Video 3 - Cardiovascular Regulation, Exercise & Hemorrhage – Comprehensive Study Notes

Cardiovascular Regulation – Core Principles

• Goal: Match tissue blood flow to metabolic need while maintaining systemic blood pressure (BP) and adequate cardiac output (CO).
• 3 co-ordinated control levels
  • Autoregulation (local, immediate)
  • Neural mechanisms (fast, CNS-driven)
  • Endocrine mechanisms (slower; minutes → hours, sometimes chronic)

Autoregulation of Blood Flow

• Triggered by changes in the interstitial fluid surrounding a capillary bed.

• Key local factors that make precapillary sphincters DILATE

• ↓ O$_2$
• ↑ CO$_2$ (→ ↓ pH)
• ↑ lactic acid (muscle anaerobic metabolism)
• ↑ inflammatory chemicals (e.g., histamine in a mosquito bite)
• ↑ local temperature

• Key local factor that makes them CONSTRICT

• Physical damage to a vessel; prevents hemorrhage by diverting flow away from a “broken pipe.”

• Net effect: Shunt blood toward active or inflamed tissue and away from damaged regions.

Example connection: Red, warm, raised, itchy skin around a mosquito bite = autoregulatory hyperemia delivering immune cells to the site.

Neural Mechanisms (Medullary Cardiovascular Centers)

1. Baroreceptor Reflexes

• Sensors: carotid sinuses, aortic sinuses, right atrium.

• If BP rises

• Parasympathetic (vagus) → ↓ HR → ↓ CO
• Widespread vasodilation

• If BP falls

• Sympathetic activation → ↑ HR & ↑ contractility → ↑ CO
• Peripheral vasoconstriction

• Protective logic: Maintain cerebral and coronary perfusion.

2. Chemoreceptor Reflexes

• Sensors: carotid bodies, aortic bodies (same physical region as baroreceptors but chemically sensitive).

• Detect

• ↑ CO$2$ (→ ↓ pH via CO2 + H2O \rightarrow H2CO_3)
• ↓ O$2$ (less potent stimulus than CO$2$)

• Response to acidosis / hypercapnia

• ↑ CO, ↑ respiratory rate, peripheral vasoconstriction → flush acidic blood to lungs, blow off CO$_2$.

• Response to alkalosis / hypocapnia (rarer)

• Essentially the opposite pattern.

Neurotransmitter – Vessel Tone Pairing

• Norepinephrine ((\alpha)-adrenergic) → vasoconstriction.
• Nitric oxide → vasodilation.

Endocrine Mechanisms

Low Blood Volume / Low BP

1. Renin–Angiotensin–Aldosterone System (RAAS)

   • Kidney releases renin → angiotensin II
   • Effects: systemic vasoconstriction, aldosterone release (Na$^+$ & water reabsorption)

2. Antidiuretic Hormone (ADH)

   • Pituitary secretion → water retention at kidneys.

3. Erythropoietin (EPO)

   • Kidney hormone → ↑ red-cell production; slow but ↑ O$_2$ carrying capacity.

High Blood Volume / High BP

• Atrial Natriuretic Peptide (ANP)

• Released by stretched atrial myocardium.
• Promotes Na$^+$ & water loss (diuresis) + peripheral vasodilation.

Ethical-clinical link: Chronic RAAS overactivation = basis for ACE-inhibitor & ARB antihypertensive drug classes.

Cardiovascular Responses to Exercise

Light (Moderate) Exercise

• General vasodilation in active skeletal muscle & skin.
• ↑ venous return via muscle pump.
• Modest rise in CO.

Heavy (Strenuous) Exercise

• Sympathetic dominance → near-maximal CO.
• Dramatic flow increase to working muscles & coronary circulation.
• Active shunting AWAY from “non-essential” viscera (GI tract, kidneys).
• Marathon‐runner example: Renal hypoxia can cause transient hematuria (blood in urine).

Post-prandial vs. Exercise Vascular Patterns

Take-home: Walking after Thanksgiving dinner redistributes some blood away from the gut → modestly lowers caloric absorption & aids glucose handling.

Quantitative Redistribution of Flow (Rest vs. Max Exercise)

• Resting flow (ml min$^{-1}$)

• Brain ≈ 750
• Heart ≈ 200
• Kidneys ≈ 1{,}100
• GI ≈ 1{,}400
• Skeletal muscle ≈ 1{,}200

• Max exercise flow

• Brain ≈ 750 (nearly unchanged; protected)
• Heart ≈ 750 (( \uparrow 275\% ))
• Kidneys ≈ 250 (( \downarrow 75\% ))
• GI ≈ 250 (( \downarrow 80\% ))
• Skeletal muscle ≈ 12{,}500 (( \uparrow \approx 10\times ))
• Skin ↑ markedly for heat loss

Mechanistic importance: Cutaneous vasodilation balances muscular vasoconstriction in core to prevent hyperthermia.

Acute Hemorrhage & Circulatory Shock

Immediate (Short-Term) Response

• ↑ HR & contractility (↑ CO)
• Peripheral vasoconstriction (↑ total peripheral resistance)
• Goal: restore BP and perfusion

If bleeding continues → Decompensation

• “Runaway train” – heart works harder against falling volume → failure risk.

Long-Term Response

• ADH + RAAS water retention
• Thirst
• Erythropoiesis to replace lost RBCs

Circulatory Shock – Causes & Hallmarks

• Causes: severe hemorrhage, dehydration, extensive burns, MI.
• Symptoms: confusion, tachycardia with weak pulse, cool clammy skin.
• Treatment: stop loss, fluid/blood replacement, vasoactive drugs if necessary.

Special Circulation Considerations

• Brain: Autoregulation keeps cerebral flow constant despite systemic BP swings.

• Coronary Vessels: Epinephrine & local hypoxia → vasodilation; ensures heart O$_2$ supply during stress.

• Pulmonary Capillaries: Constrict in low-O$_2$ regions (opposite of systemic pattern) to divert blood toward well-ventilated alveoli; optimizes gas exchange.

Transition to Next Topic – Lymphatic & Immune Systems

• Upcoming lecture covers lymphatic vessels, innate & adaptive immunity, and clinical examples (e.g., parasitic worms).
• Instructor note: Some images may be unsettling; avoid snacking on gummy worms while viewing.
• Action items before next class:
  • Complete the Circulation Quiz.
  • After immunity lectures, take the Immune/Lymphatic Quiz.
  • Begin preparing for Test 2.

Integrative Connections & Implications

• Homeostasis: Cardiovascular, renal, and respiratory systems function as an interlocking triad (BP ↔ gas exchange ↔ volume).

• Pathophysiology: Chronic hypertension often reflects maladaptive neural (sympathetic over-activity) and endocrine (RAAS) responses; therapeutic interventions target these pathways.

• Exercise Physiology: Training improves maximal CO and augments coronary reserve, explaining lower resting HR in athletes.

• Everyday Practice: Simple lifestyle choices (post-meal walks, hydration, gradual exercise warm-ups) exploit natural regulatory mechanisms for health gains.

Key Numbers & Values (quick reference)

• Right atrial pressure at rest: \approx 2\;\text{mm Hg}
• Cerebral flow protected at \approx 750\;\text{ml min}^{-1}.
• Coronary flow rest → exercise: 200 \rightarrow 750\;\text{ml min}^{-1}.
• Skeletal muscle flow rest → max: 1{,}200 \rightarrow 12{,}500\;\text{ml min}^{-1}.