Cardiovascular System Review

Main Homeostatic Variable

• Cardiac Output (Q): Measure of the flow of blood (Liters per minute).
• Cardiac Output Formula: Stroke Volume (SV) x Heart Rate (HR).

Stroke Volume Determinants

• Stroke Volume (SV): End-Diastolic Volume (EDV) - End-Systolic Volume (ESV).
  • EDV: Volume of blood in the ventricle before contraction.
  • ESV: Volume of blood remaining in the ventricle after contraction.

EDV Determinants

• Preload: Influenced by venous return and blood volume.

ESV Determinants

• Afterload: Influenced by systemic vascular resistance (SVR) or total peripheral resistance (TPR).
• Contractility: Influenced by myocardial muscle function (Frank-Starling mechanism).
  • Higher preload increases calcium sensitivity and contractility.

Factors Affecting Cardiac Output

• Heart Rate Modulation:
  • Sympathetic Nervous System (SNS): Alpha and Beta receptors.
  • Parasympathetic Nervous System (PNS): Muscarinic and Nicotinic receptors.
• Drugs: Can simulate SNS or PNS to alter heart rate.

Blood Flow and Pressure Relations

• Preload and Stroke Volume: Positive relationship; increase in preload increases stroke volume.
• Afterload and Stroke Volume: Inverse relationship; increase in afterload decreases stroke volume.
• Contractility and Stroke Volume: Direct relationship; increase in contractility increases stroke volume.

Pressure-Volume Loops

• Preload: Entire curve shifts right with decreased compliance.
• Afterload: Curve shifts up with increased resistance.
• Contractility: Curve moves up and to the left with stronger contractions.

Cerebral and Renal Blood Flow

• Cerebral Blood Flow: Constant blood flow required for brain activity.

  • Autoregulation maintains constant flow. Influenced by CO2 levels.

• Renal Blood Flow: Lower O2 extraction, high perfusion needed for filtration.

  • Not controlled by metabolic factors; autoregulated for consistent perfusion.

Other Regional Blood Flows

• Splanchnic (GI) Blood Flow: High cardiac output at rest for digestion.
  • Sympathetic dominance during activity.
• Cutaneous (Skin) Blood Flow: Regulated by thermoregulation and MAP control.
• Skeletal Muscle Blood Flow: Up to 75% of cardiac output during exercise.

Capillaries

• Largest Surface Area: Allows for nutrient and waste exchange.
• Diffusion: Primary mechanism; low speed and pressure facilitate effective diffusion.

Oncotic and Hydrostatic Pressure

• Oncotic Pressure: Pressure exerted by proteins, causing fluid reabsorption (suck).
• Hydrostatic Pressure: Pressure exerted by fluid, causing filtration (push).
• Balance:
  • High hydrostatic pressure at arteriolar end favors fluid push.
  • High oncotic pressure at venular end favors fluid reabsorption.

Lymphatic System

• Function: Removes excess fluid and proteins from interstitial space, preventing edema.
• Clinical Signs: Swollen legs/hands can indicate lymphatic blockage or increased venous pressure.

Summary

• Ensure proper balance of preload, afterload, and contractility to maintain cardiac output and effective circulation.
• Understand pressure dynamics in capillaries and the role of the lymphatic system in fluid management.