ADH, a peptide hormone, cannot cross the cell membrane. It binds to V2 receptors on the basolateral membrane (the side facing the blood) of the collecting duct cells. This binding activates a G-protein, which in turn activates adenylyl cyclase to produce cyclic AMP (cAMP). This second messenger initiates a protein kinase cascade, which results in the phosphorylation of proteins that cause vesicles containing pre-formed aquaporin-2 water channels to be translocated to and inserted into the apical membrane (the side facing the filtrate). This makes the membrane permeable to water.
A is incorrect because ADH is a hormone signal; it doesn't transport water itself. Water moves by osmosis through channels.
B is incorrect because ADH binds to the basolateral membrane and causes aquaporin insertion, not removal.
D is incorrect because ADH uses a second messenger system, not direct gene activation. Its effect is rapid, occurring in minutes, which is too fast for transcription and translation.
- Which of the following represents the correct sequence of events in the hormonal response to this situation? (HL only)
Answer: Hypothalamus detects increased blood osmolarity → posterior pituitary releases ADH → collecting ducts become more permeable to water → water reabsorption increases.
Correct: Dehydration increases the solute concentration of the blood (increased osmolarity). Osmoreceptors in the hypothalamus detect this change. The hypothalamus then signals the posterior pituitary gland to release Antidiuretic Hormone (ADH). ADH travels to the kidneys and increases the permeability of the collecting ducts and distal convoluted tubules to water by promoting the insertion of aquaporins. This leads to increased reabsorption of water back into the blood, producing a smaller volume of more concentrated urine.
A is incorrect because the anterior pituitary does not release ADH, and the final response described is for overhydration, not dehydration.
C is incorrect because the primary detectors of osmolarity are in the hypothalamus, not the posterior pituitary.
D is incorrect because the primary osmoreceptors are in the hypothalamus, not the kidney, and ADH secretion is stimulated, not inhibited, by dehydration.
- During severe heat stroke, a person's core body temperature can rise to dangerous levels (e.g., above 40°C). At this point, the normal thermoregulatory mechanisms can fail. Which statement best explains this breakdown of homeostasis?
Answer: The high temperature increases the body's overall metabolic rate, which in turn generates more heat, creating a dangerous positive feedback cycle.
Correct: All chemical reactions, including metabolic ones, speed up with temperature (up to a point). During heat stroke, as core temperature rises, the metabolic rate of all cells increases. This increased metabolism generates even more heat, which raises the body temperature further. This creates a vicious positive feedback cycle where rising temperature causes changes that lead to an even higher temperature, causing the thermoregulatory system to become overwhelmed and fail.
A is incorrect because the set point is not lowered; the body is simply unable to cool itself down to the normal set point.
B is incorrect because while sweating may become less effective or cease due to severe dehydration, the core reason for the uncontrolled rise is the metabolic positive feedback.
D is incorrect because the response to extreme heat is vasodilation, not vasoconstriction, in an attempt to lose heat.
- The liver plays a central role in maintaining blood glucose homeostasis. Which statement most accurately describes the liver's function during a period of fasting, such as overnight sleep?
Answer: The liver breaks down stored glycogen into glucose (glycogenolysis) and releases it into the blood in response to glucagon.
Correct: During fasting, blood glucose levels begin to fall. The pancreas responds by secreting glucagon. Glucagon acts on the liver, stimulating the breakdown of stored glycogen into glucose (glycogenolysis). This glucose is then released into the bloodstream to raise blood glucose levels and provide energy for other tissues, particularly the brain.
A is incorrect because this process, glycogenesis, occurs after a meal when blood glucose is high and insulin is secreted.
C is incorrect because gluconeogenesis (creating glucose from non-carbohydrate sources like amino acids) can occur, but the product is glucose for release, not glycogen for storage. Converting them to glycogen would not help raise blood glucose.
D is incorrect because the liver is the primary organ responsible for maintaining blood glucose during fasting; muscle cells can take up and store glucose but cannot release free glucose into the blood for use by other tissues.
- Homeostasis maintains a dynamic equilibrium, meaning that internal variables fluctuate within a narrow range around a set point. What is the role of the effector in maintaining this dynamic equilibrium?
Answer: To produce a response that moves the variable back towards the set point.
