Human Physiology Week 5 - Study Guide

Fluid Compartments

• Intracellular Fluid (ICF): Holds the most fluid in the body
• Extracellular Fluid (ECF):
  • Vascular Compartment: Contains plasma, the third largest fluid compartment
    • Plasma Composition:
      • 99% water with dissolved electrolytes
      • 1% proteins:
        • Albumin (most abundant)
        • Globulins (including immunoglobulins like IgG)
        • Fibrinogen (for blood clotting)
  • Interstitial Fluid (ISF): Second largest fluid compartment, between cells
    • Contains water and dissolved electrolytes
    • Exists as a gel due to interactions with structural molecules like collagen and proteoglycans
    • Typically contains no proteins
• Interstitium: The space between cells
• Interstitial Fluid: The fluid within the interstitium
• Tissue Gel: A combination of proteoglycans and trapped fluid
• Role of Proteoglycans:
  • Provide structure to the interstitium along with collagen.
  • Interact with water to form a gel, slowing water diffusion
• Free Fluid Concentration:
  • Normal tissue: Most fluid is bound within the tissue gel.
  • Edematous Tissue: Moderate to severe edema leads to free-flowing fluid not trapped between proteoglycans.

Filtration and Reabsorption

• Between Vascular and Interstitial Compartments:
  • Water and electrolytes filter out of capillaries into interstitial fluid.
  • 90% of filtered fluid is reabsorbed back into capillaries.
  • Remaining 10% enters lymphatic vessels.
  • Starling's forces determine this movement
  • Electrolytes do not significantly influence fluid movement between compartments.
• Between Interstitial and Intracellular Compartments:
  • Fluid movement occurs through channels and transporters.
  • Electrolyte concentration influences fluid movement through concentration gradients.
  • Water movement is dependent on cell osmolarity.

Fluid Filtration

• Plasma Proteins: Contribute to maintaining blood volume via osmotic pressure (oncotic pressure).
  • Colloid osmotic pressure is specifically due to plasma proteins.
  • Plasma proteins do not diffuse through capillaries, and exert a "pull" on fluid from the interstitium back into the plasma.
  • Albumin is responsible for ~80% of colloid osmotic pressure.
  • Liver dysfunction can affect albumin production and influence fluid movement.
• Starling's Forces:
  • Capillary hydrostatic pressure: Generally outward (promotes filtration)
    • Think of this as "mechanical pressure"
  • Interstitial fluid pressure: Generally inward (promotes absorption)
    • Think of this as the pressure of interstitial fluid pushing against the capillary
  • Plasma colloid osmotic pressure: Generally inward (promotes absorption)
    • Think of this as a "magnet" pulling fluid back into the capillary
  • Interstitial fluid colloid osmotic pressure: Usually negligible
    • Outward pressure (promotes filtration) if interstitial fluid contains proteins.
    • In that case, can be thought of as a "magnet" pulling fluid out of the capillary.
• Filtration vs Reabsorption:
  • Arterial End (Proximal End): Net filtration (fluid out of the capillary) due to high capillary hydrostatic pressure.
  • Venous End (Distal End): Net reabsorption (fluid into the capillary)due to increased plasma osmotic pressure and higher interstitial pressure.
  • Higher arterial pressure: Increases capillary hydrostatic pressure, promoting filtration.
  • Increased venous pressure: Reduces reabsorption causing increased filtration.
• Lymphatic System:
  • Under normal conditions, ~90% of filtered fluid is reabsorbed.
  • ~10% of filtered fluid enters lymphatic vessels.

Lymphatic System

• Lymph Flow: Greater interstitial fluid volume increases interstitial pressure, leading to increased lymph flow unless pressure exceeds ~2mmHg.
  • Edema occurs if lymphatic vessels cannot keep up with fluid removal.
• Lymphatic Pumping Mechanisms:
  • Contraction of lymph vessel walls: Smooth muscle contracts due to vessel stretch.
  • Rhythmic Compression:
    • Skeletal muscle contraction.
    • Movement.
    • Arterial pulsation.
    • External compression.

Edema

• Extracellular Edema: Accumulation of ISF.
  • Causes:
    • Increased fluid filtration: Anything increasing capillary permeability.
      • Histamine (e.g., allergic reaction)
      • Inflammatory responses.
      • Capillary damage (e.g., burns)
    • Decreased fluid reabsorption:
      • Increased venous pressure: Venous obstruction, heart failure, tourniquets.
      • Reduced lymphatic flow: Tumors, surgery, parasitic infections.
• Pitting Edema: Significant free flowing fluid in the interstitium.
  • Pressing on the edema causes an indentation that remains due to fluid displacement.
• Non-Pitting Edema: Fluid is still trapped within the tissue gel.

Vascular Function

• Arteries vs Veins:
  • Arteries and arterioles have thicker smooth muscle layers than veins and venules.
  • Veins are more distensible, expanding more for a given pressure.
  • Veins serve as reservoirs for blood: Veins hold more blood than arteries.

Volume and Pressure Relationship

• Arterial System: Small changes in volume significantly affect pressure due to lower distensibility.
• Venous System: Large changes in volume have little effect on pressure due to high distensibility.

Aortic Distensibility

• During Systole: Aorta fills with blood and distends.
• During Diastole: Aorta recoils, creating inward pressure on blood.
  • This pressure forces blood towards areas of lower pressure.
  • Aortic valve prevents backflow to the heart.
  • Elastic recoil of the aorta helps propel blood throughout the system.
  • Ensures blood flow throughout the body even during diastole.

Pulse Pressure

• Pulse Pressure: Difference between systolic and diastolic arterial pressure.
  • Bounding Pulse: High pulse pressure.
    • Could be due to high systolic, high diastolic, or low diastolic pressure.
  • Thready Pulse: Low pulse pressure.
    • Clinically, most likely due to low systolic pressure.
• Major factors influencing pulse pressure:
  • Stroke Volume: Greater SV leads to greater pressure rise and fall.
  • Arterial Compliance: Noncompliance increases pressure.
    • Sympathetic nervous system activation can cause vasoconstriction, reducing compliance.
• Mean Arterial Pressure (MAP):
  • MAP is not the average of systolic and diastolic pressure at rest, because more time is spent in diastole.
  • During exercise, MAP changes due to reduced time in diastole.

Right Atrial Pressure

• Normal right atrial pressure is approximately 0-5mmHg.
• This pressure reflects the ability of the right ventricle to pump blood out of the heart.

Explore the different fluid compartments in the human body, including intracellular fluid (ICF) and extracellular fluid (ECF). This quiz covers the composition and significance of vascular and interstitial fluids, as well as the role of proteoglycans in tissue structure. Test your understanding of how these components interact and function within biological systems.