Physiology Week 5

Pulse Pressure

• Pulse pressure is the difference between systolic and diastolic blood pressure (SBP - DBP)
• A high pulse pressure (bounding pulse) can indicate a large difference between systolic and diastolic pressure, but doesn't specify the reason.
  • Examples include a healthy exercise blood pressure, hypertension, or a normal systolic pressure with a low diastolic pressure.
• A low pulse pressure (thready pulse) is often due to a low systolic pressure. It also doesn't indicate the reason, similar to a bounding pulse.

Factors Affecting Arterial Pulse Pressure

• Stroke volume: Higher stroke volume leads to greater pressure fluctuations.
  • Stroke volume is influenced by cardiac contractility and afterload.
• Compliance of the arterial tree: Low compliance (stiffness) increases arterial pressure.

Fluid Movement Between Compartments

• Vascular and Interstitial: Fluid (water and electrolytes) is filtered from capillaries into the interstitial fluid and reabsorbed back into the capillaries.
  • About 90% of the filtered fluid is reabsorbed, with the remaining 10% returning via lymphatic vessels.
  • Starling's forces govern this movement. Electrolytes don't significantly influence fluid movement between these compartments.
• Interstitial and Intracellular: Fluid and electrolytes move across interstitial fluid and cell membranes via channels and transporters.
  • Electrolyte concentration gradient influences the direction of movement.
  • Water movement is dependent on cell osmolarity.

Fluid Filtration

• Plasma Proteins: Plasma proteins contribute to osmotic (oncotic) pressure.
  • They don't diffuse through capillaries, creating a "pulling" force that draws fluid from the interstitium back into the plasma.
  • Albumin accounts for 80% of colloid osmotic pressure.
• Starling Forces: Four forces govern fluid movement:
  • Capillary hydrostatic pressure (pushes fluid out)
  • Interstitial hydrostatic pressure (pushes fluid in)
  • Capillary osmotic pressure (pulls fluid in)
  • Interstitial osmotic pressure (pulls fluid out)
• Capillary Hydrostatic Pressure and Filtration: Increased arterial pressure leads to increased capillary hydrostatic pressure and greater filtration.
• Venous Pressure and Reabsorption: Increased venous pressure reduces fluid reabsorption and increases net filtration.
• Lymphatic Circulation: Under normal conditions, approximately 90% of filtered fluid is reabsorbed at the venous end of the capillaries, with the remaining 10% entering the lymphatic system for circulation.

Lymphatic System

• Lymph Flow: Increased interstitial fluid volume raises interstitial pressure, leading to higher lymph flow until the lymphatic system reaches its maximum capacity.
  • Excess fluid accumulation results in edema.
• Lymphatic Pumping: Lymphatic pumping mechanisms include:
  • Contraction of lymph vessel walls (smooth muscle)
  • Rhythmic compression of lymphatic vessels (skeletal muscle contraction, movement, arterial pulsation, external compression)

Edema

• Causes: Extracellular edema can be caused by:
  • Increased fluid filtration: Increased vascular permeability due to factors like histamine release, inflammation, or capillary damage.
  • Decreased fluid reabsorption: Increased venous resistance (blood clots, heart failure, external compression) or slowed/stopped lymphatic flow (tumor blockage, surgery, parasitic infections).
• Pitting vs Non-pitting Edema:
  • Pitting edema: Excess interstitial fluid accumulates, causing the gel-like structure of the interstitium to transform into free fluid.
  • Non-pitting edema: Fluid remains bound to the interstitial matrix, resulting in a solid-like consistency.

Vascular Function

• Distensibility Differences: Arteries/arterioles have thicker smooth muscle layers than veins/venules.
  • This makes veins more distensible than arteries, allowing them to expand more for a given pressure.
  • Veins serve as blood reservoirs.
• Volume and Pressure Relationship:
  • Arterial system: Small changes in volume significantly affect pressure.
  • Venous system: Large changes in volume have a minimal effect on pressure.
• Aortic Distensibility and Circulation: The aorta stretches during systole to accommodate blood.
  • During diastole, the aorta recoils, creating an inward pressure that drives blood forward throughout the systemic circulation.
  • The aortic valve prevents blood from backflow into the heart.