Correct: In a homeostatic control system, the effector (such as a muscle or a gland) is the component that receives commands from the control center and produces a response that counteracts the initial stimulus. This action actively brings the variable (e.g., temperature, blood glucose) back towards the set point, maintaining the dynamic equilibrium.
A is incorrect because this is the role of the sensor or receptor.
B is incorrect because this is the role of the control center or integrator (e.g., the hypothalamus).
C is incorrect because this describes positive feedback, which drives a variable away from the set point. Homeostasis primarily relies on negative feedback.
- What is the primary reason that a fever, a regulated increase in the body's temperature set point, can be beneficial during a bacterial infection?
Answer: The increased temperature enhances the activity of immune cells and may inhibit the replication of some pathogens.
Correct: A moderate fever is an adaptive response. The higher temperature can inhibit the growth and reproduction of many bacteria and viruses, which are optimized for the normal body temperature. Furthermore, the enzymatic activity of some immune cells, such as phagocytes and lymphocytes, is enhanced at slightly elevated temperatures, making the immune response more effective.
A is incorrect because while very high temperatures can kill bacteria, a typical fever is not high enough to sterilize the body. Its effect is more inhibitory than directly lethal.
C is incorrect because while vasodilation occurs to bring the body temperature up to the new set point (or to cool it down after the fever breaks), its primary role is not toxin flushing in this context.
D is incorrect because metabolic rate increases during a fever to generate the extra heat needed to reach the new, higher set point.
- In ectotherms like reptiles, body temperature is primarily determined by the external environment. Which statement accurately contrasts a homeostatic response to cold in a reptile versus a mammal?
Answer: The mammal will initiate physiological responses like vasoconstriction and shivering, while the reptile will primarily use behavioural responses like seeking a warm surface.
Correct: Mammals are endotherms and maintain a constant internal body temperature largely through physiological means. In the cold, they use vasoconstriction to reduce heat loss and shivering (metabolic heat production) to generate heat. Reptiles are ectotherms and have limited physiological means to generate heat. Their primary strategy for thermoregulation is behavioural—they actively move to locations in their environment (e.g., a sunlit rock) that allow them to absorb heat and reach their preferred body temperature.
A is incorrect because it reverses the roles. Mammals shiver; reptiles bask.
B is incorrect because a reptile's metabolic rate is dependent on its body temperature and will decrease in the cold. A mammal's metabolic rate will increase to generate heat.
D is incorrect because while reptiles have some ability to control peripheral blood flow, it is not their primary mechanism and is far less effective than in mammals. Their main strategy is behavioural.
- During strenuous exercise, body temperature increases. Which physiological response helps to dissipate this excess heat, and what is its relationship with the nervous system?
Answer: Sweat glands are stimulated by the autonomic nervous system to release sweat, which cools the skin by evaporation.
Correct: Exercise increases metabolic rate, generating excess heat. The hypothalamus detects this increase in blood temperature and, via the autonomic nervous system (specifically, sympathetic nerves), stimulates sweat glands. The evaporation of this sweat from the skin surface has a powerful cooling effect, dissipating heat.
A is incorrect because shivering is a mechanism to generate heat, not dissipate it.
B is incorrect because vasoconstriction conserves heat. Vasodilation (widening of arterioles) is the correct response to dissipate heat by increasing blood flow to the skin.
D is incorrect because the set point remains the same; the body initiates cooling mechanisms because the core temperature has risen above the set point. A fever involves a change in the set point itself.
- An endurance athlete is running a marathon on a hot day. They are sweating profusely and their blood glucose levels are dropping due to prolonged exertion. Which combination of homeostatic responses is expected?
Answer: Increased glucagon and increased ADH secretion.
Correct: Profuse sweating leads to water loss and dehydration, increasing blood osmolarity. This will trigger the release of ADH to promote water reabsorption by the kidneys. Simultaneously, prolonged exertion consumes blood glucose. This drop in blood glucose will trigger the release of glucagon from the pancreas to stimulate the liver to release stored glucose. Therefore, both glucagon and ADH levels will be high.
A and D are incorrect because insulin levels would decrease during exercise to prevent hypoglycemia and allow glucose to be released into the blood.
