Histology of Circulatory System

Study Notes

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).

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

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.

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.

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

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

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

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