8B Blood Vessels: Structure and Function Notes

General Plan for Blood Distribution

• Circulation Systems: Blood is distributed via systemic and pulmonary circulation.

• Blood Volume Distribution:

  • Venous system: Contains $65-70\%$ of blood volume, approximately $3.5\,\text{liters}$.

  • Other components (Heart, arterial system, capillaries): Contain $30-35\%$ of volume, approximately $1.5\,\text{liters}$.

Blood Vessel Structure

• Tunica Intima: Innermost layer consisting of endothelium, connective tissue, and elastic fibers. Arteries feature a thick layer called the Internal Elastic Membrane.

• Tunica Media: Middle layer composed of smooth muscle and connective tissue; responsible for controlling vessel diameter.

• Tunica Externa: Outermost connective tissue sheath that anchors vessels to surrounding tissue.

Arteries and Arterioles

• Arteries: Conduct blood away from the heart and are under autonomic control (Vasoconstriction and Vasodilation).

  • Elastic Arteries: High elastic fiber content in the tunica media used for conduction.

  • Muscular Arteries: High smooth muscle content in the tunica media; conduct blood to skeletal muscles and organs.

• Arterioles: Thinner smooth muscle layer than arteries. Known as Resistance vessels, as changes in their diameter significantly affect the force required to move blood.

Capillaries and Exchange Mechanisms

• General Properties: Site of exchange between blood and ISF; walls consist only of an endothelial layer. The lumen is only wide enough for a single red blood cell.

• Continuous Capillaries: Most common; feature continuous endothelial layers and tight junctions. Found in the Blood Brain Barrier.

• Fenestrated Capillaries: Contain pores for rapid exchange of fluid and solutes. Located in kidneys (filtration) and the digestive tract (absorption).

• Capillary Bed Organization:

  • Collaterals: Direct multiple arteries to one area to guarantee supply.

  • Pre-capillary sphincters: Control blood flow into the network.

  • Arteriovenous anastomoses: Direct interconnections between arterioles and venules.

Venules and Veins

• Venules: Formed from merging capillaries; lack a tunica media.

• Veins: Return blood to the heart; characterized by thinner walls, larger lumens, and reduced smooth muscle compared to arteries.

• Mechanisms for Venous Flow:

  • Blood pressure: Approximately $10\%$ of aortic pressure.

  • Valves: Prevent backflow (one-way).

  • Muscular assistance: Contraction of skeletal muscles.

  • Respiration: Assists in maintaining flow.

Fluid Dynamics and Exchange

• Exchange Sites: Occurs at the capillaries via diffusion ($O_2$, $CO_2$), osmosis (water in kidneys), concentration gradients (electrolytes), and membrane transporters (glucose and amino acids).

• Filtration and Reabsorption:

  • Arterial end: Net filtration pressure is $+10\,\text{mmHg}$ because capillary hydrostatic pressure ($35\,\text{mmHg}$) exceeds blood colloidal osmotic pressure ($25\,\text{mmHg}$).

  • Mid capillary: No net movement; pressures are equal at $25\,\text{mmHg}$.

  • Venous end: Net filtration pressure is $-7\,\text{mmHg}$, leading to reabsorption as capillary hydrostatic pressure ($18\,\text{mmHg}$) falls below blood colloidal osmotic pressure ($25\,\text{mmHg}$).