The Cardiovascular System: The Heart – Structured Study Notes

Chapter 17: The Cardiovascular System – The Heart

Heart Anatomy

The heart is a muscular organ responsible for pumping blood throughout the body via two main circuits: the pulmonary and systemic circuits. Its structure and location are optimized for efficient circulation.

• Pulmonary and Systemic Circuits:

  • Pulmonary Circuit: Right side of the heart receives oxygen-poor blood from the body and pumps it to the lungs to pick up oxygen and release carbon dioxide.

  • Systemic Circuit: Left side of the heart receives oxygen-rich blood from the lungs and pumps it to the rest of the body.

• Receiving Chambers:

  • Right Atrium: Receives blood returning from systemic circuit.

  • Left Atrium: Receives blood returning from pulmonary circuit.

• Pumping Chambers:

  • Right Ventricle: Pumps blood through pulmonary circuit.

  • Left Ventricle: Pumps blood through systemic circuit.

Site, Location, and Orientation of Heart

The heart is located in the mediastinum, between the second rib and fifth intercostal space, and rests on the superior surface of the diaphragm. It is positioned anterior to the vertebral column and posterior to the sternum.

• Base: Leans toward right shoulder.

• Apex: Points toward left hip.

• Apical Impulse: Palpable between fifth and sixth ribs, just below the left nipple.

Pericardium and Layers of the Heart Wall

The heart is enclosed in a double-walled sac called the pericardium, which protects and anchors the heart.

• Pericardium:

  • Fibrous Pericardium: Superficial layer that protects, anchors, and prevents overfilling.

  • Serous Pericardium: Deep, two-layered membrane.

  • Parietal Layer: Lines internal surface of fibrous pericardium.

  • Visceral Layer (Epicardium): On external surface of heart.

  • Layers separated by fluid-filled pericardial cavity (reduces friction).

• Layers of the Heart Wall:

  • Epicardium: Visceral layer of serous pericardium.

  • Myocardium: Circular and spiral bundles of contractile cardiac muscle cells.

  • Endocardium: Lines heart chambers and covers cardiac skeleton of valves.

Chambers and Associated Great Vessels

The heart contains four chambers: two atria and two ventricles, each with specific functions and associated vessels.

• Atria: Receiving chambers.

  • Right Atrium: Receives blood from superior/inferior vena cava and coronary sinus.

  • Left Atrium: Receives blood from four pulmonary veins.

• Ventricles: Discharging chambers.

  • Right Ventricle: Pumps blood into pulmonary trunk.

  • Left Ventricle: Pumps blood into aorta (largest artery in body).

Heart Valves

Heart valves ensure unidirectional blood flow through the heart, opening and closing in response to pressure changes.

• Atrioventricular (AV) Valves:

  • Tricuspid Valve: Right AV valve, three cusps, between right atrium and right ventricle.

  • Bicuspid (Mitral) Valve: Left AV valve, two cusps, between left atrium and left ventricle.

  • Chordae Tendineae: Anchor cusps to papillary muscles, preventing valve inversion.

• Semilunar (SL) Valves:

  • Pulmonary Valve: Between right ventricle and pulmonary trunk.

  • Aortic Valve: Between left ventricle and aorta.

Pathway of Blood Through the Heart

Blood flows through the heart in a specific sequence, ensuring oxygenation and distribution to the body.

• Right Side:

  • Superior/inferior vena cava, coronary sinus → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary trunk → pulmonary arteries → lungs

• Left Side:

  • Four pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta → systemic circulation

Equal volumes of blood are pumped to pulmonary and systemic circuits.

Coronary Circulation

Coronary circulation supplies blood to the heart muscle itself, ensuring its metabolic needs are met.

• Coronary Arteries:

  • Arise from base of aorta and supply oxygen-rich blood to heart tissue.

  • Left and right coronary arteries branch into smaller arteries serving specific heart regions.

• Coronary Veins:

  • Collect oxygen-poor blood from heart tissue and drain into coronary sinus.

  • Coronary sinus empties into right atrium.

Cardiac Muscle Fibers

Cardiac muscle fibers are striated, short, branched, and interconnected, allowing for coordinated contractions.

• Microscopic Anatomy:

  • One central nucleus (occasionally two).

  • Numerous large mitochondria for resistance to fatigue.

  • Composed of sarcomeres (contractile units).

• Intercalated Discs:

  • Connect junctions between cardiac cells, containing desmosomes (hold cells together) and gap junctions (allow ions to pass, enabling coordinated contraction).

• Endomysium: Connects cardiac muscle cells, giving them something to pull against.

Comparison: Skeletal vs. Cardiac Muscle Physiology

Cardiac muscle shares similarities with skeletal muscle but also has unique features for its role in pumping blood.

• Similarities:

  • Striated appearance.

  • Contraction via sliding filament mechanism.

  • Presence of sarcoplasmic reticulum and T tubules.

• Differences:

  • Cardiac muscle cells are self-excitable (autorhythmicity).

  • Heart contracts as a unit (functional syncytium).

  • Longer absolute refractory period in cardiac muscle.

  • Calcium influx from extracellular fluid triggers calcium release from SR.

  • Cardiac muscle cannot function without oxygen; skeletal muscle can use fermentation.

Table: Comparison of Skeletal and Cardiac Muscle

Feature	Skeletal Muscle	Cardiac Muscle
Striations	Present	Present
Cell Shape	Long, cylindrical	Short, branched
Number of Nuclei	Multiple	One (occasionally two)
Intercalated Discs	Absent	Present
Autorhythmicity	Absent	Present
Contraction	Individual fibers	As a unit (syncytium)
Oxygen Requirement	Can use fermentation	Must have oxygen

Key Equations

• Cardiac Output: The amount of blood pumped by each ventricle in one minute. $\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$

Example: Pathway of Blood Through the Heart

Blood enters the right atrium from the superior and inferior vena cava, passes through the tricuspid valve into the right ventricle, is pumped through the pulmonary valve into the pulmonary trunk, travels to the lungs, returns via pulmonary veins to the left atrium, passes through the mitral valve into the left ventricle, and is pumped through the aortic valve into the aorta for systemic distribution.

Additional info: These notes expand on the original bullet points to provide definitions, examples, and a comparison table for clarity and completeness.