Blood Vessels: Composition and Types

Overview of the Circulatory System

• The circulatory system includes both the cardiovascular and lymphatic systems.
• The cardiovascular system consists of the heart (covered in week 3), blood (covered in week 1), and blood vessels (covered in week 2).
• Blood vessel types are arteries, capillaries, and veins.
• The lymphatic system consists of lymphatic vessels, lymph nodes, and tissues (covered in week 5).
• There is an inter-relationship between the lymphatic and cardiovascular systems.

Blood Vessel Composition (Tunics)

• Blood vessels are composed of concentric layers or tunics of different tissue types.
• The tunica intima/interna is the inner layer, being a very thin lining of endothelium (simple squamous epithelium) plus a thin layer of supporting connective tissue.
• Usually only the nuclei are noticeable in the tunica intima.
• The tunica media is the middle layer, often the thickest, and contains smooth muscle arranged circumferentially and elastic fibers in varying proportions.
• The tunica adventitia/externa is the outer layer, made of fibrous connective tissue continuous with stromal connective tissue, collagen, and elastic fibers.
• Nerve tissue is inconspicuous and involved in regulating smooth muscle and mediating pain sensation.

Blood Vessel Types

• Blood vessels are categorized by function.
• Arteries conduct blood away from the heart.
• Elastic (conducting) arteries transition into muscular (distributing) arteries, then into arterioles.
• Capillaries communicate between arteries and veins, and can be continuous, fenestrated, or discontinuous.
• Veins return blood to the heart and include the transition from venules to medium-sized veins to large veins.

Arteries

• Arteries carry blood away from the heart.
• Elastic arteries "conduct" blood to specific anatomical regions.
• Function to maintain a constant pressure gradient despite pumping action.
• Major collateral or terminal vessels include the pulmonary trunk, brachiocephalic trunk, subclavian, carotid, and common iliac arteries.
• Muscular arteries 'distribute' blood to muscles or organs and are named after what they supply.
• These contain mostly smooth muscle in the wall and are under mostly sympathetic regulation.
• Examples of muscular arteries are the renal, brachial, radial, femoral and popliteal arteries.

Veins Structural Differences

• The low pressure system in veins measures approximately 5-10 mmHg.
• Blood at rest (64%) is largely within the systemic veins and venules, highlighting their role as a blood reservoir.
• Valves aid in venous return by preventing backflow of blood.
• The musculovenous pump helps return blood by pushing against gravity through contraction of skeletal muscle.
• Paired structures are located in distal body regions like the hands, feet, forearm, and leg, with companion veins (X2) traveling with a single artery.
• Pulsations of the artery aid with venous return.
• There are both superficial and deep veins.
• Deep veins course with the arteries.
• Superficial veins run beneath the skin in the subcutaneous space and are easy to visualize/access.
• Sympathetic impulses can cause venoconstriction of the tunica media during exercise or trauma.
• This decreases the volume of blood in reservoirs and redirects it to skeletal muscle/tissue.

Histology of Arteries versus Veins

• Arteries typically have a round shape in sectional view and a relatively thick wall against a vein with a flattened or collapsed shape and a relatively thin wall.
• The endothelium in arteries is usually rippled while in veins it is often smooth.
• Internal elastic membrane is present in arteries but absent in veins.
• The tunica media is thick in arteries, dominated by smooth muscle cells and elastic fibers, but thin in veins, dominated by smooth muscle cells and collagen fibers.
• Arteries have a present external elastic membrane while veins have an absent external elastic membrane.
• Both arteries and veins have a tunica externa made of collagen and elastic fibers.

Elastic or Conducting Arteries

• The tunica media in aorta and its larger branches is predominated by elastic tissue which facilitates compliance, stretch, recoil, and pressure reservoir functions.
• The tunica media contains elastic laminae, collagen, and smooth muscle fibers.
• The tunica adventitia layer is made of collagen, some elastic fibers, small blood vessels (vasa vasorum) .

