(4.8) HTN PHYSIOLOGY

Lecture #31: Hypertension Physiology

• Julia Hum, PhD, Primary Course Instructor
• Course meets Monday/Wednesday/Friday, 2:00-2:50pm
• Office hours Monday/Wednesday/Friday, 11:00am-12:00pm (317B or WebEx)
• Lecture materials are from Lippincott Illustrated Reviews: Physiology 1e Wilson (Ch. 20)

L31: Learning Objectives

• Define major factors influencing blood pressure
• Differentiate sensors aiding in arterial pressure control
• Explain how cardiovascular control centers maintain arterial pressure control
• Diagram baroreceptor reflex and the RAAS
• Compare short- and long-term mechanisms for controlling blood pressure
• Diagram sympathetic nervous system and RAAS response to blood pressure changes
• Define hypertension and describe general treatment strategies

L31: "Take Home" Slide

• Arterial blood pressure is affected by cardiac output, peripheral resistance, heart rate, filling pressure, contractility, blood volume, and venous tone.
• Baroreceptor reflex is a short-term fix for changes in arterial blood pressure.
  • Drop in aortic pressure triggers reflex
  • Resistance vessels constrict to limit outflow
  • Increased inotropy and heart rate to increase cardiac output
  • Venoconstriction to increase venous return
• RAAS (Renin-Angiotensin-Aldosterone System) is a long-term fix for changes in arterial blood pressure
  • Renin released from kidney afferent arterioles in response to decreased blood pressure
  • Renin catalyzes Angiotensin I (Ang I) formation
  • Ang I converted to Angiotensin II (Ang II) by Angiotensin-Converting Enzyme (ACE)
  • Ang II stimulates aldosterone release from adrenal glands
  • Aldosterone stimulates water and Na+ recovery from urine, increasing blood volume and therefore, increasing blood pressure

Major Factors Influencing Blood Pressure

• MAP: Mean Arterial Pressure
• CO: Cardiac Output
• SVR: Systemic Vascular Resistance
• Preload: Ventricular filling pressure
• Inotropy: Contractility of the myocardium

Arterial Pressure Control

• Cardiovascular system uses two pathways:
  • Baroreceptor reflex (short-term):
    • Sensors: Arterial baroreceptors, Cardiopulmonary receptors, and Chemoreceptors
    • Integrator: Medulla oblongata's Cardiovascular Center
    • Effectors: Changes in cardiac output and peripheral resistance
  • Mean arterial pressure (MAP) (long-term):
    • Changes in circulating blood volume by modifying renal function

Arterial Pressure Control: Sensors

• Three main sensors:
  • Arterial baroreceptors
  • Cardiopulmonary receptors
  • Chemoreceptors (discussed during pulmonary section)

Arterial Baroreceptors

• Bare sensory nerve endings in the aorta and carotid sinus
• Respond to changes in Mean Arterial Pressure (MAP)
• Increases in MAP stretch the nerve endings, increasing their firing rate

Cardiopulmonary Receptors

• Baroreceptors also in regions of the cardiovascular system
• Provide information on fullness indication
• Found in the walls of vena cava, pulmonary artery, vein and atria
• Relay information to CNS via vagal nerve trunk

Central Integrator: Medulla Oblongata

• Sensory afferents converge on the medulla oblongata
• Cardiovascular Center (input/output control)
  • Vasomotor center
  • Cardioacceleratory center - increases Heart Rate (HR) and myocardial contraction
  • Cardioinhibitory center - slows HR

Baroreceptor Reflex - Short Term Fix

• MAP falls, triggering the baroreceptor reflex.
• Heart rate increases, moving blood back into the arterial system.
• Systemic vascular resistance increases
• Inotropy increases (myocardium contracts harder).
• LV preload increases.
• Venoconstriction reduces venous system capacity.

MAP: Long-term Control Pathways

• Require 24-48 hours for full effectiveness
• Pathways converge on kidneys to control vascular fullness (circulating blood volume)
• Regulate water output and intake
• Regulate Na+ levels (controls water partitioning in cells)

MAP: Long-term Control Pathways - Renin-Angiotensin-Aldosterone System (RAAS)

• Renin released from kidney afferent arterioles during decreased blood pressure.
• Renin converts angiotensinogen to angiotensin I.
• Angiotensin I converted to angiotensin II (Ang II) by ACE.
• Ang II stimulates aldosterone release.
• Aldosterone promotes Na+ and water recovery from the urine, increasing blood volume

Response of Sympathetic NS to Changes in Blood Pressure

• Sympathetic nervous system activity increases (decreased BP) or decreases (increased BP)
• Activation of different receptors that promote either vasoconstriction, increased heart rate, and increased cardiac output

Hypertension: Treatment Strategies

• Goal: reduce cardiovascular and renal morbidity and mortality
• Current recommendations
  • Start monotherapy (e.g., thiazide diuretic, ACE inhibitor, ARB, or calcium channel blocker) if blood pressure (BP) is >130/80 mmHg
  • Still uncontrolled: add additional medication, selecting based on adverse effects to achieve goal BP
  • If BP >160/100 mmHg: dual therapy should be started

What is Hypertension?

• Blood pressure categories in adults (Current Guidelines):
  • Normal: <120 and <80 mmHg
  • Elevated: 120-129 and <80 mmHg
  • Hypertensive Stage 1: 130-139 or ≥80-89 mmHg
  • Hypertensive Stage 2: ≥140 or ≥90 mmHg

How Does Hypertension Occur?

• Most hypertension is secondary to other diseases
• 90% of patients have essential hypertension
• Known influences: family history, age, race, education, income, diet, stress

FYI: Concomitant Disease and Drug Classes in Hypertension Treatment

• Drug classes for various concomitant diseases (e.g., high coronary disease risk, diabetes, recurrent stroke, heart failure, previous myocardial infarction, chronic renal disease) are listed

Factors Tipping the Scale Towards Hypertension

• Factors that contribute to the development of hypertension are discussed.

This quiz covers the key aspects of hypertension physiology as detailed in Lecture 31. You will explore factors influencing blood pressure, the role of various sensors, and the mechanisms of cardiovascular control. Additionally, you'll understand the baroreceptor reflex and RAAS in the context of blood pressure regulation.