Lesson 86- Structures of the heart

Systems 1: Cardiovascular & Respiratory Lesson Overview


Title: The Topography and Structure of the HeartInstructor: Dr. Mahesh Shriram Deokar

Learning Outcomes

By the end of this session, students will be able to:

  • Integrate the big picture of the structure and function of the heart using clinically relevant examples such as heartworm disease.

  • Outline key topics:

    • Prework linkage

    • Functional considerations

    • Internal structure and valves

    • Blood and nerve supply

    • Heartworm disease

The Heart - Location and Topography

Location

  • Extends from the 3rd to 6th Rib (3rd to 5th Intercostal Space)

  • Obliquely oriented with apex slightly to the left side, giving it its characteristic shape and position in the thorax.

  • Great Vessels: Found at the base, which are critical for systemic and pulmonary circulation, and supported by the phrenicopericardiac/Sternopericardiac ligament.

Images:

  • Lateral radiograph of the horse's thorax

  • Schematic and dissection images demonstrating the heart's location in dogs

The Heart - Relations

Surrounding Structures

  • Lungs: Positioned on either side with a thinner pulmonary tissue layer on the left side.

  • Cardiac Notches: Present only on the right side in dogs, which are depressions in the lung margins that accommodate the heart's shape.

Diagram Illustrations:

  • Transverse section of canine thorax highlighting heart and lung relations.

  • Lateral views with heart anatomy labeled (e.g., auricles, ventricles, and major vessels)

Structures In Relation to the Heart

  • Trachea and Esophagus: Located at the base of the heart, providing essential conduits for air and food, respectively.

  • Nerve Relations: Phrenic and Vagus nerves are situated laterally, playing a significant role in diaphragmatic movement (phrenic) and autonomic control (Vagus).

The Pericardium

Composition:

  • Completely invested by the pericardium comprising three layers:

    • Visceral Pericardium (Epicardium): Covers the heart wall, offering a protective layer.

    • Parietal Pericardium: The outer layer attached to the mediastinum, providing structural integrity.

    • Pericardial Cavity: Lies between visceral and parietal pericardium and contains serous fluid that reduces friction between the heart and surrounding structures.

    • Fibrous Pericardium: A tough connective tissue structure anchoring the parietal pericardium to the mediastinum and forming ligaments at the apex of the heart.

    • Mediastinal Pleura: Outermost layer that connects to the fibrous pericardium, providing additional support.

Functions of the Pericardium

  • Provides an isolated environment for the heart

  • Contains lubricating fluid to reduce friction as the heart pumps

  • Prevents overstretching/dilation of the heart during vigorous activity

Heart Surface Anatomy

Heart Chambers

  • Four Chambers: 2 Ventricles and 2 Atria

  • Ventricles: Thick-walled chambers responsible for pumping blood out of the heart under high pressure.

  • Atria: Thin-walled chambers that receive blood from the body (right atrium) and lungs (left atrium).

  • Auricles: Blind sacs protruding from each atrium, increasing the capacity of the atria.

  • Key Locations: Base (atria) and Apex (located predominantly in the left ventricle)

  • Coronary Groove: Defines the boundary between the atria and ventricles, essential for identifying structural relationships.

  • Interventricular Grooves: Separate the ventricles, aiding in identification and understanding of heart morphology.

  • Conus Arteriosus: Cone-shaped section of the right ventricle leading to the pulmonary trunk, crucial for directing blood flow.

  • Grooves: Paraconal and subsinuosal interventricular grooves, which are surface markers for coronary blood supply.

Great Vessels of the Heart

  • Pulmonary Trunk: Carries deoxygenated blood to the lungs for oxygenation

  • Aorta: The largest artery that supplies oxygenated blood to the entire body, branching off to various systemic arteries.

  • Cranial and Caudal Vena Cava: Major veins collecting deoxygenated blood from the upper and lower parts of the body, respectively.

  • Azygos Vein: Collects blood from the thoracic wall and drains into the cranial vena cava.

Internal Structure of the Heart

Septa and Valves

  • Septa: Comprising the interventricular and interatrial septa, these structures separate the chambers and prevent mixing of oxygenated and deoxygenated blood.

  • Atrioventricular Openings: Contain AV valves (Tricuspid and Mitral) that ensure unidirectional blood flow into the ventricles.

  • Heart Wall Layers:

    • Endocardium: Smooth inner lining of heart chambers

    • Myocardium: Thick muscular layer responsible for contraction

    • Epicardium: Outermost layer, which is also the visceral pericardium.

