Physiology of the Cardiovascular System

Overview

• Understand the structure and function of the cardiovascular system, with focus on the heart and blood flow.

Anatomy of the Heart

Location and Orientation

• Sits in the mediastinum of the thoracic cavity.

• May vary in position with individual height, obesity, or pregnancy.

Chambers of the Heart

• Four Chambers:

  • Right and left atrium (plural: atria)

  • Right and left ventricle

External Features

• Superior point of maximal intensity (PMI): where the heartbeat is strongest.

• Major arteries include the aorta and pulmonary trunk, connecting the heart to systemic and pulmonary circulation.

Blood Flow Through the Heart

Deoxygenated Blood Flow

• Enters the right atrium via the superior and inferior vena cava, indicating low oxygen levels (less than 50%).

Oxygenated Blood Flow

• After passing through the lungs, oxygenated blood returns to the left atrium via pulmonary veins:

  • Essential for systemic circulation from the left ventricle through the aorta.

Blood Pressure and Valve Function

• Atrioventricular (AV) Valves:

  • Located between atria and ventricles, preventing backflow during contraction.

  • Right AV valve: Tricuspid; Left AV valve: Bicuspid (Mitral).

• Papillary Muscles and Chordae Tendinae:

  • Prevent valve prolapse during ventricular contraction by maintaining tension on valve leaflets.

Cardiac Cycle Phases

Diastole (Filling Phase)

• Atria fill with blood; ventricles relax, preparing for next contraction.

• AV valves open, allowing blood flow into ventricles from atria, aided by gravity.

Systole (Contraction Phase)

• Ventricular Systole:

  • Ventricles contract, pushing deoxygenated blood to the lungs and oxygenated blood to the body.

  • Semi-lunar valves open during this phase to allow blood ejection.

Heart Sounds

• First Heart Sound (S1 - Lub):

  • Occurs when AV valves close at the onset of ventricular contraction.

• Second Heart Sound (S2 - Dup):

  • Occurs when semi-lunar valves close at the onset of ventricular relaxation.

Myocardial Contraction and Action Potentials

Cardiac Muscle Characteristics

• Cardiac muscle contains 99% contractile cells and 1% autorhythmic cells (pacemaker cells).

• Mitochondria-rich cells enable aerobic metabolism to support sustained contractions.

Pacemaker Cells

• Located in sinoatrial (SA) node; responsible for initiating action potentials autonomously.

• AV node acts as a secondary pacemaker if SA node fails.

Action Potential Phases

• Resting Membrane Potential:

  • Stable at -90 mV for contractile cells.

• Depolarization:

  • Rapid influx of Na+ followed by Ca2+ entry during contraction.

• Repolarization:

  • Ca2+ channels close, K+ channels open to reset the resting potential.

Clinical Considerations

Myocardial Infarction

• Occurs due to inadequate blood flow (ischemia) leading to muscle death.

• Plaque buildup in coronary arteries can lead to heart attacks.

Atherosclerosis

• Characterized by plaque buildup in arteries, influenced by cholesterol levels.

  • HDL (High-Density Lipoprotein): "Healthy" cholesterol.

  • LDL (Low-Density Lipoprotein): "Lethal" cholesterol.

Electrocardiogram (ECG)

ECG Basics

• Reflects electrical activity across the heart, interpretable through surface electrodes.

• Waveforms:

  • P Wave: Atrial depolarization.

  • QRS complex: Ventricular depolarization.

  • T Wave: Ventricular repolarization.

Clinical Interpretation

• Assess heart rate, rhythm, and the relationship between components (e.g., each P wave followed by QRS complex).

Influence of Autonomic Nervous System

• Sympathetic stimulation increases heart rate.

• Parasympathetic (via acetylcholine) decreases heart rate.

Summary of Blood Flow Dynamics

• Blood flow is influenced by:

  • Hydrostatic pressure gradients.

  • Resistance due to vessel radius.

• Poiseuille’s Law: Resistance increases with a decrease in radius, dramatically affecting blood flow.

Conclusion

• The understanding of cardiovascular physiology is integral to recognizing heart function, potential pathological states, and overall health.