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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
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.
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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?
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.
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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?
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.
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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?
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.
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Which hormones act as antagonists to bone growth?
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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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?
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.
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What happens to the levels of FSH and LH in a human female, once her ovaries have begun to atrophy during menopause?
This question requires knowledge of the negative feedback loop between the anterior pituitary hormones (FSH and LH) and the sex hormones (estrogen and progesterone). Prior to menopause, FSH and LH production is inhibited by estrogen production in the ovaries. When ovarian estrogen production decreases, during menopause, FSH and LH levels increase uninhibited.
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What two hormones have opposite effects?
Calcitonin decreases blood calcium levels, while parathyroid hormone increases blood calcium levels.
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A patient goes to his doctor for a normal check-up, and the doctor sees that he ihas very high levels of Na+ and Cl- in his blood. He runs some tests, and determines that:
Aldosterone functions to make the collecting duct and distal convoluted tubule in the kidneys to make them more permeable to Na+, Cl–, K+, and H+. When Na+ and Cl– are reabsorbed, water follows them into the blood, thus increasing total blood volume and raising blood pressure.
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All of the following hormones exhibit periods of spiked concentration in the mestrual cycle EXCEPT __________.
FSH, LH, estrogen, and progesterone all increase around ovulation to make an environment in the uterus that will be healthy if fertilization occurs. Prolactin promotes milk production at the end of pregnancy and while a mother is breast feeding a child.
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All of the following are expected to increase after a meal has been consumed except __________.
When an a meal is consumed, the digestive system is stimulated. This causes the release of secretin and cholecystokinin, hormones that are involved in digestion. Once glucose is in the bloodstream, it interacts with beta cells and causes the release of insulin. Insulin enters the bloodstream and stimulates widespread expression of the GLUT-4 receptor. Glucagon, on the other hand, counters the effects of insulin in a negative feedback loop and would not be expected to increase after a meal is consumed.
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Antidiuretic hormone (ADH) is released from the posterior pituitary in response to increased plasma osmolarity. It acts on the kidney nephrons to increase water permeability in the distal convoluted tubule and collecting duct. This allows for greater water reabsorption from the filtrate, and results in higher body fluid volume.
Which of the following would NOT be reasonably associated with increased levels of ADH?
Release of ADH would not be associated with release of atrial naturitic hormone. ADH increases blood pressure, while atrial naturitic hormone decreases blood pressure.
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Gigantism is a rare condition that is the result of hypersecretion of growth hormone from the anterior pituitary during childhood. Symptoms of gigantism include abnormal height (several standard deviations above average) and abnormal growth of the face, hands, and feet.
Functions of growth hormone are best decribed by which of the following?
Growth hormone (GH) causes increased calcium retention and stimulation of the immune system. It reduces liver uptake of glucose and increases lipolysis. Growth hormone also causes increased protein synthesis, muscle mass, bone mineralization, and _gluco_neogenesis.
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An increase in aldosterone will __________.
An increase in aldosterone will cause an increase in sodium reabsoption, an increase in blood osmolarity, an increase in antidiuretic hormone (ADH) release, an increase in water reabsoption, and an increase in blood volume.
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A lack of which of the following hormones could cause high plasma calcium levels?
If the lack of a certain hormone can cause high calcium levels, that means the hormone acts to lower plasma calcium when it is present. Only one of the choices, calcitonin, decreases blood calcium. Parathyroid hormone (PTH) and calcitriol increase plasma calcium levels. Aldosterone and glucagon are unrelated to calcium regulation—aldosterone regulates kidney activity to increase blood pressure and glucagon increases blood sugar levels.
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What is the significance of the LH surge in the menstrual cycle?
The LH surge, a spike in levels of luteinizing hormone, immediately precedes the luteal phase of the menstrual cycle, which begins with ovulation and ends with degeneration of the corpus luteum.
Estradiol, an estrogen, is secreted by theca cells and promotes growth of the endometrium to prepare for implantation.
Follicle-stimulating hormone is important for follicle maturation.
Degeneration of the corpus luteum into corpus albicans occurs in the absence of implantation. This degeneration process typically occurs 14 days after ovulation.
Human chorionic gonadotropin is only released during pregnancy and is responsible for maintaining the corpus luteum.
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John goes to the doctor and is told that his anterior pituitary is not functioning. Which of the following hormones is likely at lower levels than normal?
Of all the choices, the only one released by the anterior pituitary is TSH (thyroid-stimulating hormone). Glucagon is released from the pancreas, parathyroid hormone is released from the parathyroid glands, melatonin comes from the pineal gland, and atrial natriuretic hormone is a heart hormone.
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Recall that parathyroid hormone (PTH) is a peptide hormone secreted by the parathyroid glands that serves to increase serum calcium.
Which of the following is not a mechanism by which PTH increases serum calcium?
Parathyroid hormone (PTH) does not enhance absorption of vitamin D through the skin.
It does, however, act independently on the bones, kidneys, and gut, to increase blood calcium. Interestingly, cancer cells in various types of cancers (breast and lung) can secrete parathyroid hormone-related protein (PTHrP), which acts similarly to PTH and can cause hypercalcemia (high serum calcium) in cancer patients.
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Which anterior pituitary hormone is most responsible for facilitating sperm development in the testes?
The following answer choices are all examples of anterior pituitary hormones, however, follicle-stimulating hormone (FSH) is the hormone that acts on Sertoli cells in the testes to nourish sperm and facilitate spermatogenesis.
Luteinizing hormone (LH) acts on the testes, but works on Leydig cells in producing testosterone. Adrenocorticotropic hormone (ACTH) and thyroid-stimulating hormone (TSH) do not act on the testes.
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Which hormone is most responsible for decreased levels of blood calcium?
Calcitonin lowers plasma calcium, while parathyroid hormone (PTH) increases it. Insulin and glucagon are responsible for blood glucose, not calcium.
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