Physiology Week 3: Lecture 1

Overview of the Cardiovascular System

• The cardiovascular system includes the heart, vessels, and blood.

• It works closely with the respiratory system to maintain homeostasis of oxygen and carbon dioxide in the body.

• Key functions include transporting oxygenated blood to tissues and removing carbon dioxide and waste.

Integrative Physiology

• The cardiovascular and respiratory systems are deeply interconnected, often referred to together as cardiorespiratory fitness.

• Respiratory system is responsible for oxygen intake and carbon dioxide removal.

• Cardiovascular system delivers oxygenated blood and nutrients to tissues while transporting waste for disposal.

Anatomy of the Heart

• The heart has four primary chambers:

  • Right Atrium & Right Ventricle (Pulmonary Pump): Pumps blood to the lungs.

  • Left Atrium & Left Ventricle (Systemic Pump): Pumps blood to the rest of the body.

• The left ventricle has a thicker wall than the right ventricle due to higher workload in systemic circulation.

• The interventricular septum separates right and left chambers, preventing blood mixing.

• Cardiac muscle tissue is interconnected, allowing electrical signals to spread efficiently through intercalated discs.

Heart's Conduction System

• Specialized conduction cells allow for the spread of electrical signals:

  • SA Node: Primary pacemaker that initiates the heartbeat.

  • AV Node: Relays electrical signal to ventricles.

  • Bundle of His: Further spreads signals to right and left bundle branches.

  • Purkinje Fibers: Distributes electrical impulses across the ventricles for coordinated contraction.

Cardiac Cycle

• The cardiac cycle consists of two main phases:

  • Diastole: Heart relaxes and fills with blood.

  • Systole: Heart contracts and pumps blood out.

• Four mechanical phases are involved:

  1. Ventricular Filling Period: Blood fills the ventricles; AV valves are open.

  2. Isovolumetric Contraction Period: All valves are closed; pressure builds without change in volume.

  3. Ventricular Ejection Period: Pressure in ventricles exceeds pressure in aorta/pulmonary vessels; blood is ejected.

  4. Isovolumetric Relaxation Period: Valves are closed; pressure drops as heart relaxes.

Key Cardiovascular Variables

• Heart Rate (HR): Frequency of heartbeats per minute.

• Stroke Volume (SV): Amount of blood ejected from the ventricles during each contraction.

• Cardiac Output (CO): Total volume of blood pumped by the heart per minute (CO = HR x SV).

Vascular System

• Arteries: High-pressure vessels carrying oxygenated blood away from the heart; regulate flow via smooth muscle (vasoconstriction/vasodilation).

• Capillaries: Site for nutrient and gas exchange, characterized by narrow diameter, high total cross-sectional area, and low blood velocity.

• Veins: Return deoxygenated blood to the heart; contain valves to prevent backflow, and can expand to act as reservoirs.

Blood Pressure

• Systolic Blood Pressure (SBP): Pressure during heart contractions (typical ~120 mmHg).

• Diastolic Blood Pressure (DBP): Pressure during heart relaxation (typical ~80 mmHg).

• Mean Arterial Pressure (MAP): Average blood pressure in the circulation, calculated as DBP + 0.33(SBP - DBP).

Principles of Blood Flow

• Governed by the Hagen-Poiseuille Law, which states that:

  • Blood flow is influenced by the vessel radius, blood viscosity, and vessel length.

  • Wider vessels allow for faster flow; increased viscosity/slower flow; longer vessels slow flow and increase resistance.

Regulation of the Cardiovascular System

• Controlled by the Autonomic Nervous System:

  • Parasympathetic Nervous System: Slows heart rate, acting as a brake via acetylcholine release.

  • Sympathetic Nervous System: Increases heart rate and contraction force via epinephrine release.

• Other influences on heart rate include hormones, baroreceptors responding to blood pressure, stretch receptors in the atria, and chemoreceptors assessing metabolic byproducts.

Blood Volume Regulation

• Influenced by hormones:

  • Antidiuretic Hormone (ADH): Signals kidneys to retain water, increasing blood volume.

  • Aldosterone: Signals for sodium retention, increasing water retention and blood volume.

Homeostatic Distribution of Blood Flow at Rest

• Cardiac Output: ~5.8 liters/minute.

• Blood distribution includes:

  • Heart: 4%

  • Skeletal Muscle: 20%

  • Skin: 9% (thermoregulation)

  • Splanchnic System: 20% (digestive organs)

  • Kidney: 20%

  • Brain: 7.5%

Conclusion

• The cardiovascular system integrates with other bodily systems, ensuring proper function and regulation, adapting to physiological demands.