MCAT Biology : Endocrine Regulatory Mechanisms

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Endocrine Regulatory Mechanisms

Which of the following are examples of positive feedback mechanisms in the body?
 
I. Fever during illness
II. Loss of bone mass in osteoporosis
III. Contraction during childbirth
Possible Answers:

I, II, and III

I and III only

III only

I and II only

Correct answer:

I and III only

Explanation:

Positive feedback describes an event in which a pathway generates a response that further triggers the pathway, increasing the pathway effects. In contrast, negative feedback occurs when a pathway generates a response to inhibit the pathway origin, diminishing the pathway effects. Negative feedback is a common control mechanisms in the body to maintain homeostasis, while positive feedback is inherently designed to disrupt homeostasis.

Fever during illness is enhanced via a positive feedback system that only ends once the illness begins to alleviate. Similarly, contractions during labor will intensify via positive feedback oxytocin stimulation until the child is born. Osteoporosis is caused by an imbalance in the negative feedback system that controls blood calcium. This imbalance simply means that bone is lost more than it is gained, and is still an example of negative feedback.

Example Question #2 : Endocrine Regulatory Mechanisms

The body attempts to closely regulate the free hormone concentration in the blood through a variety of factors, including binding of free hormones in the blood by albumin. This causes inhibition of additional hormone release and increased degradation of existing hormone when levels are too high. For example, triiodothyronine (T3) is able to prevent additional release of T3 from the thyroid when its levels are too high.

What kind of feedback mechanism does T3 employ?

Possible Answers:

Neutral feedback

T3 does not employ a feedback mechanism

Negative feedback

Positive feedback

Correct answer:

Negative feedback

Explanation:

Triiodothyronine (T3) employs a negative feedback mechanism, meaning that when blood serum concentrations of T3 become too high, receptors on the thyroid gland inhibit the release of additional T3.

In contrast, a positive feedback mechanism would encourage additional release of a hormone when levels are high, resulting in an exponential increase in the hormone effects. An example of a positive feedback mechanism is the release of oxytocin during childbirth to help the uterus contract.

Example Question #3 : Endocrine Regulatory Mechanisms

The body attempts to closely regulate the free hormone concentration in the blood through a variety of factors, including binding of free hormones in the blood by albumin. This causes inhibition of additional hormone release and increased degradation of existing hormone when levels are too high. There are times, however, when the body needs to continue having high levels of certain hormones. For example, oxytocin, a hormone released during childbirth, stimulates the uterus to contract. Existing levels of oxytocin in the blood encourage additional oxytocin release.

What kind of feedback mechanism does oxytocin employ?

Possible Answers:

Positive feedback

Negative feedback

Neutral feedback

Oxytocin does not employ a feedback mechanism

Correct answer:

Positive feedback

Explanation:

Oxytocin employs a positive feedback mechanism, meaning that existing levels of oxytocin encourage additional release of oxytocin. This results in an exponential increase in the hormone's effects.

In contrast, a negative feedback mechanism would prevent additional release of a hormone when levels of the existing hormone were too elevated. This results in stable homeostasis around a constant hormone concentration in the blood.

Example Question #4 : Endocrine Regulatory Mechanisms

Grave's Disease is an autoimmune disorder that causes antibodies to bind onto thyroid-stimulating hormone (TSH) receptors on the thyroid. This causes the thyroid to constantly release T3 and T4. What effect would this have on TSH levels?

Possible Answers:

TSH levels would increase, and then decrease

TSH levels would be decreased

TSH levels would not be affected

TSH levels would decrease, and then increase

TSH levels would be increased

Correct answer:

TSH levels would be decreased

Explanation:

Thyroid-stimulating hormone (TSH) levels would be decreased.

The thyroid hormones T3 and T4 act in a negative feedback loop to regulate thyroid activity. Release of TSH increases thyroid activity, while release of the thyroid hormones decreases thyroid activity by suppressing further release of TSH. Since the thyroid is overstimulated in Grave's Disease, excess T3 and T4 are being produced. These hormones will act on the feedback loop to suppress TSH release, lowering TSH levels in individuals with this disease. Since the antibodies bind to the TSH receptors, there are increased levels of T3 and T4 in the body, which negatively inhibit the production of TSH because the body is already flooded with the products of TSH action.

Example Question #5 : Endocrine Regulatory Mechanisms

Insulin is a peptide hormone responsible for lowering blood glucose levels. In some forms of diabetes mellitus, insulin is lacking. Why do diabetics inject insulin into their bodies rather than consuming an insulin pill?

Possible Answers:

Injecting insulin directly into the bloodstream is much safer than consuming an insulin pill

Injecting insulin allows for it to be dispensed in a slow-release fashion, while consuming an insulin pill releases the hormone rapidly

Injecting insulin delivers it directly into the bloodstream, while consuming a pill would require it to travel through the gastrointestinal tract before being utilized

There is no advantage to injecting insulin versus consuming an insulin pill; insulin is injected simply because that is how it is manufactured

Injecting insulin delivers it directly to the brain, while consuming a pill would require it to travel through the gastrointestinal tract first

Correct answer:

Injecting insulin delivers it directly into the bloodstream, while consuming a pill would require it to travel through the gastrointestinal tract before being utilized

Explanation:

Injecting insulin into the body delivers it directly into the bloodstream, while consuming an insulin pill would require it to pass through the gastrointestinal tract. In the gut, the pill would be degraded first and then absorbed into the bloodstream. This entire process would result in a small amount of insulin circulating in the blood, and therefore be much less effective than an insulin injection. Additionally, injecting insulin directly allows more rapid administration of a more consistent dose. 

Example Question #6 : Endocrine Regulatory Mechanisms

Which hormones act as antagonists to bone growth?

Possible Answers:

Androgens and estrogens

Adrenalines and cortisols

Epinephrines and norepinephrines

Gonadotropins and luteins

Vasopressin and erythropoietin

Correct answer:

Androgens and estrogens

Explanation:

Androgens and estrogens, the sex hormones, feedback to inhibit somatotropins, the growth hormones. In humans, the sex hormones are produced in puberty, disabling the growth plates and halting growth.

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