Blood Vessel Structure and Function

Blood Vessel Structure and Function

• All tissues have an arterial supply and venous drainage
• Different types of blood vessels include arterioles, venules, veins, arteries, and capillaries.
• Blood vessels must be resilient, flexible, and always remain open to maintain function.
• Different blood vessel types have different functions.

Blood Vessel Structure

• Blood vessels are composed of three layers: tunica intima, tunica media, and tunica externa/adventitia.
• Tunica intima: simple squamous epithelium (endothelium), basal lamina, subendothelial connective tissue.
• Tunica media: smooth muscle fibers in connective tissue and may contain elastic fibers.
• Tunica externa/adventitia: connective tissue merging with surrounding connective tissue, may contain vaso vasorum.

Arteries vs Veins

• Arteries: blood under high pressure, thick walls, resemble garden hoses.
• Veins: blood under low pressure, thin walls, resemble fire hoses, may have valves to prevent backflow.
• When comparing corresponding arteries and veins, the artery has a thicker wall, a smaller lumen and maintains its shape. The artery is more resilient and doesn't contain valves, while veins contain many valves.

Types of Arteries

• Elastic (conducting) arteries: e.g., aorta, brachiocephalic, common carotid. Diameter: up to 2.5 cm
  • Withstand changes in pressure, ensure continuous blood flow.
  • Structural adaptations: thick tunica media with many elastic fibers and few smooth muscle cells.
• Muscular (distributing) arteries: e.g., brachial & femoral. Diameter: 0.5 mm – 0.4 cm.
  • Distribute blood to muscles and organs.
  • Capable of vasodilation and vasoconstriction to regulate blood flow.
  • Structural adaptations: smooth muscle cells in tunica media, distinct internal & external elastic laminae, thick tunica externa.
• Arterioles (resistance vessels): Diameter: ≤ 30 μm
  • Capable of vasoconstriction and vasodilation regulating blood flow to organs.
  • Involved in blood pressure control. -Structural adaptations: one to two layers of smooth muscle cells in tunica media; poorly defined tunica externa.

Capillaries

• Connect arterioles and venules (microcirculation).
• Site of gaseous exchange.
• Thin walls facilitate diffusion.
• Blood flow is slow through capillaries.
• Structure permits 2-way exchange.
• 8-µm (micrometers) in diameter.
• Found near almost every cell.

Different Types of Capillaries

• Continuous capillaries: Majority, found in skeletal and smooth muscle, CT and the lungs.
• Fenestrated capillaries: Pores penetrate the endothelial lining; rapid exchange of water and larger solutes (e.g. small peptides). Found in absorption sites (kidney, choroid plexus, endocrine glands).
• Sinusoidal capillaries: Spaces between endothelial cells; incomplete or absent basement membrane; exchange of large solutes (e.g., plasma proteins). Found in special locations like the liver and engulfing damaged RBCs or in tissues where large molecules need to pass easily.

Capillary Beds

• Capillaries are organized into groups called capillary beds.
• A metarteriole supplies a single capillary bed and continues as a thoroughfare channel leading directly to a vein, having numerous capillaries branching off it.
• Metarteriole constriction reduces blood flow to the entire capillary bed.
• Precapillary sphincters guard entrances to capillaries; contraction narrows entrance and reduces blood flow; relaxation dilates entrance, increasing blood flow.
• Arteriovenous anastomoses form direct connections between arterioles and venules. When dilated, blood bypasses the capillary bed and goes directly to venous circulation.

Venules

• Collect blood from capillary beds and deliver it to small veins.
• Diameter varies, average 20 μm.
• Small venules: endothelium on a basement membrane.
• Larger venules: increasing numbers of smooth muscle cells outside endothelium.

Veins

• Classified by size: small (<2mm), medium (2-9mm), large (>9mm), e.g., superior and inferior vena cavae.
• Low-pressure system.
• Easily distensible (capacitance vessels).
• Structural adaptations: thin walls, tunica externa is predominant.
• Valves aid in blood flow.

Valves and Musculovenous Pump

• Valves prevent backflow in veins.
• The musculovenous pump, assisted by skeletal muscle contraction, moves blood from the limbs back to the heart.

Pressure Changes

• Systolic and diastolic pressures are illustrated throughout the various vessels, showing the pressure changes in each type of vessel.

Distribution of Blood

• The percentage of blood volume in different parts of the circulatory system is shown in a pie chart.

Anatomical Terminology

• Anatomical position: a standard reference point to describe the location of body parts.
• Directional terms (anterior/ventral, posterior/dorsal, superior, inferior, medial, lateral, proximal, distal) are used to describe location relative to other structures.
• Plane terms (coronal/frontal, horizontal/transverse, sagittal) are used to describe sections of the body.