Capillary Exchange

Capillary Exchange

• Occurs between blood and interstitial fluid
• Solutes enter and exit capillaries through diffusion, vesicular transport, and mediated transport

Diffusion

• The major route of solute movement in all tissues except the brain
• Substances such as CO2, O2, ions, amino acids, glucose, and hormones utilize diffusion
• Movement typically occurs between endothelial cells through fenestrations
• Hydrophobic solutes move directly through endothelial cells
• Cells and proteins are too large to move by diffusion

Vesicular Transport

• Transcytosis involves endocytosis from blood into endothelial cells, followed by exocytosis into interstitial fluid
• It facilitates absorption of large, hydrophilic molecules
• Proteins like albumin and antibodies are transported via this method

Mediated Transport

• Vital in the brain, where capillaries lack fenestrations, forming the blood-brain barrier (BBB)
• Requires a membrane carrier protein

Fluid Exchange

• Fluid enters the capillary through absorption and exits via filtration
• Both processes are facilitated by osmosis and bulk flow

Bulk Flow

• Occurs due to pressure differences; several pressures are involved:
  • Blood hydrostatic pressure (BHP): 35mmHg at the arterial end, 16mmHg at the venous end
  • Blood colloid osmotic pressure (BCOP): 26mmHg due to plasma proteins
  • Interstitial fluid hydrostatic pressure (IFHP): 0mmHg
  • Interstitial fluid osmotic pressure (IFOP): 1mmHg due to protein in the interstitial fluid
  • Net filtration pressure (NFP) is calculated as: (BHP + IFOP) – (IFHP + BCOP)
    • Positive NFP promotes filtration
    • Negative NFP promotes absorption

Fluid Reabsorption

• Approximately 90% of fluid filtered from the arterial end of the capillary into the interstitial fluid is reabsorbed back into the capillary at the venous end
• The remaining 10% enters the lymph system, maintaining a constant interstitial fluid volume

Edema

• Accumulation of fluid in the interstitial space, resulting in swelling
• Can be caused by high blood pressure (increased BHP), leakage of plasma protein due to increased capillary permeability (increased IFOP), decreased plasma protein (decreased BCOP) from malnutrition or burns, and obstruction of lymphatic vessels
• Obstruction of lymphatic vessels can be caused by elephantiasis or lymph node removal in cancer patients

Circulatory Shock

• Results from inadequate blood flow to tissues, compromising oxygen and nutrient delivery

Hypovolemic Shock

• Due to loss of blood volume from hemorrhage, severe diarrhea/vomiting, or extensive burns

Vascular Shock

• Blood volume is normal, but vessels are dilated causing vasodilation and systemic vasodilation leading to a drop in blood pressure
  • Anaphylactic shock: triggered by allergic reactions, leads to histamine release from basophils and mast cells, causing vasodilation
  • Septic shock: caused by bacterial toxins, such as Staphylococcal TSST-1

Cardiogenic Shock

• Caused by pump failure, where the heart cannot sustain blood flow, often due to myocardial damage from a heart attack

Stages of Shock

• Compensatory Stage:
  • Homeostasis is restored through mechanisms like baroreceptors, chemoreceptors, and response to ischemia in the medulla
  • These mechanisms trigger the sympathetic nervous system, increasing heart rate and causing general vasoconstriction (except in vessels supplying the heart or brain)
  • Decreased blood flow to the kidneys releases renin which then converts plasma angiotensinogen into angiotensin I, ACE then converts angiotensin I into angiotensin II
  • Angiotensin II causes vasoconstriction and stimulates aldosterone release from the adrenal gland and ADH release from the posterior pituitary gland, leading to increased Na+ reabsorption, water retention, and thirst to correct blood pressure

Progressive Stage

• Mechanisms are inadequate to restore homeostasis, necessitating intervention
• Cardiac output decreases and blood pressure in the cardiac circuit declines, diminishing cardiac activity
• Reduced blood flow to the brain impairs cardiovascular control and viscera become damaged due to inadequate blood flow
• Kidneys are especially vulnerable, leading to renal failure

Irreversible Stage

• Characterized by decreased cardiac output and insufficient blood supply to the heart, creating a self-perpetuating cycle that leads to death

Blood Flow

• Blood flow refers to the volume of blood moving through tissue per minute

Blood Flow Factors

• Blood flow depends on pressure gradient, which is determined by blood pressure difference between two points
• Blood typically flows from high pressure (arteries near the heart) to low pressure (arterioles)
• Resistance opposes blood flow and is caused by friction between blood and vessel walls
  • Influenced by vessel length, blood viscosity, and arteriolar radius

Arterioles

• Arterioles Are major resistance vessels containing smooth muscle
• Smooth muscle is innervated by sympathetic nervous system (SNS) and regulates arteriolar radius
  • Vasodilation increases radius and decreases resistance
  • Vasoconstriction decreases radius and increases resistance
  • Arterioles regulate blood pressure upstream and blood flow downstream

Blood Flow Regulation

• Vasoconstriction results in:
  • Decreased radius
  • Increased resistance
  • Decreased flow
  • Increased pressure in artery
  • Decreased pressure in organ
• Vasodilation results in:
  • Increased radius
  • Decreased resistance
  • Increased flow
  • Decreased pressure in artery
  • Increased pressure in organ
• Localized vasoconstriction/vasodilation does not affect systemic blood pressure
• Systemic vasoconstriction/vasodilation does

Blood Flow Control

• Changes in arteriolar radius control blood flow to organs through intrinsic and extrinsic mechanisms

Intrinsic Regulation

• Allows organs to control their own blood flow
  • Myogenic regulation: smooth muscle contracts when stretched; increased systemic blood pressure causes vasoconstriction
  • Metabolic regulation:
    • Decreased O2, increased CO2, and decreased pH cause endothelial cells to release nitric oxide (NO), leading to vasodilation
    • Increased O2, decreased CO2, and increased pH cause endothelial cells to release endothelin, leading to vasoconstriction
• These mechanisms maintain blood gas concentration within a homeostatic range, especially in the brain, heart, and skeletal muscle

Extrinsic Control

• Sympathetic Nervous System:
  • Increased SNS activity causes arteriolar vasoconstriction everywhere but the brain, also causes venoconstriction
  • Decreased SNS activity causes arteriolar vasodilation and maintains intrinsic regulation
  • The parasympathetic nervous system does not innervate arteriolar smooth muscle, except in the penis and clitoris
• Hormonal control:
  • SNS causes release of epinephrine, leading to:
    • Vasoconstriction of arterioles in skin and viscera
    • Vasodilation in heart, liver, and skeletal muscle
  • Metabolic regulation determines response
  • Other hormones include: Angiotensin II and ADH (vasoconstriction), and histamine (vasodilation)