Blood Vessels Structure and Function (Part C)

Control of Blood Flow

Tissue Perfusion

• Defined as blood flow through body tissues.

• Involves:

  • Delivery of O₂ and nutrients to tissue cells.

  • Removal of wastes from tissue cells.

  • Gas exchange in the lungs.

  • Absorption of nutrients in the digestive tract.

  • Urine formation in kidneys.

• Rate of flow is precisely regulated to ensure proper function of tissues or organs.

Factors Controlling Blood Flow

• Rate of blood flow is controlled by two types of factors:

  1. Extrinsic Control

     • Involves the sympathetic nervous system and hormones.

     • Controls blood flow throughout the body.

     • Acts on arteriolar smooth muscle to reduce flow to regions that need it the least.

  2. Intrinsic Control (Autoregulation)

     • Refers to local control of blood flow that adjusts flow to meet specific requirements of tissues.

     • Local arterioles can modify their diameter (constriction or dilation).

     • Organs regulate their own blood flow by altering the resistance of their arterioles.

Summary of Control Mechanisms

• Intrinsic vs. Extrinsic Control:

  • Intrinsic Control deals with local blood flow regulation.

  • Extrinsic Control involves systemic regulation to maintain mean arterial pressure (MAP) and adapt to whole-body demands.

Example: Redistribution of Blood During Exercise

• At rest, skeletal muscles receive about 20% of total blood flow but can receive over 70% during exercise.

• Intrinsic controls: Skeletal muscle arterioles dilate, allowing increased blood flow to muscles.

• Extrinsic controls: Blood flow is reduced to other organs (e.g., kidneys, digestive organs).

• Mean Arterial Pressure (MAP) is maintained despite muscle dilation.

Blood Flow Distribution

At Rest:

• Brain: 750 ml/min

• Heart: 250 ml/min

• Skeletal Muscles: 1200 ml/min

• Skin: 500 ml/min

• Kidneys: 1100 ml/min

• Abdomen: 1400 ml/min

• Other: 600 ml/min

• Total blood flow: 5800 ml/min

During Strenuous Exercise:

• Total blood flow: 17,500 ml/min

Autoregulation: Intrinsic Control of Blood Flow

• Reactive Hyperemia: Increased blood flow to an area due to intrinsic factors.

• Two types of mechanisms determining final autoregulatory response:

  1. Metabolic Controls:

     • Increase in tissue metabolic activity leads to:

     • Declining levels of O₂.

     • Increasing levels of metabolic products such as H⁺, K⁺, adenosine, and prostaglandins.

     • These changes cause:

     • Direct relaxation of arterioles.

     • Relaxation of precapillary sphincters.

     • Release of nitric oxide (NO), a powerful vasodilator.

  2. Myogenic Controls:

     • Local vascular smooth muscle responds to changes in Mean Arterial Pressure (MAP) to maintain perfusion.

     • Passive Stretch: Increased MAP stretches the vessel wall, leading to muscle constriction and reduced blood flow.

     • Reduced Stretch: Decreased MAP causes less stretch, resulting in muscle dilation and increased blood flow.

Long-Term Autoregulation

• Occurs when short-term autoregulation cannot meet tissue nutrient requirements.

• May take weeks or months to develop.

• Results in:

  • Increase in the number of vessels in the region (angiogenesis).

  • Enlargement of existing vessels.

• Common in the heart when coronary vessels are occluded and in individuals living at high altitudes.

Summary of Intrinsic vs. Extrinsic Controls

• Intrinsic controls include metabolic and myogenic controls that distribute blood flow to individual organs based on immediate needs.

• Extrinsic controls involve hormonal and neural mechanisms:

  • Maintain Mean Arterial Pressure (MAP).

  • Redistribute blood during exercise and thermoregulation.

Blood Flow in Special Areas

Skeletal Muscles:

• Blood flow varies with fiber type and activity.

• At rest, myogenic and neural mechanisms maintain flow at approximately 1L/min.

• During exercise, blood flow increases in direct proportion to metabolic activity (active or exercise hyperemia).

  • Local controls override sympathetic vasoconstriction, allowing flow to increase up to 10 times.

Active Hyperemia

• Definition: Increased blood flow due to enhanced metabolic demand during physical activity.

• Involves factors such as O₂, CO₂, and H⁺ in extracellular fluid leading to vasodilation of arterioles overriding sympathetic signals.

Capillary Exchange

Velocity of Blood Flow

-Velocity Changes in Systemic Circulation:

• Fastest in the aorta;

• Slowest in capillaries;

• Increases again in veins.

  • Speed is inversely related to total cross-sectional area.

  • Capillaries have the largest area, resulting in the slowest flow, which is optimal for nutrient exchange.

Capillary Exchange Mechanisms

• Molecules pass by diffusion between blood and interstitial fluid along concentration gradients.

• There are four routes for molecules to cross capillaries:

  1. Direct Diffusion through Endothelial Membranes:

  • Especially for lipid-soluble molecules such as respiratory gases.

  1. Passage through Clefts:

  • For water-soluble solutes.

  1. Fenestrations:

  • For water-soluble solutes.

  1. Active Transport via Pinocytotic Vesicles or Caveolae:

  • For larger molecules like proteins.

Capillary Transport Mechanisms Illustration

• Illustrates:

  • Red blood cells in the lumen.

  • Endothelial cells structures like intercellular clefts and fenestrations.

  • Tight junctions, basement membranes, and pinocytotic vesicles allow for transport.

Summary of Transport Mechanisms

• Routes of Transport:

  1. Diffusion through plasma membranes: for lipid-soluble substances.

  2. Through intercellular clefts: for water-soluble substances.

  3. Through fenestrations: for certain water-soluble substances.

  4. Via vesicles or caveolae: for larger substances like proteins.