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Chapter 20c: Physiology of Circulation

Overview of Regulation of Blood Pressure (BP)

• Hormonal Mechanism: Hormones play a critical role in regulating BP through various actions, both short-term and long-term.

• Renal Mechanism: The kidneys regulate BP by controlling blood volume and filtration rates.

• Tissue Perfusion: Essential for delivering oxygen and nutrients and removing waste products.

Factors Influencing Blood Pressure

• Long-term Changes: Influenced by kidney function (renal changes) and blood volume adjustments.

• Short-term Changes: Influenced by neural and hormonal factors that can rapidly modify BP.

Hormonal Controls

• General Function: Regulate BP in short-term by adjusting peripheral resistance and in long-term by altering blood volume.

• Key Hormones Influencing BP:

  • Norepinephrine & Epinephrine

  • Atrial Natriuretic Peptide (ANP)

  • Antidiuretic Hormone (ADH)

  • Angiotensin II

Detailed Hormonal Mechanisms

1. Norepinephrine & Epinephrine

• Source: Released by adrenal medulla upon stress or exposure to nicotine.

• Effects:

  • Enhances sympathetic response, increasing Cardiac Output (CO) by raising Heart Rate (HR) and Stroke Volume (SV).

  • Causes generalized vasoconstriction, increasing peripheral resistance.

• Treatment Insight: Beta blockers inhibit these hormones to lower BP.

2. Atrial Natriuretic Peptide (ANP)

• Source: Secreted by atrial myocytes in response to excessive stretching (distension).

• Effects:

  • Promotes sodium (Na+) and water excretion leading to decreased blood volume.

  • Acts as a vasodilator, further reducing BP.

3. Antidiuretic Hormone (ADH)

• Source: Produced by the hypothalamus, released from the posterior pituitary in response to low BP.

• Effects:

  • Increases water retention by the kidneys, raising blood volume and subsequently BP over time.

  • High levels can induce vasoconstriction, which acts to rapidly increase BP.

4. Angiotensin II

• Trigger: Low blood flow to kidneys prompts release of renin.

• Effects:

  • Renin converts angiotensinogen to angiotensin II, a potent vasoconstrictor.

  • Stimulates secretion of aldosterone and ADH, enhancing fluid retention and increasing BP.

Renal Mechanisms of BP Regulation

Direct Renal Mechanism

• Function: Stabilizes BP independently of hormones by adjusting urine output based on blood volume.

• Process:

  • Increased BP → Increased kidney filtration → Increased urine output → Decreased blood volume → Decreased BP.

Indirect Renal Mechanism

• Renin-Angiotensin-Aldosterone Mechanism: Activates with low BP to stabilize BP by modulating fluid retention and arteriolar constriction.

Pathology of Blood Pressure Regulation

Hypotension

• Definition: BP less than 90/60 mmHg, which may lead to insufficient oxygen delivery.

• Types:

  • Acute Hypotension

  • Orthostatic Hypotension

• Symptoms: Dizziness upon standing, particularly after prolonged periods of sitting or lying down.

Hypertension

• Definition: Defined as systolic BP > 130 mm Hg or diastolic BP > 80 mm Hg.

• Implications: Can stress the heart and damage arteries; often result from genetic and environmental factors.

Tissue Perfusion

• Importance: Essential for ensuring adequate nutrient delivery and waste removal.

• Autoregulation: Allows blood flow to adjust based on metabolic needs independently within tissues.

Autoregulation Mechanisms

Metabolic Controls

• Function: Adjust blood flow based on oxygen and nutrient availability; high levels of CO2 and low O2 prompt vasodilation.

• Effects: Damage and inflammatory mediators can trigger vasodilation.

Myogenic Controls

• Function: Vascular smooth muscle responds to stretch – increased stretch leads to vasoconstriction and decreased stretch leads to vasodilation.

• Contrast with Baroreceptor Reflex: Baroreceptors regulate systemic BP while myogenic responses manage local blood flow.

Long-term Autoregulation: Angiogenesis

• Process: When immediate autoregulation fails, the body can increase the number of blood vessels or enlarge existing ones to ensure tissue nutrient supply.

Circulatory Shock

• Definition: Inability to meet O2 and nutrient demands of tissues, leading to inadequate perfusion.

• Types:

  • Hypovolemic Shock: Due to substantial fluid loss.

  • Cardiogenic Shock: Heart dysfunction.

  • Vascular Shock: Maintained blood volume but extreme vasodilation leads to low BP.

Key Questions for Review

1. Predicting Changes in Peripheral Resistance: How does increased blood vessel diameter affect resistance?

2. Effects of Skeletal Muscle Inactivity: What happens to blood flow during prolonged inactivity?

3. Baroreceptor Function: What do baroreceptors detect in the cardiovascular system?

4. Blood Flow in Capillaries: Why does blood velocity decrease in capillary beds?

5. Autoregulation and Oxygen Levels: How does low oxygen impact blood flow to skeletal muscle?