Regulation of Blood Pressure

1. Regulation of Blood Pressure Dr. AmelEassawi

2. OBJECTIVES The student should be able to: • List the normal at rest values of systolic blood pressure, diastolic blood pressure, pulse pressure and mean arterial pressure. • List the factors that determine the level of the arterial blood pressure. • Understand the involvement of systems in multiple times-frames in blood pressure regulation. • Describe the components of Renin-Angiotensin-Aldosterone system. • Understand the role of Renin- Angiotensin-Aldosterone system in the regulation of extracellular fluid and blood pressure. • Classify types and causes of blood pressure abnormalities.

3. Blood Pressure Blood Pressure is the force exerted by blood against a vessel wall • Depends on • Volume of blood contained within vessel • Compliance of vessel walls Systolic Pressure • Maximum pressure exerted by ejected blood against vessel walls during cardiac systole. • Averages 120 mm Hg Diastolic Pressure • Minimum pressure in arteries when blood is draining off into vessels downstream • Averages 80 mm Hg

4. Blood Pressure Pulse Pressure Pressure difference between systolic and diastolic pressure • If blood pressure is 120/80, pulse pressure is 40 mm Hg (120mm Hg – 80mm Hg). • Pulse that can be felt in artery lying close to surface of skin is due to pulse pressure. Mean Arterial Pressure (MAP) Average pressure driving blood forward into tissues throughout cardiac cycle. Mean arterial pressure = diastolic pressure + ⅓ pulse pressure At 120/80 Mean arterial pressure = 80 mm Hg + ⅓ (40 mm Hg) = 93 mm Hg

5. Blood Pressure • Mean arterial pressure is the blood pressure that is monitored and regulated in the body Primary Determinants of the Mean Arterial Pressure is: • Cardiac output • Total peripheral resistance Mean arterial pressure = cardiac output x total peripheral resistance

6. Factors Affecting Total Peripheral Resistance

7. Total Peripheral Resistance Factors Affecting Total Peripheral Resistance: The primary determinant of total peripheral resistance is arteriolar radius. Tow Major Factors Influence Arteriolar Radius: • Local Control (intrinsic): is the primary mechanism that matches the blood flow to the metabolic needs of the organ. 2. Extrinsic Control

8. Total Peripheral Resistance Local Control Of Total Peripheral Resistance: • Local Chemical Influences on Arteriolar Radius • Local metabolic changes • Histamine release • Local Physical Influences on Arteriolar Radius • Local application of heat or cold • Chemical response to shear stress • Myogenic response to stretch

9. Total Peripheral Resistance • Extrinsic control • Accomplished primarily by sympathetic nerve influence. • Accomplished to lesser extent by hormonal influence over arteriolar smooth muscle. Sympathetic Control of Arteriolar Radius: • Sympathetic supplies arteriolar smooth muscle all over body except brain. • Increase sympathetic stimulation causes arteriolar vasoconstriction. • Decrease sympathetic stimulation causes arteriolar vasodilation. • There is no parasympathetic innervations to arterioles.

10. Total Peripheral Resistance Factors Causes Vasoconstriction - Sympathetic Stimulation - Increase secretion of epinephrine and norepinephrine - AngiotensinII - Vasopressin - Endothelin release - Increase oxygen (O2) - Decrease carbon dioxide (CO2) - Increase myogenic activity - Cold

11. Total Peripheral Resistance Factors Causes Vasodilation • Decreased O2 • Increased CO2 • Increased acid • Increased K+ • Increased osmolarity • Adenosine release • Prostaglandin release • Nitric oxide (NO) • Heat

12. Determinants of Mean Arterial Blood Pressure

13. Regulation of BLOOD Pressure • Blood pressure is controlled in two ways: 1. Short Term Control Adjustments • Occur within seconds • Adjustments made by alterations in cardiac output and total peripheral resistance • Mediated by means of autonomic nervous system influences on heart, veins, and arterioles 1. Baroreceptor reflex 2. Chemoreceptor 3. Hormones

14. Regulation of BLOOD Pressure 2. Long Term Control Adjustments • Require minutes to days • Involve adjusting total blood volume by restoring normal salt and water balance through mechanisms that regulate urine output and thirst • Mediated by the kidneys Renin—Angiotensin— Aldosterone System

15. Regulation of BLOOD Pressure The Cardiovascular Center (CVC) is located within the Medulla Oblongata and Pons. The CVC receives, integrates and coordinates inputs from: Baroreceptors (pressure receptors) Chemoreceptors Higher centers in the brain. • The CVC sends autonomic nerves (both sympathetic and parasympathetic) to the heart and blood vessels. It controls blood pressure by slowing down or speeding up the heart rate and by dilating or constricting blood vessels.

