Circulatory System Overview

Blood Flow, Pressure & Resistance

• Blood flow (F) is directly proportional to the blood pressure gradient (ΔP) and inversely proportional to peripheral resistance (R).
• Resistance (R) is key in influencing local blood flow due to its ease of change by altering blood vessel diameter.
• Resistance is increased by abrupt vessel diameter changes and obstacles like fatty plaques.

Arterial Blood Pressure

• Blood pressure near the heart is pulsatile.
• Systolic pressure: pressure during ventricular contraction (~120 mm Hg).
• Diastolic pressure: aortic pressure during heart rest.
• Pulse pressure: difference between systolic and diastolic pressure.
• Mean arterial pressure (MAP): pressure propelling blood to tissues.
• MAP is calculated by adding diastolic pressure + 1/3 pulse pressure.
• Pulse pressure and MAP decrease with distance from the heart.

Capillaries

• Capillaries are the smallest vessels facilitating gas, nutrient, waste, hormone exchange between blood and interstitial fluid.
• Only one RBC can pass through at a time due to the narrow diameter.
• Capillary walls are composed of endothelial cells with tight junctions and intercellular clefts.
• Pericytes stabilize capillary walls, control permeability, and participate in vessel repair.
• Slow capillary flow allows for adequate time for exchange.

Veins

• Large-diameter lumens offer low resistance.
• Blood pressure is lower than in arteries.
• Venous valves prevent backflow.
• Venous sinuses are flattened veins with thin walls composed of only endothelium.

Anastomoses

• Interconnections of blood vessels provide alternate pathways (collateral channels) for continuous flow, even if one artery is blocked.
• Arterial anastomoses are common in joints, abdominal organs, brain, and heart but absent in retina, kidneys, and spleen.
• Arteriovenous anastomoses (shunts in capillaries) include the metarteriole-thoroughfare channel.
• Venous anastomoses are abundant and rarely block blood flow if a vein is occluded.

Velocity of Blood Flow

• Velocity of blood flow is fastest in the aorta, slows in capillaries, and speeds up in veins.
• Speed is inversely related to total cross-sectional area.
• Capillaries have the largest area resulting in the slowest flow.

Capillary Exchange Mechanisms

• Diffusion through intercellular clefts (water-soluble substances).
• Diffusion through the membrane (lipid-soluble substances).
• Movement through fenestrations (water-soluble substances).
• Transport via vesicles or caveolae (large substances)

Fluid Movements: Bulk Flow

• Hydrostatic pressure (HP) is the force exerted by a fluid pressing against a wall.
• Capillary hydrostatic pressure (HPc): capillary blood pressure forcing fluids through capillary walls.
• Interstitial fluid hydrostatic pressure (HPif): pressure pushing fluid back into a vessel; usually assumed to be zero.
• Colloid osmotic pressure (OPc): "sucking" pressure created by nondiffusible plasma proteins pulling water back into the capillary.
• Interstitial fluid colloid osmotic pressure (OPif): inconsequential due to low protein content.

Net Filtration Pressure (NFP)

• NFP comprises all forces acting on the capillary bed.
• NFP = (HPc + OPif) − (HPif + OPc)
• Net fluid flow is outward at the arterial end (filtration).
• Net fluid flow is inward at the venous end (reabsorption).
• More fluid leaves at the arterial end than returns at the venous end.
• Excess interstitial fluid is returned to the blood via the lymphatic system.

Edema

• Abnormal increase in interstitial fluid.
• Caused by increased outward pressure or decreased inward pressure.
• Increased capillary hydrostatic pressure accelerates fluid loss from blood.