Cardiovascular System Notes

Heart Overview

• The heart is roughly the size of a closed fist, weighing about:
  • 250g in adult women
  • 300g in men
• Location:
  • Rests on the diaphragm near the midline of the thorax, located within the mediastinum.
  • The mediastinum contains:
  • Heart
  • Trachea
  • Esophagus
  • Lies between the sternum and thoracic vertebrae.
• Orientation:
  • Base of the heart is more superior and to the right.
  • Apex is more inferior and to the left.
  • Approximately 2/3 of the heart's mass is located to the left of the midline.

Pericardium

• A double-walled sac that surrounds and protects the heart, composed of two layers:
  • Fibrous Pericardium (outer layer):
  • Made of tough, inelastic tissue.
  • Serous Pericardium:
  • Thin, delicate membrane with two parts:
    • Parietal Layer
    • Visceral Layer (epicardium)
  • Contains the pericardial cavity, which produces fluid for lubrication, reducing friction between the layers.

Layers of the Heart Wall

1. Epicardium:
   • Outermost layer, also the visceral layer of the serous pericardium.
   • Composed of delicate connective tissue, providing a smooth, slippery consistency.
2. Myocardium:
   • Middle, thickest layer made of cardiac muscle tissue.
   • Responsible for heart contractions.
3. Endocardium:
   • Innermost layer, composed of thin endothelium and connective tissue lining the heart chambers.

Blood Flow Through the Heart

• The heart has four chambers:
  • Two superior chambers (atria)
  • Two inferior chambers (ventricles)

Right Atrium

• Receives deoxygenated blood from:
  • Superior Vena Cava (from head and shoulders)
  • Inferior Vena Cava (from lower limbs)
  • Coronary Sinus (from the heart itself).
• Blood flows from:
  • Right Atrium → Right Ventricle via the Tricuspid Valve.
• The myocardial layer is thinner, accommodating lower pressures.

Right Ventricle

• Pumps deoxygenated blood into the:
  • Pulmonary Trunk → Pulmonary Arteries to the lungs.
• Structure:
  • Forms most of anterior surface of the heart.
  • Contains papillary muscles and chordae tendinae connected to tricuspid valve.
• Thick myocardium due to higher pressures.

Left Atrium

• Receives oxygenated blood from the lungs via:
  • Pulmonary Veins.
• Blood flows from:
  • Left Atrium → Left Ventricle via the Bicuspid Valve (Mitral Valve).
• Fairly thin myocardial layer.
• Forms most of base of the heart.

Left Ventricle

• Pumps blood through:
  • Aortic Valve → Ascending Aorta → body.
• Contains chordae tendinae and papillary muscles anchoring the bicuspid valve.
• Very thick myocardial layer due to high pressure needed for systemic circulation.

Circulatory Pathways

Deoxygenated Blood Flow (Body to Lungs)

1. Body → Blood enters the heart via:
   • Superior & Inferior Vena Cava
2. Enters Right Atrium
3. Passes through Tricuspid Valve
4. Enters Right Ventricle
5. Passes through Pulmonary Valves
6. Into the Pulmonary Trunk/Ateries → Travels to lungs for gas exchange.

Oxygenated Blood Flow (Lungs to Body)

1. Lungs → Blood returns via:
   • Pulmonary Veins
2. Enters Left Atrium
3. Passes through Bicuspid (Mitral) Valve
4. Enters Left Ventricle
5. Passes through Aortic Valve
6. Into the Aorta → Distributed throughout the body.

Circulation Types

• Systemic Circulation:

  • Left side of the heart:
  • Receives oxygen-rich blood from lungs, pumps into aorta to body tissues (except lungs).
  • Pathway:
  • Left Atrium → Left Ventricle → Aorta → Arteries → Arterioles → Systemic Capillaries → Systemic Venules → Systemic Veins → Right Atrium.

• Pulmonary Circulation:

  • Right side of the heart:
  • Pumps deoxygenated blood to the lungs.
  • Pathway:
  • Right Atrium → Right Ventricle → Pulmonary Trunk → Pulmonary Arteries → Pulmonary Capillaries → Pulmonary Veins → Left Atrium.

Cardiac Action Potential

• Initiated by the SA node and travels through the conduction system, facilitating atrial and ventricular contraction.

Phases of Cardiac Action Potential

1. Depolarization:

   • Resting membrane potential ≈ -90 mV.
   • Rapid depolarization triggered by action potential from SA node:
     • Na⁺ channels open, influx leads to +20 mV.

2. Plateau:

   • Maintained depolarization:
     • Ca²⁺ channels open, Ca²⁺ influx triggers muscle contraction.
     • The balance of K⁺ outflow allows for sustained depolarization.

3. Repolarization:

   • Closure of Ca²⁺ channels, increased K⁺ channels allow for outflow, returning potential to -90 mV.

EKG (Electrocardiography)

• Recording of electrical currents produced by heart muscle fibers during each heartbeat.
• Three main waves:
  1. P wave:
  • Represents atrial depolarization (small deflection).
  1. QRS complex:
  • Represents ventricular depolarization (large upward/downward deflection).
  1. T wave:
  • Represents ventricular repolarization (small upward deflection).