16. Summary of the Main Mechanisms in Blood Pressure Control

18. Regulation of BLOOD Pressure Baroreceptors • Situated in the arch of the aorta and in the carotid sinuses. Sensitive to pressure changes (stretch) within the vessel. A rise in blood pressure in the arteries stimulates the baroreceptors, increasing their input to the CVC. • The CVC responds by increasing parasympathetic nerve activity to the heart; this slows the heart down. At the same time, sympathetic stimulation to the blood vessels is inhibited, causing vasodilatation. The net result is a fall in systemic blood pressure.

19. Baroreceptors Reflex

20. Baroreceptor Reflexes to Restore Blood Pressure to Normal

21. Functions of the Baroreceptors • Maintains relatively constant pressure despite changes in body posture. Decrease Central Blood Volume Supine Standing Sympathetic Nervous Activity Decrease Cardiac Output Vasomotor Center Sensed By Baroreceptors Decrease Arterial Pressure

22. Regulation of BLOOD Pressure Chemoreceptors: • Situated in the carotid and aortic bodies. • They are sensitive to changes in the levels of carbon dioxide, oxygen and the acidity of the blood (pH). • Their input to the CVC influences its output only when severe disruption of respiratory function occurs or when arterial blood pressure falls to less than 80 mmHg. • Divided into the peripheral and central Chemoreceptors. • The peripheral chemoreceptors are located in a distinct structures known as the carotid and aortic bodies. • The carotid body is located on the bifurcation of the common carotid artery just above the carotid sinus (baroreceptors). • The aortic bodies are located above and below the aortic arch. • The central chemoreceptors are located in medulla in the area of the respiratory center in the brain.

23. Chemoreceptors

24. CNS Ischemic Response Carotid and Aortic Chemoreceptor

25. CNS Ischemic Response • CNS Ischemic response is activated in response to cerebral ischemia. • Reduced cerebral blood flow causes CO2 buildup which stimulates vasomotor center thereby increasing arterial pressure. • CNS Ischemic response is one of the most powerful activators of the sympathetic vasoconstrictor system. Vasomotor Center Sympathetic Activity Arterial Pressure CO2 Cerebral Ischemia

26. Regulation of BLOOD Pressure Cardiopulmonary Receptors (Low Pressure): - Sense changes in blood volume or fullness of vascular system - Located in atria ,veins & pulmonary artery If there is increase in blood volume these receptors causes the following changes: • Increase secretion of atrialnatriuretic peptide (ANP) • Decrease secretion of antidiuretichormone (ADH) • Renal vasodilatation • Increase heart rate

27. Renin—Angiotensin— Aldosterone System • It works in minutes and fully active in 20 mins. • Renin is enzyme released by kidneys when arterial blood pressure becomes low. • Renin is synthesized by Juxta glomerular (JG cells) of the kidneys. • Renin enters the blood and acts on Angiotensinogin.

28. Renin-Angiotensin-Aldosterone System

29. Vasopressin Secretion

30. Blood Pressure Variations • Age: At birth blood pressure is 50/30 mmHg. It rises during the first few weeks to 90/60 rnmHg. Then gradually increases with age up to the adult age to be about 120/80 mmHg. • Sex: Women have a slight lower blood pressure than men. • Constitution: Blood pressure of obese people is usually above the normal. • Gravity: In the recumbent position. The blood pressure in the brachial artery increases by about 25 mmHg if the arms are hanged below the bed. Blood pressure decreases if the arm is raised. 5. Emotion: Increases the blood pressure by more than 30 mmHg 6. Muscular exercise: Increases the by about 30-50 mmHg

31. Blood Pressure Abnormalities • Hypertension • Blood pressure above 140/90 mm Hg • 2 broad classes • Primary hypertension • Secondary hypertension • Hypotension • Blood pressure below 100/60 mm Hg

32. Blood Pressure Abnormalities • Most common of blood pressure abnormalities • Primary hypertension Accounts for about 90% of cases • Catch all category for blood pressure elevated by variety of unknown causes rather than by a single disease entity • Potential causes being investigated • Defects in salt management by the kidneys • Excessive salt intake • Diets low in K+ and Ca2+ • Plasma membrane abnormalities such as defective Na+-K+ pumps • Variation in gene that encodes for angiotensinogen • Endogenous digitalis-like substances • Abnormalities in NO, endothelin, or other locally acting vasoactive chemicals • Excess vasopressin

33. Blood Pressure Abnormalities • Secondary hypertension • Accounts for about 10% of hypertension cases • Occurs secondary to another known primary problem • Examples of secondary hypertension • Renal hypertension • Endocrine hypertension • Neurogenic hypertension

34. Blood Pressure Abnormalities • Complication of hypertension • Congestive heart failure • Stroke • Heart attack • Spontaneous hemorrhage • Renal failure • Retinal damage

35. References • Human Physiology by Lauralee Sherwood, seventh edition. • Text book physiology by Guyton & Hall,11th edition. • Physiology by Berne and Levy, sixth edition.