BSC2086: The Circulatory System: The Heart Lecture Notes

BSC2086 LECTURE 4: THE CIRCULATORY SYSTEM: THE HEART

Instructor: Hernan Aviles

1. Introduction to the Heart

Photograph: A human heart intended for transplant shown in a TransMedics Organ Care System, allowing it to stay viable longer than traditional methods such as immersion in saline solution and icing.

2. Overview of the Circulatory System

Definition: The circulatory system, also known as the cardiovascular system, consists of the heart and blood vessels, responsible for transporting blood to and from the body's organs.
Pulmonary Circuit:
  - Function: Transports blood to the lungs for gas exchange.
  - Process:
    - Right side of the heart receives blood from the body rich in carbon dioxide (CO₂) and waste, and poor in oxygen (O₂).
    - Blood travels via the pulmonary trunk to the lungs (alveoli) where CO₂ is exchanged for O₂.
    - Blood returns to the left side of the heart through pulmonary veins.
Systemic Circuit:
  - Function: Supplies oxygen-rich blood to all body organs.
  - Process:
    - O₂-rich blood is pumped through the aorta to the body.
    - O₂-poor blood returns to the right atrium via the superior vena cava (for the upper body) and inferior vena cava (for the lower body).

3. Position & Pericardium & Heart Wall

Location: The heart is located in the mediastinum between the lungs.
Heart Anatomy:
  - Base: The broad superior portion of the heart.
  - Apex: Inferior end that tilts to the left.
  - Dimensions: 9 cm wide at the base, 13 cm from base to apex, 6 cm anterior to posterior; weighs about 10 oz.
Pericardium: Protective sac that allows the heart to beat without friction and provides room to expand while resisting over-expansion.
  - Layers of the Pericardium:
    - Parietal Layer: Outer tough layer (fibrous and serous).
    - Pericardial Cavity: Filled with pericardial fluid, providing lubrication.
    - Visceral Layer (Epicardium): Inner, thin, smooth layer covering the heart surface.
Heart Wall Layers:
  - Epicardium: Also known as the visceral pericardium.
  - Myocardium: Thick muscular layer responsible for heart contractions.
  - Endocardium: Smooth inner lining of the heart chambers.
  - Fibrous Skeleton: Provides structural support, includes septa around valves, connects cardiac muscle, acts as insulation between atria and ventricles, aids in refilling the heart with blood.

4. Chambers, Sulci, and Septa

Chambers:
  - Right and Left Atria: Superior chambers that receive blood returning to the heart.
    - Auricle: Small, ear-like extensions of each atrium.
  - Right and Left Ventricles: Inferior chambers that pump blood into arteries.
Septa: Divides the heart chambers into right and left using interventricular and interatrial septa.
Sulci: Fat-filled grooves on the heart surface that contain coronary arteries:
  - Coronary Sulcus: Encircles the heart.
  - Anterior Interventricular Sulcus: On the anterior side.
  - Posterior Interventricular Sulcus: On the posterior side.
Muscle Ridges:
- Pectinate Muscles: Ridges of myocardium in the RA and both auricles.
- Trabeculae Carneae: Ridges of myocardium in both ventricles.

5. Heart Valves

Atrioventricular (AV) Valves:
  - Right AV Valve (Tricuspid Valve): Composed of 3 cusps.
  - Left AV Valve (Bicuspid or Mitral Valve): Composed of 2 cusps.
  - Chordae Tendineae: Tendinous cords connecting AV valves to papillary muscles in the ventricles.
Semilunar Valves:
  - Functions to control blood flow into great arteries:
    - Pulmonary Valve: Controls flow from right ventricle into the pulmonary trunk.
    - Aortic Valve: Controls flow from left ventricle into the aorta.

