CARDIOVASCULAR SYSTEM (SYSTEMIC HISTOLOGY)

Cardiovascular System

The cardiovascular system has two main components:

1. Lymphatic System
2. Blood Vascular System

1. Lymphatic System

• The lymphatic system contains lymph organs and lymph channels (or vessels).
• Lymph cells are studied alongside the blood vascular system, whereas lymphoid organs are studied under the immune system or lymphatic system.

2. Blood Vascular System

• The blood vascular system structurally includes the heart and the blood vessels.
• Blood vessels are divided into:
  • Macroscopic vasculature: Visible to the naked eye.
  • Microscopic vasculature: Seen under a microscope.

Heart Structure

The heart is made up of three layers, from the inside to the outside:

1. Endocardium (inner lining):
   • This layer is continuous with the tunica intima of the blood vessels.
   • It is subdivided into three layers:
     • Endothelium
     • Subendothelium
     • Subendocardium
2. Myocardium (middle layer):
   • This is the thick muscular layer of the heart.
3. Epicardium (outer layer):
   • This is the external layer that covers the surface of the heart.

Endocardium

• The endocardium is the innermost lining of the heart.
• It is continuous with the tunica intima of the blood vessels.
• The endocardium is subdivided into three layers:

1. Endothelium

• Composed of simple squamous epithelium with bulging nuclei.

2. Subendothelial Layer

• This layer is made up of dense connective tissue.

3. Subendocardium

• Located outer to the subendothelial layer.
• Composed of loose connective tissue, blood vessels, nerves, and fat cells.
• This layer also contains conducting myofibres, which are known as Purkinje fibres.

Myocardium

• The myocardium is made up of:
  • Cardiac muscle cells (myocytes)
  • Loose connective tissue
  • Blood capillaries
• It also contains the conducting system and the cardiac skeleton.
• Myocardial cells (myocytes) are arranged in interweaving and anastomosing bundles.
• Atrial myocytes are smaller and thinner compared to ventricular myocytes.

Epicardium

• The epicardium is the outermost layer of the heart.
• It is a serous membrane lined by simple squamous epithelium.
• The epicardium represents the visceral pericardium.
• Together with the parietal pericardium, it forms the pericardial cavity.

Conducting System of the Heart

• The conducting system consists of structures that generate and conduct electrical impulses across the heart.
• It includes:
  1. Sinoatrial (SA) Node
  2. Atrioventricular (AV) Node
  3. Atrioventricular Bundles (Bundle of His)

1. SA Node (Sinoatrial Node)

• The SA node is located in the wall of the right atrium, near the anterior or cranial part of the sinus venarum.
• It is composed of nodal cells, which are modified cardiac myocytes.
  • Nodal cells are small, thin, and branching.
  • They are also called pacemaker cells.
• The SA node is surrounded by numerous autonomic nerve endings and ganglia of the 10th cranial nerve (vagus nerve).
• It generates electrical impulses, which propagate radially to the AV node.

2. AV Node (Atrioventricular Node)

• The AV node is located on the tricuspid valve in the right atrium, within the septal wall (interventricular septum).
• The cells of the AV node are structurally similar to those of the SA node.
• The AV node gives rise to large cells called Purkinje cells, also known as Purkinje fibres due to their longer length.

3. AV Bundle (Bundle of His)

• The AV bundle arises from the AV node and penetrates the cardiac skeleton (cardiac ring).
• It divides into two branches:
  • Left bundle branch
  • Right bundle branch
• Each bundle branch travels within the subendocardium of the respective side of the interventricular septum.
• These branches extend to the apex of the heart, distributing fibres to different parts of the ventricles during their course.

Cardiac Skeleton

• The cardiac skeleton is made up of fibrous tissue, cartilage, bone, or a combination of these, depending on the age and species of the animal.
• Functions:
  • It provides attachment to the atrial and ventricular muscles.

Structure of the Cardiac Skeleton

The cardiac skeleton consists of three main parts:

1. Fibrous Rings (Annuli Fibrosi)

• These are four fibrous rings located around:
  • The atrioventricular orifices.
  • The aorta.
  • The pulmonary artery.

2. Fibrous Trigone (Trigonum Fibrosum)

• The fibrous trigone is a triangular layer of connective tissue located between the aortic bulb and the atrioventricular orifices.
• There are two fibrous trigones:
  • Left fibrous trigone
  • Right fibrous trigone

3. Fibrous Part of the Interventricular Septum

• This is the fibrous portion present within the interventricular septum.

Species Variations

The composition of the cardiac skeleton varies among animal species:

• Pig and cat: Made up of dense irregular connective tissue.
• Dog: Composed of fibrocartilage.
• Large ruminants (e.g., cattle): Contains ossified bone (called os cordis).
• Other species: May contain varying amounts of cartilage or fibrocartilage.