lesson 86

Lesson Overview

Systems 1: Cardiovascular & Respiratory

Lesson #86: 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 essential for a comprehensive understanding of cardiac anatomy and physiology.

Key Topics:

• Prework linkage to connect previous knowledge to the current lesson

• Functional considerations regarding the heart's role in the circulatory system

• Internal Structure and the significance of valves in blood flow

• Blood and nerve supply critical to heart function

• Overview of Heartworm disease and its impact on cardiovascular health

Location and Topography of the Heart

Position:

• The heart is located between the 3rd to 6th rib, specifically from the 3rd to 5th intercostal spaces.

• It is oriented more or less obliquely, with the apex slightly deviating to the left side of the thorax, contributing to its unique positioning.

• The great vessels (such as the aorta and pulmonary trunk) are situated at the base of the heart, providing a critical connection to systemic and pulmonary circulation.

• The heart is supported by phrenicopericardiac and sternopericardiac ligaments, which provide structural stability.

Visual References:

• Lateral radiograph of the thorax of the horse demonstrating the positioning of the heart

• Schematic representation of the heart structure in the dog for comparative anatomy

• Dissection image showing the orientation and location of the heart in the dog, emphasizing anatomical landmarks

Relations of the Heart

Surrounding Structures

• Lungs: Positioned laterally on either side, critical for gas exchange. Notably, there is a thinner pulmonary tissue layer on the left side of the heart, allowing for easier access to the heart for certain clinical procedures. Cardiac notches are present only on the right side in dogs, influencing surgical approaches.

• Other Relations:

  • Trachea: Located at the base of the heart, allowing for air passage between lungs and the upper respiratory tract.

  • Esophagus: Also located at the base, running close to the heart, which can be relevant during surgical interventions.

• Nerves:

  • The phrenic and vagus nerves are related laterally, playing crucial roles in innervating the diaphragm and regulating heart rate and vascular resistance, respectively.

The Pericardium

Structure:

• The heart is completely enveloped by the pericardium, which is composed of three distinct layers:

  • Visceral pericardium (Epipericardium): This innermost layer tightly adheres to the heart wall, providing a protective interface.

  • Parietal pericardium: The outer layer, which is adhered to the mediastinum, provides additional structural support.

  • Pericardial cavity: A potential space filled with serous fluid that lubricates the heart, minimizing friction during heartbeats.

  • Fibrous pericardium: A tough connective tissue layer that attaches to the pleura and forms ligaments at the apex, maintaining the heart's position within the thorax.

Function:

• The pericardium serves several essential functions, including isolating the heart, providing lubrication to reduce friction, and preventing overstretching during intense physical activity.

Internal Structure of the Heart

Chambers and Valves

• The heart consists of four chambers: two ventricles and two atria (including their extensions, the auricles).

  • Ventricles: Thick-walled chambers responsible for pumping blood away from the heart under high pressure. The left ventricle has the thickest walls due to the higher systemic pressure it must generate.

  • Atria: Thin-walled chambers that receive blood returning to the heart; their structure allows for volume accommodation.

  • Auricle: A blind sac extended from each atrium that increases capacity for receiving blood.

  • Coronary Groove: This groove separates the atria from the ventricles, serving as an anatomical landmark for coronary artery placement.

  • Interventricular Grooves: These demarcate the right and left ventricles and contain blood vessels that supply the heart muscle itself.

  • Valves: Cardiac valves, including the atrioventricular (AV) and semilunar valves, ensure unidirectional blood flow; AV valves are supported by chordae tendineae, preventing backflow.

Great Vessels

Function & Structure

• Pulmonary Trunk: Carries deoxygenated blood from the right ventricle to the lungs for oxygenation.

• Aorta: The main artery distributing oxygenated blood to the body, branching into systemic circulation.

• Cranial and Caudal Vena Cava: These veins collect deoxygenated blood returning from the body, entering the right atrium.

• Azygos Vein: Drains the thoracic wall, connecting systemic venous return to the superior vena cava, providing an alternative route for blood flow if obstructions occur.

Blood Supply

Coronary Arteries

• Right Coronary Artery: Typically smaller, providing blood primarily to the right side of the heart and parts of the left ventricle.

• Left Coronary Artery: Larger and supplies more blood to the heart; it branches into:

  • Paraconal Interventricular Branch: Supplies the interventricular septum and parts of both ventricles.

  • Circumflex Branch: Encircles the heart, providing collateral supply.

  • Subsinuosal Interventricular Branch: Supplies the posterior aspects of the ventricles.

• Venous Drainage: The Great Coronary Vein drains into the right atrium, complemented by the middle cardiac vein and right cardiac veins collecting deoxygenated blood from the heart tissue.

Nerve Supply

Autonomic Nervous System

• Sympathetic: Innervation is provided by stellate and middle cervical ganglia, which increase heart rate and contractility.

• Parasympathetic: Supplied by right and left vagus nerves, which decrease heart rate and promote digestive activities.

Heartworm Disease

Overview

• Causative Agent: Dirofilaria immitis (filarial nematode) transmitted by mosquito bites, impacting both dogs and cats.

• Effects: The disease leads to the occlusion of right heart chambers and the pulmonary trunk/arteries due to the presence of adult worms, resulting in increased workload for the heart.

• Clinical Signs:

  • Coughing after exercise: Suggesting pulmonary congestion.

  • Weight loss and lethargy: Results from the heart's reduced efficiency.

  • Strength loss: Impacting overall quality of life.

  • Exercise intolerance: Due to reduced cardiac output.

Lesson Summary

• Understanding heart anatomy emphasizes its oblique orientation in the thorax and its encapsulation within the pericardium, which forms a protective cavity.

• The cardiac valves ensure unidirectional blood flow, assisted by the chordae tendineae, which prevent backflow during contraction.

• The coronary arteries supply oxygen-rich blood to the heart muscle; however, they lack extensive collateral circulation, making them critical in cardiac health.

• Heartworm disease poses significant vascular and functional disturbances in the cardiac system, necessitating awareness in veterinary practices to prevent and treat effectively.