Parameters of Vascular Function

Transcription

1 Parameters of Vascular Function Model 1: Relationships between Pressure and Flow in a Single Vessel The following data was collected by perfusing individual arterioles and measuring the relationship between pressure entering the arteriole (i.e. pressure at the proximal end of the vessel, where blood is flowing in), and the flow rate through the arteriole. Pressure at the distal end (where blood is flowing out) of the arteriole was 10 mmhg in all cases. 1. What does the proximal end of the blood vessel refer to? The end where blood is flowing in. 2. What does the distal end of the blood vessel refer to? The end where blood is flowing out.

2 3. Complete Table 1 using values for the normal vessel. Blood flow rate (ml/min) Approximate pressure at the proximal end of the vessel (mmhg) Table 1 Pressure at the distal end of the vessel Pressure change (also knowns as pressure drop) along the length of the vessel (mmhg) Plot the pressure drop and flow rate data from Table 1 on the following axes. 5. Although not completely linear, the relationship between pressure drop and flow rate is typically approximated by a straight line. Draw a best fit line through your data points. 6. Based on what you already know, write the algebraic equation for a straight line. Indicate which variables in your equation represent pressure drop and which represents flow rate. Y = mx + b, Y represents pressure drop. X represents the flow rate.

3 7. The slope of the best fit line represents the vascular resistance created by the friction between the flowing blood and the wall of the blood vessel. Assuming that the y-intercept is 0 (although hard to tell from the graph, the relationship is less linear at the very low end of flow rate), write the equation showing the relationship between pressure drop, blood flow and vascular resistance. ΔP = Resistance x Flow 8. Complete the following table (table 2): Table 2 Blood flow rate (ml/min) Approximate pressure drop across the dilated vessel Approximate pressure drop across the constricted vessel Plot the data from Table 2 on your graph in #4, including best fit lines. 10. Complete the following table (table 3). Table 3 Vessel Vessel diameter (1 = smallest, 3 = largest) Vascular resistance (1 = smallest, 3 = largest) Constricted 1 3 Normal 2 2 Dilated Describe the relationship between vessel diameter and vascular resistance. The larger the vessel diameter, the lower the resistance. 12. What affect does increased sympathetic nervous system tone (level of activation) generally have on vascular resistance. It increases vascular resistance by causing a decrease in blood vessel diameters.

4 Model 2: Pressures in the Circulation This figure shows the approximate pressures in each segment of the vasculature of a horizontal individual who has a cardiac output of 5 L/min. The solid line represents the mean pressures under normal conditions; the dotted line indicates how pressures might change under conditions of arteriolar constriction caused by the sympathetic nervous system. 13. How does the mean pressure compare in each subsequent order of the systemic circulation? It decreases along each segment. 14. How does the mean pressure compare in each subsequent order of the pulmonary circulation? It decreases along each segment. 15. How does the flow rate compare in each subsequent order of the systemic circulation? What about the pulmonary circulation? It is the same, 5 L/min in each 16. What is responsible for the increase in pressure imparted to the blood between the pulmonary veins and aorta? What about the systemic and pulmonary circulations? Contraction of the left and right ventricle, respectively.

5 17. Based on your previous answers, explain what you think is necessary for blood to flow through the vascular system. A pressure gradient. Pressure is generated by the heart and then decreases as blood moves through the circulation. 18. The relationship described in your answer to question 7, ΔP = F x R, is not only applicable to a single vessel, it is also applicable to either a vascular segment or the entire circulation (although in this case, the resistance represents a weighted total of all individual vessel resistances). Using this relationship, calculate the approximate resistance across the systemic circulation. Calculate the approximate resistance across the pulmonary circulation. Systemic: ΔP = 95 mmhg, F = 5 L/min, ΔP/F = R; 95 mmhg/5 L per minute = 19 mmhg/l/min Pulmonary: ΔP = 20 mmhg, F=5 L/min, 20/5 = 4 mmhg/l/min 19. Fill in the following table that shows the normal state and the arteriolar constricted state of the systemic vessels caused by sympathetic stimulation: Vessel Order Large arteries Table 4 Approximate pressure change Approximate resistance along along the vessel order each vessel order Normal Sympathetic Normal Sympathetic stimulation stimulation 1-2 mmhg 2 mmhg 2/5 =.4 2/5 =.4 Arterioles /5 = 9 75/5 = 15 Capillaries /5 = 4 20/5 = 4 Venules /5 = 2 6/5 = Which vessel order is responsible for the greatest amount of resistance in the systemic circulation? Which vessel order increases its resistance the most during an increase in sympathetic tone? The arterioles contribute the largest amount of systemic resistance. They also increase their resistance the most during an increase in sympathetic tone.

6 Extension questions 21. Hypertension, or high blood pressure, is sometimes treated with vasodilator drugs. Explain how these would help lower someone s blood pressure? Vasodilators relax blood vessels, increasing their diameter and lowering their resistance. In model 2, this would move someone from the dotted line to the solid line. 22. If the left heart only generated 60 mmhg pressure, predict how the graph of the pressure through the systemic circulation would change. You may elect to draw a new curve on Model 2 with your prediction. The curve would start at 60 mmhg and follow a shape similar to that in model 2, only more flattened. Any curve needs to show a consistent decrease in pressure along each segment. 23. Assume that vascular diameters are similar between the systemic and pulmonary circulations. If vascular diameter is does not explain the difference in resistance between the two circulations, what else might be responsible? The resistance of the pulmonary circulation is much less because of the short pathway for blood to flow. Thus length of the vessels also contributes to resistance. 24. Capillaries have smaller diameters than arterioles, but collectively have les resistance (as determined in Table 4). What might explain this? Capillaries are shorter than arterioles. Also, because resistance of a vessel order is a weighted average of all the blood vessels involved, the much larger number of capillaries leads to less overall resistance.