The Circulatory System

The Circulatory System

# Arteries

General Structure:

• Inner coat:

• Endothelium: includes endothelial cells, subendothelial layer (loose connective tissue), and internal elastic membrane (with fenestrae that allow for the passage of substances).

• Middle coat:

• Thickest layer, containing circularly arranged smooth muscle fibers, elastic lamellae, elastic fibers, collagen fibers, and proteoglycans.

• Large arteries have vasa vasorum and external elastic membrane.

• Outer coat:

• Connective tissue with many collagen fibers and elastic fibers running longitudinally along the artery.

Classification: (based on size and predominant component of the MIDDLE COAT)

• Elastic arteries:

• Located in: pulmonary artery, brachiocephalic artery, subclavian artery, common carotid artery, common iliac artery.

• Characteristics:

• The inner coat has an atypical internal elastic membrane.

• The middle coat has elastic lamellae, elastic fibers, smooth muscle cells, collagen fibers, and proteoglycans.

• The outer coat is relatively thin; in large arteries, there are vasa vasorum.

• Muscular arteries:

• Characteristics:

• The inner coat in small arteries lacks a subendothelial layer; the internal elastic membrane has numerous fenestrae.

• The middle coat has densely packed circular smooth muscle fibers, elastic lamellae, and circular elastic fibers.

• The outer coat consists of fibrous connective tissue, elastic fiber network, loose connective tissue, adipose cells, fibroblasts, and mast cells.

• Arterioles:

• Characteristics:

• The inner coat has a very thin subendothelial layer; some terminal arterioles lack an internal elastic membrane.

• The middle coat has 1-5 layers of circularly arranged smooth muscle fibers.

• The outer coat is thin and poorly developed.

Role:

• Elastic arteries: Change the elasticity of the vessel.

• Muscular arteries: Contraction, elasticity, regulation of blood flow to different regions.

• Arterioles: Reduce pressure, decrease blood flow velocity.

# Capillaries

General Structure:

• Endothelium:

• Connected by tight junctions, gap junctions, and overlapping areas.

• Has endothelial pores (fenestrae), caveolae.

• Basement membrane:

• Pericytes:

• Capable of contraction to regulate blood flow, have long cytoplasmic branches.

• Perivascular cells are capable of phagocytosis.

Classification:

• Continuous capillaries:

• Located in: adipose tissue, muscle tissue, central nervous system.

• Characteristics: Endothelium and basement membrane lack fenestrae; the cell membrane has caveolae; cytoplasm contains pinocytotic vesicles.

• Fenestrated capillaries:

• Located in: renal glomeruli, intestinal mucosa, endocrine glands, choroid plexus, ciliary body.

• Characteristics: Have fenestrae, lack pericytes.

• Sinusoidal capillaries:

• Located in: liver, spleen, bone marrow.

• Characteristics: Tortuous, wide lumen, wide intercellular spaces between endothelial cells, numerous endothelial pores, lack a basement membrane, many phagocytic perivascular cells.

# Veins

General Structure:

• Thinner walls than arteries, lack internal elastic membrane, poorly developed circular elastic fibers, less muscle than arteries, longitudinal collagen fibers are well-developed, thick outer coat with many capillaries.

• Inner coat: Has venous valves.

Classification:

• Muscular veins: The main component of the middle coat is smooth muscle.

• Fibrous veins: The middle coat only contains collagen fibers, no muscle.

• Mixed veins:

• Fibroelastic veins

• Fibromuscular veins

• Musculoelastic veins

Distribution:

• Veins of the brain and meninges: Fibrous veins

• Axillary, subclavian, and jugular veins: Fibroelastic veins

• Veins in the deep region of the arm: Fibromuscular veins

• Veins of the lower limb: Musculoelastic veins

Portal vein system: One vein lies between two capillary networks.

Portal vein systems:

• Portal vein system in the liver.

• Hypothalamic-hypophyseal portal vein system.

Portal system of the arteries: Unique to the kidneys.

# Lymphatic System

Begins with: Blind-ended lymphatic capillaries.

Not present in: Central nervous system, cartilage, bone, bone marrow, thymus, teeth, and placenta.

Characteristics of lymphatic capillaries:

• Only one layer of flattened endothelial cells.

• No basement membrane.

• Reinforced by small bundles of fibers that connect the outer surface of the endothelial cells and collagen fibers.