Answers     Problem Set #8

1. Pressure on neck exerts pressure on carotid sinus where baroreceptor is located. This increases firing rate of afferent neuron to cardiovascular center in medulla oblongata, which in turn, sends more parasympathetic stimulation to the SA node and the heart rate decreases.

2. a    3. c    

4. Framingham: Longitudinal (long-term) study: Advantages: see development of disease over time in same individual. All people here lived in same geographical area, so environmental and ethnic variables were minimized. Disadvantage: takes long time, expensive. Can't see effects of diet and race, since group was so homogeneous. Seven Countries (crosscultural) Advantages: get results more quickly, probably more representative of "people in general". Disadvantages: use of several cultures mean that the variable of interest (dietary fat intake) is confounded with other varialbes (genetic differences, climate, stresses, etc). Also, this looks at current behavior only, and it's possible that what's important is what people ate 10-20 years before they developed the disease, which can't be ascertained from this study.

5. A. They were interested in the effect of dietary lipid on behavior. Had they varied lipids over a small range, say, 300 to 500 mg/day, they would have had to study the animals for a very long time, or to use a lot of animals, before they could expect to see statistically significant results. By using extreme categories, they're more likely to see an effect, especially when the effect is small. (Also, the Framingham study had found that there was no relationship between dietary fat and heart disease in a population where there were only small variations in fat content, while the Seven Countries study had found tht larger variations in fat content were related to development of heart disease. B. Not necessarily. Could be that there are species differences and monkeys but not humans become more aggressive when cholesterol is low. However, it's hard to draw such conclusions without additional information: How old were the monkey? (turns out they were juveniles, and data from young animals may not be applicable to adults, even in the same species) What were the endpoints that were used as measurements of "aggression"? (Behavior patterns of aggression differ in different species, and turns out that for the monkeys, the researchers used "amount of time spent sitting near another animal" as an indication of how friendly/aggressive the subjects were.) How was the sample of Finns selected? What other dietary/climatic/social factors affected the behavior of this group of humans? (Note: This question was actually referring to a research article that the students read last year.)

6. Increase.  Activation of these neurons by high blood pressure normally causes heart rate to decrease.  Cutting these neurons will prevent further stimulation of the medulla, and cause heart rate to increase.

7.The rate of flow of fluid in a tube is inversely proportional to the Resistance, or directly proportional to the radius to the 4th, so a small increase in r will lead to a large increase in blood flow.  So even a small decrease in blockage of arteries would lead to a large increase in blood flow through the coronary arteries.

8. Pressure of exercising muscles ==> compresses veins==> increase blood returning to heart ==> increase EDV ==> increase stretch on muscle ==> increase force of contraction ==> increase stroke volume. (following Frank-Starling law of the heart)

9. Usually, standing up causes a sudden decrease in blood pressure, sensed by baroreceptors, which send messages to the medulla oblongata, which in turn send messages via sympathetic neurons to arterioles and veins, constricting them, and to the heart, increasing its heart rate and stroke volume.  All this leads to a reversal of the falling blood pressure.  These messages are conveyed by sympathetic neurons, which release norepinephrine from their terminals.  With DBH deficiency, there is a decrease in the ability to rapidly reverse this fall in blood pressure and the person feels dizzy because there is less blood flow to the brain.

10. a.     

11. An enzyme found in the liver catalyzes the rate determining step in the synthesis of cholesterol. If these people have a particular allele that produces a slow-working enzyme or an enzyme that does not work at all, they’ll produce less cholesterol. This in turn will reduce the down regulation of LDL receptors. The presence of these extra receptors will increase the amount of dietary cholesterol that will be absorbed form the blood.

12.a. decrease in cardiac output. Beta-1 receptors are the receptors on the heart for norepinephrine from the sympathetic nervous system. Norepinephrine will cause an increase in HR, and blocking the receptors will decrease HR and will lower the CO because CO= SV x HR

b.decrease in peripheral resistance – Nitric oxide causes local vasodialtation, an increase in the radius of the arterioles. Resistance is proportional to 1/r^4 so an increase in the radius greatly decreases peripheral resistance.

c. decrease in cardiac output. Aldosterone usually increases sodium reabsorption in the kidney. Here it is blocked by an antagonists, so less sodium will be reabsorbed, less water will follow, and the blood volume will decrease. The less blood entering the heart, the lower the stroke volume (Frank Starling law) and the lower the CO. CO= SVxHR.


14. Y. Frank Starling Law -> The greater the heart muscle is stretched, i.e the greater volume of blood that enters, the stronger the contraction, and the greater the volume of blood ejected in one stroke.

15. A. Adrenal cortex. B. Cholesterol is needed there to synthesize steroid hormones such as cortisol. HDL may bring cholesterol to adrenal,

16. For this question, I expected students to include a summary of some of the mechanisms by which stress might increase the risk of heart disease or cancer, but to also consider this quantitatively, that is, the magnitude of the stress that results from steak-avoidance is probably not sufficient to cause the long-term ill effects we discussed.

17. decrease, decreases, sympathetic, arterioles, veins, increase

18. decreases. decrease. Angiotensin-converting enzyme usually converts angiotensin I into angiontensin II. Angiotensin II acts on the arterioles to cause vasoconstriction (in addition to its role in increasing sodium reabsorption). Without angiontensinII, the radius of the arterioles will increase. Resistance is proportional to 1/r^4 (where r = radius), so an increase in radius cause a large decrease in resistance (or you could use R=(8*P*n*L)/(Pi*r^4). Since Blood Pressure = COxResistance a drop in resistance will also cause a drop in BP. Summary: Block ACE-> less angio II -> a larger radius-> lower resistance-> lower bp.

19. Not affected because Epinephrine receptors on the heart are Beta one receptor (not alpha–1 )

20. Possible answers include: could be one group of patients has poorer memory for food intake; sugar is often found in high fat foods (cakes, ice cream), and could be those eating much sugar are also eating much fat; those people eating those cakes and ice cream may also be smoking cigarettes afterwards; sample size is relatively small.

21. Usually 60% of the blood is stored in the veins. Body regulates amount in different regions by vasoconstriction. Usually some overall vasoconstriction, if suddenly have extensive vasodilation, the same amount of blood is now spread over a much larger area, leading to a decrease in blood pressure.

22. decreased. The increase in blood pressure is sensed by the baroreceptor in the carotid sinus, which sends a message to the medulla oblongata to relax arteriolar smooth muscle, and to decrease rate of the heart beat, both of which will lead to a decrease in blood pressure. These effects are dependent on increased parasympathetic activity, and decreased sympathetic activity.