6. Functioning of Heart Valves

Mechanics:
  - Valves Opening and Closing: Depends on pressure gradients in the heart.
  - When Ventricles Relax: Pressure drops, semilunar valves close, and AV valves open; blood flows from atria to ventricles.
  - When Ventricles Contract: AV valves close, pressure rises, semilunar valves open; blood is ejected into great vessels.
Valvular Insufficiency: Impairment causing reflux of blood.
Mitral Valve Prolapse (MVP):
- Affects 1 in 40, mostly young women, may require treatment with an artificial or pig valve.
Valvular Stenosis: Constricted valve opening due to scar tissue (often from rheumatic fever) leading to cardiac hypertrophy and potential heart failure.
Murmurs: Sounds created by turbulent blood flow due to valvular insufficiency.

7. Blood Flow Through the Heart

Blood enters right atrium from superior and inferior venae cavae.
Blood flows through right AV valve into right ventricle.
Contraction of right ventricle forces pulmonary valve open; blood flows into the pulmonary trunk.
Blood is distributed to lungs via right and left pulmonary arteries for gas exchange (unloading CO₂, loading O₂).
Blood returns via pulmonary veins into the left atrium.
Blood flows through left AV valve into left ventricle.
Contraction of left ventricle forces aortic valve open; blood flows into ascending aorta.
Blood is distributed to the body for oxygen delivery and carbon dioxide removal.

8. Coronary Circulation

Heart Rate: 75 beats per minute (~3 billion heartbeats in a lifetime).
Left Coronary Artery:
- Comprised of the anterior interventricular branch supplying blood to the interventricular septum and anterior walls of ventricles, and circumflex branch supplying the left atrium and posterior wall of the left ventricle.
Right Coronary Artery:
- Comprised of marginal branch supplying the lateral right atrium and ventricle, and posterior interventricular branch feeding the posterior walls of ventricles.
Myocardial Infarction: Caused by blockage leading to tissue necrosis; usually due to fat deposits or clots.

9. Heart Disease - Angina & Heart Attacks

Angina Pectoris: Temporary chest pain from ischemia (insufficient blood flow and O₂ supply), leading to muscle shifting to anaerobic metabolism.
Myocardial Infarction (MI): Sustained obstruction causes cardiac cell death in the affected area.
- Symptoms include chest pain radiating to the left arm.
Heart Attack Consequences: Infarctions can weaken heart walls and disrupt electrical conduction, risking fibrillation or cardiac arrest.
- MI accounts for approximately 50% of deaths in the USA.

10. Venous Drainage of Heart

Thebesian Veins: 20% of blood drains directly into the right atrium and ventricle.
Coronary Sinus: Collects 80% of blood returning to the right atrium from:
  - Great Cardiac Vein: Drains blood from the anterior interventricular sulcus.
  - Middle Cardiac Vein: Drains blood from the posterior area.
  - Left Marginal Vein: Drains from the left region of the heart.

11. Autonomic Nerve Supply to the Heart

Pacemaker: The heart's intrinsic pacemaker can be modified by the autonomic nervous system.
Sympathetic Nerves: From the lower cervical and upper thoracic spinal cord; can accelerate heart rate up to 230 bpm.
Parasympathetic Nerves: Via the vagus nerve, which reduces heart rate under normal conditions (70-80 bpm). It can sometimes drop to 20 bpm.

12. Cardiac Conduction System

Autorhythmic Myocytes: Certain cells can spontaneously depolarize, losing contraction ability, forming a specialized conduction system:
  - Sinoatrial (SA) Node: Located in the right atrium; acts as the primary pacemaker.
  - Atrioventricular (AV) Node: Acts as the electrical gateway to the ventricles.
  - AV Bundle: Pathway for signals to the ventricles branching to the right and left bundles along the interventricular septum.
  - Purkinje Fibers: Conduct signals upward from apex throughout the ventricular myocardium.

13. Structure of Cardiac Muscle

Distinct Features:
  - Short, thick, branched cells with a single central nucleus.
  - Less developed sarcoplasmic reticulum and larger T-tubules allow for higher extracellular calcium intake.
  - Larger mitochondria compared to other muscle types.
Intercalated Discs: Structures connecting myocytes, consisting of:
- Mechanical Junctions: Such as fascia adherens linking actin and transmembrane proteins; desmosomes for physical connections.
- Electrical Junctions: Gap junctions facilitating ion flow for coordinated heart contractions.