Control of Arterial Pressure II

Control of Arterial Pressure II

• Objectives: Understand blood pressure regulation, components of the baroreceptor reflex, and the role of kidneys in regulation.
• Blood Pressure Definition: Pressure exerted by blood on vessel walls while flowing.
• Blood Pressure Formula: BP = CO × TPR (Cardiac output x Total peripheral resistance).
• Resistance Factors: Resistance is directly proportional to length and viscosity, inversely proportional to radius.
• Blood Pressure Measurement: Force applied to artery walls.
• Clinical Blood Pressure Units: Systolic over Diastolic (e.g., 120/80 mmHg).
• Normal Blood Pressure Ranges: Non-diabetic adults: Systolic 100-140, Diastolic 60-90 mmHg; Diabetic adults: Systolic 100-130, Diastolic 60-85 mmHg.
• Mean Arterial Pressure (MAP): 90-100 mmHg.
• Diabetic Patients: Slightly lower blood pressure, diabetes is a major coronary artery disease risk factor.
• Postural Hypotension: Low blood pressure during transitions from sitting/lying to standing.
• Autonomic Nervous System in BP Regulation: The sympathetic nervous system constricts blood vessels (vasoconstriction), hormones such as epinephrine, norepinephrine, and angiotensin II also cause vasoconstriction.
• Viscosity and Angiogenesis in Hypertension: Increased viscosity (e.g., in polycythemia/hyperproteinemia) and angiogenesis increase resistance.
• Poiseuille's Law: Smaller radius results in higher resistance.
• Short-Term Blood Pressure Control: Regulates mean arterial pressure constantly. This includes baroreceptors, chemoreceptors, CNS ischemic response, abdominal compression reflex, and Cushing reaction.
• Baroreceptors: Pressure-sensitive receptors in carotid and aortic arteries; respond to pressure changes (MAP); involved in maintaining MAP during posture changes.
• Chemoreceptors: Located in carotid & aortic bodies, respond to changes in blood chemistry (pH, O2, CO2).
• CNS Ischemic Response: Activation of sympathetic vasoconstrictor system when blood flow to brain decreases (MAP <60mmHg).
• Long-Term Blood Pressure Regulation: Kidneys play a crucial role (RAAS).
• Renin-Angiotensin-Aldosterone System (RAAS) :Renin converts angiotensinogen to angiotensin I; then to angiotensin II (strongest vasoconstrictor); stimulates aldosterone & ADH release.
• Cardiac Output: Dependent on heart rate (HR) and stroke volume (SV).
• Stroke Volume: Volume of blood pumped in one ventricular contraction, influenced by EDV (end-diastolic volume).

Baroreceptor Reflex

• Receptor: Located in the carotid sinus and aortic arch.
• Afferent Pathway: Nerve impulses to the cardiovascular center in the medulla oblongata.
• Integrating Center: The cardiovascular center in the medulla oblongata processes the information.
• Efferent Pathway: Sends signals to the heart and blood vessels via the sympathetic and parasympathetic nervous systems.
• Effector Organ: The heart, blood vessels.
• Mechanism of Action : When blood pressure decreases, baroreceptors fire less frequently, and sympathetic activity increases, causing vasoconstriction and increasing heart rate to elevate blood pressure. If blood pressure is high, baroreceptor activity increases, and sympathetic activity decreases, promoting vasodilation and decreasing heart rate to reduce blood pressure.

Types of Baroreceptors

• Type A (carotid sinus): High sensitivity, high firing rate.
• Type C (aortic): Lower sensitivity, lower firing rate, higher threshold.

Chemoreceptor Reflex

• Stimulus: Responds to changes in blood chemistry (O2, CO2, H+).
• Mechanism: Chemoreceptors signal the cardiovascular center when blood flow (due to lowered MAP) is reduced; stimulating sympathetic activity, which causes vasoconstriction in attempt to elevate blood pressure.
• Response: Increased respiratory activity (breathing) to reduce CO2 and H+ levels, increase in vasoconstriction to maintain blood pressure.

CNS Ischemic Response

• Stimulus: Reduced blood flow to the brain (low MAP).
• Mechanism: Leads to increased sympathetic activity & vasoconstriction, especially in kidney arterioles.
• Response: Prevents further decrease in blood pressure to the brain.

Importance of Blood Pressure Regulation

• Supply of Nutrients & Gases: Ensures adequate delivery of essential substances to cells.