Histology of Circulatory System

Circulatory System

• The cardiovascular system is subdivided into two functional parts: blood vascular system and lymph vascular system.
• Blood vascular system:
  • Distributes nutrients, gases, hormones to all parts of the body.
  • Collects wastes produced during cellular metabolism.
  • Consists of a continuum of blood vessels (arteries, arterioles, capillaries, venules, veins) and a muscular pump (heart).
• Lymph vascular system:
  • Collects tissue fluid from tissues and returns it to the blood vascular system.
  • Consists of blind-ended capillaries (lymphatic capillaries) connected to venous vessels (lymphatic vessels) and various lymphoid organs (e.g., lymph nodes).

Structure of the Heart Wall

• The heart wall is a three-layered structure:
  • Inner layer: endocardium
  • Middle layer: myocardium
  • Outer layer: epicardium (also called the pericardium)

Heart Wall Layers

• Endocardium:
  • Inner layer of the heart wall.
  • Consists of the endothelial lining and underlying connective tissue layers.
  • Endothelium consists of a typical simple squamous epithelium.
  • Connective tissue region separates the endothelium from the myocardium in humans.
• Myocardium:
  • Middle layer of the heart wall.
  • Contains cardiac muscle throughout most of the heart.
  • Cardiac muscle cells are arranged in strands whose ends attach to the dense connective tissue which surrounds the valves.
• Epicardium:
  • Outer layer of the heart wall.
  • Consists of a connective tissue region covered by a mesothelium on its outer surface.

Special Features of the Heart

• Valves:
  • Outgrowths from the endocardium that prevent backflow of blood.
  • Contain three components: cardiac skeleton, dense regular CT (heavily collagenous), and cardiac muscle in the myocardium.
• Pacemakers:
  • Modified cardiac muscle cells in the myocardium of the sinoatrial (SA) node.
  • Have a high leakage rate, giving them the fastest intrinsic contraction rate among the populations.

Blood Vessels

• Walls of larger blood vessels contain three major layers:
  • Tunica intima (luminal layer)
  • Tunica media (contains layers of elastic fenestrated sheets or CT alternating with layers of smooth muscle)
  • Tunica adventita (contains loose to moderately dense CT, scattered smooth muscle cells)
• Types of blood vessels:
  • Large arteries (also called elastic arteries or conducting arteries)
  • Medium to small arteries (also called muscular arteries)
  • Arterioles
  • Capillaries

Blood

• Type of connective tissue that contains living and non-living components.
• Purpose: transport nutrients and waste throughout the body.
• Components:
  • Blood cells (45% of volume)
  • Plasma (liquid part of blood)

Blood Cells

• Types:
  • Erythrocytes (RBCs): carry oxygen to tissues
  • Leukocytes (WBCs): defend against infectious agents
  • Platelets: clotting factors

Erythrocytes (RBCs)

• Purpose: carry oxygen to tissues
• Hemoglobin:
  • Type of protein
  • Contains iron which binds with oxygen
  • Concentration determines availability of oxygen to cells

Leukocytes (WBCs)

• Purpose: defend against infectious agents
• Mechanisms:
  • Diapedesis: move across/through blood vessels
  • Positive chemotaxis: locate area of damage/infection by responding to released immune/bacterial chemicals
• Types:
  • Neutrophil
  • Eosinophil
  • Basophil
  • Lymphocytes
  • Monocytes