Cardiovascular System: The Integrated System for Blood Pressure Regulation Mary Christenson, PT, PhD DPT 732 – Managemen

1. Cardiovascular System: The Integrated System for Blood Pressure Regulation Mary Christenson, PT, PhD DPT 732 – Management Applications of Physiology II Spring 2009

2. Kidney Facts • ~50 gallons of blood pass through the 2 kidneys every day • ~1.3 quarts of urine produced from the 50 gallons • Kidneys about the size of a computer mouse • Several important functions including role in maintaining BP

3. Objectives • Compare and contrast the integrated system of both short-term and long-term (including kidney involvement) regulation of arterial blood pressure • Compare and contrast the effects of various physiologic stressors on the integrated regulation of the cardiovascular system

4. Objectives (cont.) • Describe components in measuring cardiac output • Describe factors that contribute to the homeostatic disruption of normal circulatory function

5. What We Already Know • Rapidly Acting Arterial Pressure Control Mechanisms • SNS: effect on total peripheral vascular resistance and capacitance and cardiac pump • Shift of fluid through the capillary walls

6. What We Are Missing • Long-term control mechanisms for arterial blood pressure

7. Long-term mechanisms for BP Regulation • Related to maintaining homeostasis of body fluid volume • Based on maintaining a balance between intake and output of body fluid • Overall regulation of kidney excretion of H2O and Na+ • Variables account for variation in blood volume

8. Simple Concept • Increase in extracellular fluid results in increased blood volume and arterial pressure • Normal body response: kidneys excrete excess extracellular fluid and returns the pressure to normal • Mechanism reverses if reduced blood volume

9. Terminology/General Concepts • Pressure diuresis • Pressure natriuresis • blood volume blood pressure • blood volume blood pressure • Excess salt intake: increase H2O retention – increase MAP

10. Volume X Pressure {Guyton & Hall, 2006} 8 Urinary Volume Output (x normal) 1 20 200 Arterial Pressure mmHg

11. Renal Output Curve and Net Water/Salt Intake • Over long-term, water and salt intake must equal output • Demonstrated at equilibrium point of curve • Two determinants of long-term arterial pressure • Location of renal output curve (shift?) • Level of intake line

12. TPR, Arterial Pressure and Kidney Function • Arterial Pressure = CO X TPR • If increase TPR: • Get acute rise in arterial pressure • However, normal kidney function will respond by returning arterial pressure to the pressure level of the equilibrium point – Why?

13. Effect of Fluid Volume on Arterial Pressure • Increased extracellular fluid volume • Increases blood volume • Increased mean circulatory filling pressure • Increased venous return • Increased CO • Increased arterial pressure

14. CO: Two Mechanisms to Increase Arterial Pressure • Direct effect • Increased CO increases pressure • Indirect effect • Autoregulation

15. Salt Intake • Effect of Na+ greater than effect of H2O • Why? • Amount of salt accumulation in body is main determinant of extracellular fluid volume

16. Chronic Hypertension • MAP > 110 mmHg • Results of pathology • With dialysis, what happens if patient’s body fluid level is not kept at a normal level?

17. Renal Mechanisms for Control of BP • Review: 1st mechanism of kidney control of arterial pressure • 2nd system: Renin-Angiotensin

18. Renin-Angiotensin System • Renin – hormone that acts as an enzyme; released when arterial pressure drops – i.e., when renal perfusion is inadequate • Helps raise arterial pressure • Can be life-saving system in circulatory shock

20. Renin-Angiotensin Pathway Renin (kidney) Decreased Arterial Pressure Angiotensin I Renin substrate (angiotensinogen) Angiotensin II Vasoconstriction Inactivation Retention (salt/H2O) Increased Arterial Pressure

21. Angiotensin and Salt/Water Retention • Direct effect: on kidneys to retain salt and water • Indirect effect: causes adrenal glands to secrete aldosterone which increases salt/water reabsorption by kidneys

22. Renin-Angiotensin and Salt Regulation • Allows body to deal with widely varying Na+ intake and maintain normal BP salt intake extracellular volume arterial pressure renin and angiotensin renal retention of Na+ and H2O Return of extracellular volume almost to normal Return of arterial pressure almost to normal

24. Primary Hypertension = “Silent Killer” • Unknown Cause – i.e., not secondary to a known cause • Influence of weight gain and sedentary lifestyle • PT role?

25. Weight Gain and Obesity Role in HTN • Cardiac output increased • SNS activity increased • Angiotensin II/Aldosterone levels increased

26. Treatment Options in HTN • Lifestyle modifications • Pharmacological • Vasodilator drugs • Natriuretic or diuretic drugs

27. Summary of Mechanisms to Control Arterial Pressure • Rapid (seconds) • Semi-rapid (minutes/hours) • Long-term (hours/days/months/years)

29. Additional Circulatory Factors • Cardiac Output • Venous Return

30. Cardiac Output and Venous Return • Cardiac output controlled by venous return under most normal unstressful conditions • Factors in the peripheral circulation affecting venous return to the heart (not heart itself) • Sum of local blood flows contribute to venous return • CO inversely related to TPR

31. Heart Influence on CO • Frank-Starling Law • Receptors • Heart is limiting factor if receives more venous return than it can handle

32. Cardiac Output • Normal: ~5L/min • Normal CO plateaus at ~13 L/min without any special stimulation • Hypereffective heart • Hypoeffective heart

33. How Can We Measure CO? • Fick Principle • CO (L/min) = O2 absorbed per minute by the lungs (ml/min)/A-VO2 difference (ml/L of blood)