Homeostasis

Which of the following correctly describes how positive and negative feedback are different?

• Negative feedback requires the detection of stimuli, while positive feedback functions independently of stimuli.

• Positive feedback moves a system closer to its set point, while negative feedback moves a system further from its set point.

• Positive feedback occurs only in response to stimuli, while negative feedback can occur in the absence of stimuli.

• Negative feedback reverses changes in a system, while positive feedback amplifies changes in a system.

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Which of the following statements about feedback loops is most accurate?

• Positive feedback loops amplify changes to the set point of a system.

• Negative feedback loops amplify changes to the set point of a system.

• Positive feedback loops reverse changes to the set point of a system.

• Negative feedback loops maintain changes to the set point of a system.

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Thyroxin is a hormone that increases metabolic activities within various tissue targets. Low levels of circulating thyroxin trigger the secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary. TSH secretion then stimulates thyroxin production and release by the thyroid gland. The increased level of circulating thyroxin inhibits further secretion of TSH from the anterior pituitary.

Based on the information provided, which of the following can most likely be concluded about the TSH-thyroxin loop?

• A person taking thyroxin to supplement low thyroxin secretion will produce more TSH.

• Increased thyroxin production would cause elevated ribosomal activity in the anterior pituitary.

• The structure of the loop would lead to elevated thyroid and tissue activity due to positive feedback.

• The feedback mechanism would maintain relatively constant levels of thyroxin throughout tissue targets.

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The endocrine system incorporates feedback mechanisms that maintain homeostasis. Which of the following demonstrates negative feedback by the endocrine system?

• During labor, the fetus exerts pressure on the uterine wall, inducing the production of oxytocin, which stimulates uterine wall contraction. The contractions cause the fetus to further push on the wall, increasing the production of oxytocin.

• After a meal, blood glucose levels become elevated, stimulating beta cells of the pancreas to release insulin into the blood. Excess glucose is then converted to glycogen in the liver, reducing blood glucose levels.

• At high elevation, atmospheric oxygen is more scarce. In response to signals that oxygen is low, the brain decreases an individual’s rate of respiration to compensate for the difference.

• A transcription factor binds to the regulatory region of a gene, blocking the binding of another transcription factor required for expression.

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In response to stress, the hypothalamic–pituitary–adrenal (HPA) axis coordinates the release of the steroid hormone cortisol, as summarized in the figure above. Inhaled corticosteroids (ICS) are a class of synthetic steroid hormones that are commonly used in the long-term treatment of persistent asthma.

Assuming that ICS function in the same way as naturally occurring cortisol, it is most likely that long-term, high-dose ICS use would

• reduce cortisol production

• stimulate CRH production

• stimulate ACTH production

• stimulate cortisol production

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The figure depicts how the hormone oxytocin functions during the onset of labor. Which of the following claims is best supported by the figure?

• Oxytocin levels in the bloodstream remain constant as labor progresses.

• The frequency of uterine contractions decreases as labor progresses.

• The frequency of uterine contractions increases as labor progresses.

• Oxytocin levels in the bloodstream decrease as labor progresses.

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The diagram illustrates feedback control as exerted by the hormone thyroxine. Following surgical removal of the thyroid gland, the level of TSH in the blood will increase. Which of the following best explains this increase?

• Residual blood thyroxine, from prior to thyroid gland removal, will bind to cells in the anterior pituitary, signaling more TSH secretion.

• Thyroxine will remain bound to thyroxine receptors on various body cells, and these body cells will secrete additional hormones that stimulate the anterior pituitary to secrete TSH.

• Thyroxine that was stored in the anterior pituitary prior to thyroid gland removal will signal more TSH secretion.

• A decrease in thyroxine levels means a loss of inhibition to the hypothalamus and anterior pituitary, leading to increased TSH secretion.

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Antidiuretic hormone (ADH) is important in maintaining homeostasis in mammals. ADH is released from the hypothalamus in response to high tissue osmolarity. In response to ADH, the collecting duct and distal tubule in the kidney become more permeable to water, which increases water reabsorption into the capillaries. The amount of hormone released is controlled by a negative feedback loop.

Based on the model presented, which of the following statements expresses the proper relationship between osmolarity, ADH release, and urine production?

• As tissue osmolarity rises, more ADH is released, causing less water to be excreted as urine.

• As tissue osmolarity rises, less ADH is released, causing less water to be excreted as urine.

• As tissue osmolarity rises, more ADH is released, causing more water to be excreted as urine.

• As tissue osmolarity rises, less ADH is released, causing more water to be excreted as urine.

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The model shown in the figure represents the role of two hormones, calcitonin and parathyroid hormone (PTH), in maintaining normal blood calcium levels in humans. If a dietary change results in an increase in blood calcium concentration above normal levels, which of the following is the most likely effect on calcium homeostasis?

• Calcitonin levels will decline, thus stimulating the release of PTH.

• Calcitonin levels will rise, thus promoting the deposit of calcium into bones.

• PTH levels will decline, thus stimulating the loss of calcium from bones.

• PTH levels will increase, thus preventing the release of calcitonin.

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The coagulation cascade controls blood clot formation in response to blood vessel injury. Thrombin is an enzyme that plays a key role in regulating the coagulation cascade. A simplified model of thrombin’s role in regulating the coagulation cascade is represented in Figure 1.

Argatroban is a competitive inhibitor of thrombin. Which of the following effects on the coagulation cascade is most likely to result from inhibiting thrombin activity with argatroban?

• The activation of clotting factors will be blocked.

• The rate of fibrin formation will decrease.

• Thrombin will be converted to prothrombin.

• The rate of blood clot formation will increase.