Distributing or Muscular Arteries

• The tunica media is predominated by smooth muscle for regulation in distributing or muscular arteries like the brachial, radial, femoral, and coronary arteries.
• The tunica externa is mainly collagen with diffuse external elastic lamina, continuous with the fascia of the organ in which the vessel is found.
• The tunica media is the thickest of the three tunics with circumferentially arranged smooth muscle and collagen fibers.
• Muscular arteries receive nerve supply from both alpha receptors of the ANS.
• Increased stimulation via epinephrine or norepinephrine causes vasoconstriction.
• Decreased stimulation leads to vasodilation.
• Damage to a vessel triggers vascular spasm, limiting blood loss.

Arterioles

• Arterioles immediately precede a capillary bed
• Tunica media has single-few layers of smooth muscle which regulates blood flow to capillaries providing resistance

Capillaries

• They are the smallest (microscopic) vessels and serve as communication pathways between arteries and veins.
• Diameter is less than 10 μm, allowing red blood cells to pass in a single file.
• Capillaries are formed by thin, flattened endothelial cells forming the endothelium.
• The endothelium is often inconspicuous (usually only the nuclei are visible) and is supported by a basement membrane.
• Pericytes are wrapped at intervals around capillaries.
• They have contractile properties and regulate diameter or capillary blood flow.
• Gases pass through endothelial cells by diffusion in continuous capillaries.
• Small molecules pass through small gaps between neighboring cells (except in the brain), known as intercellular clefts.
• Large molecules are transported by rapid vesicular transcytosis.
• Continuous capillaries are found in muscle, lungs, CNS (with tight junctions), skin, and fat.
• The endothelial plasma membrane is contains pores (fenestrations) in fenestrated capillaries.
• These are more permeable than continuous capillaries, facilitating quick exchange of substances.
• Fenestrated capillaries are found in kidneys, intestines, gallbladder, and endocrine glands.
• Sinusoidal capillaries have very large fenestrations and incomplete basement membranes, making them leaky.
• Sinusoidal capillaries facilitate exchange of large macromolecules and cells between the blood and surrounding tissue.
• The capillaries are also more voluminous.
• Sinusoidal capillaries are found in the liver, bone marrow, spleen, and adrenal medulla.

Venules

• Post-capillary venules consist of endothelium and pericytes.
• Larger venules have endothelium plus 1-2 layers of tunica media.
• They are easily distinguishable from arterioles by the size of the lumen relative to wall thickness
• Medium-sized veins have an intima, media and adventitia.

Veins

• Veins are capacitance vessels and act as a blood reservoir.
• The tunica intima forms valves with each valve is a leaflet.
• The tunica media has less muscle and elastin compared to similar diameter arteries.
• The tunica adventitia is the thickest layer and consists of collagen and some elastic fibers.
• In veins, lack visible elastic laminae, are often flattened or collapsed in cross-section , and have a larger lumen compared to arteries.

Large Vein (Superior and Inferior Vena Cava)

• The tunica adventitia is the thickest layer with bundles of smooth muscle arranged longitudinally.
• The tunica media consists of collagen and some smooth muscle cells

Heart Wall

• Endocardium is tunica intima, myocardium is tunica media, and epicardium is tunica adventitia.

Lymphatic Vessels

• Plasma leakage is balanced by re-absorption via lymphatic vessels.
• 'Leakiness' of capillaries is determined by endothelial lining.
• Lymphatic vessels return tissue fluid and leaked proteins to circulation.
• Imbalance leads to oedema (inflammation, obstruction).
• Lymphatic vessels resemble venules, but often contain a pink precipitate, few red blood cells along with many white blood cells.
• Very flattened endothelial cells form the vessel lining, with rudimentary or no basement membrane.

Terminology Checklist

• Endothelium
• Subendothelium
• External elastic lamina/lamella
• Internal elastic lamina/lamella
• Pericyte
• Tunica Adventitia
• Tunica Media
• Tunica Intima
• Valve
• Vasa Vasorum
• Conducting/elastic artery
• Distributing/muscular artery
• Arteriole
• Capillary (continuous, fenestrated, sinusoidal)
• Post-capillary venule
• Venule
• Vein (medium, large)
• Sphincter
• Lymphatic vessel