HAP201 Jenkins Chpt 19 Heart (3)

Chapter 19: The Cardiovascular System - The Heart

Introduction

The heart is a complex muscular organ vital for maintaining life by pumping blood throughout the body. It works tirelessly, ensuring that oxygen and essential nutrients reach body cells while facilitating the removal of carbon dioxide and other metabolic wastes.

Function of the Heart:

• Pumps Blood Throughout Body: Ensures continuous circulation, vital for tissue health.

• Supplies Oxygen & Nutrients to Body Cells: Enriches cells with oxygen and essential nutrients derived from food.

• Removes Carbon Dioxide: Collects and transports carbon dioxide, a metabolic waste product, to the lungs for exhalation.

Double Pump Structure:

• Left Side: Pumps oxygen-rich blood systemically through the body to nourish cells and organs.

• Right Side: Pumps oxygen-poor blood to the lungs for essential gas exchange (pulmonary circulation), where it becomes oxygenated.

Location of the Heart

• Mediastinum: Located in the thoracic cavity, positioned centrally between the lungs, and extends from the sternum anteriorly to the vertebral column posteriorly.

• Orientation:

  • Apex: The pointed end of the heart, which points inferiorly and towards the left side.

  • Base: The broad upper surface positioned superiorly and towards the right side.

Pericardium

• Protective Membrane: Encases the heart, providing a barrier against infection and trauma.

• Parts:

  • Fibrous Pericardium: A tough, outer fibrous layer that anchors the heart to surrounding structures and prevents overstretching.

  • Serous Pericardium: A delicate, double-layered membrane that secretes pericardial fluid;

    • Parietal Layer: Fused to the inner surface of the fibrous pericardium.

    • Visceral Layer: Adheres directly to the heart, also known as the epicardium.

• Pericardial Fluid: The fluid fills the pericardial cavity, lubricating the heart to reduce friction during contraction and relaxation.

Layers of the Heart Wall

• Epicardium: The outermost layer composed of connective tissue and epithelial cells, nourishes the heart itself through its vessels.

• Myocardium: The middle layer, predominantly cardiac muscle tissue that makes up the bulk of heart mass and is responsible for contracting to pump blood.

• Endocardium: The inner layer that lines the heart chambers and covers the heart valves; composed of a smooth layer of endothelial cells to minimize turbulent blood flow.

Heart Chambers

• Structure: The heart consists of four chambers:

  • Two Atria: Upper chambers that receive blood returning to the heart.

    • They feature pouch-like auricles that help increase their capacity to hold blood.

  • Two Ventricles: Lower chambers that pump blood out of the heart into the circulation.

    • Surface grooves (sulci) on the heart surface contain blood vessels and fat, providing protection and nourishment.

Right Atrium

• Receives Blood From: Superior vena cava, inferior vena cava, and coronary sinus.

• Features: Contains a fossa ovalis, an important landmark as it indicates a remnant of fetal circulation.

• Blood flows through the tricuspid valve into the right ventricle.

Right Ventricle

• Receives Blood From: The right atrium.

• Function: Pumps blood through the pulmonary valve into the pulmonary trunk, which leads to the lungs for oxygenation.

• It is separated from the left ventricle by the interventricular septum.

Left Atrium

• Receives Blood From: Four pulmonary veins that transport oxygen-rich blood from the lungs.

• Blood passes through the bicuspid (mitral) valve into the left ventricle.

Left Ventricle

• Receives Blood From: The left atrium.

• Function: Pumps oxygen-rich blood through the aortic valve into the aorta, initiating systemic circulation.

• Notably, it has a thicker muscular wall compared to the right ventricle due to the higher pressure required for systemic circulation.

Myocardial Thickness and Function

• The thickness of the myocardium varies among the heart chambers based on functional requirements:

  • Atrial walls are comparatively thinner than those of ventricular walls, suitable for their less forceful contraction.

  • The left ventricle has the thickest wall, optimized for the intense pumping pressure required for systemic circulation.

Heart Valves

• Function: Ensure unidirectional blood flow through the heart, preventing backflow during heart contractions.

• Types:

  • Atrioventricular Valves:

    • Tricuspid Valve: Located on the right side, between the right atrium and right ventricle.

    • Bicuspid (Mitral) Valve: Located on the left side, between the left atrium and left ventricle.

  • Semi-lunar Valves:

    • Pulmonary Valve: Controls blood flow from the right ventricle to the pulmonary artery.

    • Aortic Valve: Controls blood flow from the left ventricle to the aorta.

• Operation: Valves function primarily based on pressure changes throughout the cardiac cycle, ensuring efficient blood flow.

Cardiac Cycle

• Definition: Represents all events occurring during a single heartbeat, encompassing two main phases: systole (contraction) and diastole (relaxation).

• Heart Sounds Produced by Valve Closure:

  • Lubb (S1): Produced by the closure of atrioventricular valves at the start of ventricular contraction.

  • Dupp (S2): Produced by the closure of semi-lunar valves at the end of ventricular contraction.

Cardiac Output

• Definition: The total volume of blood ejected from each ventricle per minute.

• Calculation: Expressed as CO = stroke volume (SV) x heart rate (HR).

• Variability: Cardiac output adjusts based on the body's demands, increasing significantly during exercise or physical exertion.

Regulation of Stroke Volume and Heart Rate

• Stroke Volume Regulation Factors:

  • Preload: The degree of stretch on the heart muscle before contraction, influenced by venous return.

  • Contractility: The inherent strength of cardiac muscle contraction, influenced by neural and hormonal cues.

  • Afterload: The resistance the ventricles must overcome to eject blood during contraction.

• Heart Rate Regulation: Managed primarily by the medulla oblongata, responding to various physiological signals.

  • Influenced by inputs such as proprioceptors and hormones (e.g., epinephrine) that modulate activity based on the body's demands.

Autonomic Regulation of Heart Rate

• Sympathetic Nervous System: Increases heart rate via the release of norepinephrine, often during stress or physical activity.

• Parasympathetic Nervous System: Decreases heart rate through the release of acetylcholine, promoting a resting state.

Cardiac Conduction System

• Key Components:

  • Sinoatrial (SA) Node: The natural pacemaker of the heart leading the electrical signal initiation.

  • Atrioventricular (AV) Node: Delays the signal allowing for atrial contraction before ventricular contraction.

  • AV Bundle, Bundle Branches, Purkinje Fibers: Conduct electrical impulses swiftly, coordinating the contractions of the heart chambers.

Electrocardiogram (ECG or EKG)

• Purpose: Records the electrical activity of the heart during heartbeat cycles.

• Key Waves:

  • P Wave: Represents atrial depolarization and contraction.

  • QRS Complex: Indicates ventricular depolarization and contraction, the primary electrical event of a heartbeat.

  • T Wave: Reflects ventricular repolarization, allowing the ventricles to prepare for the next contraction.

Summary of Key Circulations

• Pulmonary Circulation: The right side of the heart sends oxygen-poor blood to the lungs for oxygenation.

• Systemic Circulation: The left side of the heart pumps oxygen-rich blood throughout the body to maintain cellular metabolism.

• Coronary Circulation: Provides the heart muscle with its own blood supply via the coronary arteries, essential for maintaining heart health.

Conclusion

The heart functions as a vital double pump, orchestrating blood flow through two primary circuits while being regulated by complex mechanisms. It adapts to the body’s evolving demands efficiently, illustrating the importance of cardiovascular health for overall well-being.