C is incorrect because both dehydration and falling blood glucose are stimuli that would cause an increase, not a decrease, in ADH and glucagon secretion, respectively.
- In the negative feedback control of a hormone, the hormone itself often inhibits its own production by acting on the hypothalamus or pituitary gland. What would happen if a tumour in an endocrine gland began producing this hormone uncontrollably?
Answer: The pituitary and hypothalamus would stop producing their stimulating hormones, but the target hormone level would remain high.
Correct: The uncontrollably high level of the hormone from the tumour would create a strong negative feedback signal to the hypothalamus and pituitary. This would shut down the production of the normal stimulating hormones (like releasing hormones and tropic hormones). However, since the tumour's secretion is autonomous and not under the control of these stimulating hormones, it would continue to secrete the hormone, leading to persistently high levels. The normal control loop is effectively broken.
A is incorrect because the source of the hormone (the tumour) is outside the control of the feedback loop.
C is incorrect because chronic overstimulation typically leads to downregulation (a decrease) of receptors, not an increase.
D is incorrect because tumour cells, by definition, do not respond to normal regulatory signals.
- What is the expected immediate homeostatic response to this large intake of water? (HL only)
Answer: Decreased secretion of ADH, leading to less permeable collecting ducts and the production of large volumes of dilute urine.
Correct: Drinking large amounts of water decreases blood osmolarity (dilutes the blood). Osmoreceptors in the hypothalamus detect this decrease and signal the posterior pituitary to reduce or stop the secretion of ADH. Without ADH, the collecting ducts of the kidneys remain relatively impermeable to water. Therefore, less water is reabsorbed into the blood, resulting in the excretion of a large volume of dilute urine to restore normal blood osmolarity.
A is incorrect because ADH secretion is inhibited, not increased, by low blood osmolarity.
C is incorrect because glucagon regulates blood glucose, not overall blood osmolarity in this direct way. Its release is triggered by low blood glucose, not low osmolarity.
D is incorrect because the body's response is to excrete the excess water, which requires maintaining or increasing the glomerular filtration rate, not reducing it.
- During labour, the hormone oxytocin stimulates uterine contractions. These contractions, in turn, stimulate stretch receptors in the cervix, which signal the brain to release more oxytocin, leading to stronger contractions. This cycle continues until the baby is delivered. How is this process correctly classified?
Answer: As a positive feedback loop, because the output of the system (contractions) enhances the original stimulus (stretch), driving the system further from its initial state.
Correct: This is a classic example of a positive feedback loop. The response (uterine contraction) amplifies the stimulus (cervical stretch), which leads to an even greater response. This mechanism is used to rapidly complete a process, in this case, childbirth. It pushes the body away from its normal state until an external event (delivery) breaks the loop.
A is incorrect because the defining feature is amplification of the stimulus, not the eventual return to a set point. Negative feedback counteracts the stimulus.
C is incorrect because while it deviates from a resting state, it is a controlled, physiological process, not a failure of regulation. Positive feedback is a normal, though less common, regulatory mechanism.
D is incorrect because while heat is generated, the primary purpose and regulatory mechanism are related to uterine contractions, not temperature control.
- A doctor is investigating two patients with hyperglycemia. Patient X shows no improvement in blood glucose levels after an insulin injection. Patient Y shows a significant drop in blood glucose levels after an identical insulin injection. Which conclusion is most consistent with these results? (HL only)
Answer: Patient X has Type 2 diabetes, while Patient Y has Type 1 diabetes.
Correct: Patient Y responds to an insulin injection, indicating their body's cells are sensitive to insulin but their own pancreas is not producing it. This is characteristic of Type 1 diabetes. Patient X does not respond to insulin, indicating that their cell receptors are insensitive or resistant to insulin's effects, which is the hallmark of Type 2 diabetes.
A is incorrect because it reverses the diagnoses. A Type 1 diabetic would respond to insulin, while a Type 2 diabetic would not (or would respond poorly).
C is incorrect because a key feature of Type 1 diabetes is responsiveness to exogenous insulin. Lack of response points to a different mechanism.
D is incorrect because Patient Y's strong response to insulin is more indicative of a lack of endogenous production (Type 1) rather than early-stage insulin resistance (Type 2).