Cardiac Cycle

  • Consists of rhythmic contractions, allowing for effective pumping of blood:

    • Systole: Contraction phase where blood is pumped out of the chambers.

    • Diastole: Dilation/relaxation phase allowing chambers to fill with blood.

  • Unidirectional flow through heart chambers is maintained by the presence of valves, which open and close depending on pressure changes within the heart.

Right Heart Chambers and Valves

  • Openings of Right Atrium Include:

    • Coronary Sinus (drains blood from heart muscle)

    • Fossa Ovalis (remnant of fetal circulation)

    • Intervenous tubercle (directs blood flow into right ventricle)

    • Pectinate Muscles: Muscular ridges that help in atrial contraction.

  • Right AV Opening: Guarded by the tricuspid valve, crucial for preventing backflow during ventricular contraction.

Pulmonary Trunk and Valves

  • Pulmonary Valve: Composed of three semilunar cusps and functions without needing chordae tendineae, facilitating effective blood flow to lungs.

Left Heart Chambers and Valves

  • Left Atrium and Ventricle: Show cases of left AV (bicuspid) valve, responsible for maintaining forward flow into the aorta.

  • Supplied by Pulmonary Veins: Which return oxygenated blood from the lungs.

The Mitral Valve and Aortic Valve

  • Mitral Valve: Also known as the left AV valve, contains chordae tendineae that anchor its leaflets to the myocardium, preventing prolapse.

  • Aortic Valve: Has three semilunar cusps similar to pulmonary valve, playing a vital role in systemic circulation.

Blood Supply to the Heart

Coronary Arteries

  • Right and Left Coronary Arteries: Arising from the bulb of the aorta, supply oxygenated blood to the myocardium.

  • Location: Found in coronary and interventricular grooves on the heart surface.

  • Left Coronary Artery (LCA): Predominantly larger in dogs, indicative of their higher myocardial oxygen demand.

  • Right Coronary Artery (RCA): Larger in horses, adapted to their unique circulatory requirements.

Venous Drainage of the Heart

  • Great Cardiac Vein: Major venous drainage pathway, opening into the right atrium along with tributaries from coronary arteries.

  • Other cardiac veins drain into the heart, helping to return deoxygenated blood to the right atrium efficiently.

Nerve Supply to the Heart

Autonomic Nervous System

  • Autonomic supply: Includes both sympathetic and parasympathetic divisions, regulating heart rate and force of contraction.

  • Cardiac Nerves: Known collectively as the cardiac plexus, these nerves play a crucial role in heart function.

  • Sympathetic Innervation: Comes from cervical ganglia, increasing heart rate and contractility during stress-related situations.

  • Parasympathetic Innervation: Supplied by Vagus nerves, responsible for slowing the heart rate during resting states.

Heartworm Disease Overview

  • Vector-borne parasitic disease affecting dogs, cats, ferrets, and some wildlife

  • Causative Parasite: Dirofilaria immitis (filarial nematode), which can cause significant cardiovascular impairment.

  • Transmission: Occurs via mosquito bites (Aedes, Anopheles, Culex), making preventative measures crucial in endemic regions.

  • Adult worms: Primarily occlude the right heart side and pulmonary arteries, leading to various complications.

Implications of Heartworm Infestation

  • Causes inflammation and trauma to the pulmonary vasculature, which can lead to significant health issues.

  • Increased pulmonary artery resistance: Results in clinical signs such as cough, lethargy, and heart failure, which can progress if untreated.

Clinical Observations

Symptoms to look for include:

  • Coughing after exercise or excitement

  • Lethargy and weakness, especially during routine activities

  • Loss of appetite and accompanying weight loss, which can indicate severe issues

Lesson Summary

  • The heart is obliquely oriented between ribs 3 and 6, crucial for thoracic anatomy understanding.

  • Completely invested by a pericardium with a pericardial cavity that protects and supports heart motion.

  • AV valves supported by chordae tendineae to ensure proper blood flow direction.

  • Pulmonary and aortic valves are self-supported, demonstrating advanced evolutionary adaptations.

  • Coronary arteries and veins supply the heart; collateral circulation is limited, indicating the importance of primary coronary supply.

  • Autonomic innervation through sympathetic and Vagus nerves regulates heart functionality.

  • Heartworm disease impacts pulmonary circulation and heart function, making awareness and prevention vital for at-risk pets.