Blood & Body Fluid Volumes Quiz

Blood & Body Fluid Volumes

• Blood comprises a small portion of total body fluid.
• Two-thirds of body water is inside cells (intracellular compartment).
• One-third of body water is in the extracellular compartment.
• Eighty percent of the extracellular fluid is interstitial fluid (11-13 liters).
• Blood plasma volume is 3.0-3.5 liters.
• Water excretion per 24 hours is variable.
  • Kidneys: 0.6-1.5 liters
  • Lungs: 0.3-0.4 liters
  • Sweat glands (skin): 0.2-1.0 liters
  • Gastrointestinal tract: 0.1-0.2 liters.
• Water intake is 1.5-2.5 liters per day (from drinking and food).

Exchange of Fluid between Capillaries & Tissues

• Extracellular fluid (ECF) distribution between blood and interstitial compartments is dynamically balanced.
• Movement out of capillaries is driven by hydrostatic pressure against the capillary wall.

Net Filtration Pressure

• Net filtration pressure = hydrostatic pressure in capillary (17-37 mm Hg) - hydrostatic pressure of ECF (1 mm Hg)
• This pressure promotes tissue fluid formation.

Colloid Osmotic Pressure

• Movement is also influenced by colloid osmotic pressure.
• This is the osmotic pressure exerted by proteins in the fluid.
• Difference in osmotic pressure between inside and outside the capillaries (oncotic pressure) affects fluid movement.
• Plasma osmotic pressure is 25 mm Hg, interstitial osmotic pressure is 0 mm Hg

Overall Fluid Movement

• Overall fluid movement is determined by net filtration pressure and the opposing forces (Starling forces).
• Net filtration pressure = hydrostatic pressure in capillary + Colloid osmotic pressure of interstitial fluid - hydrostatic pressure in interstitial fluid - Colloid osmotic pressure of blood plasma

Edema

• Normal filtration, osmotic reuptake, and lymphatic drainage maintain proper ECF levels.
• Edema is excessive ECF accumulation resulting from:
  • High blood pressure
  • Venous obstruction
  • Plasma protein leakage into ECF
  • Myxedema (excess glycoprotein production in the extracellular matrix from hypothyroidism)
  • Low plasma protein levels (liver disease)
  • Lymphatic drainage obstruction

Regulation of Blood Volume by Kidney

• Urine formation begins with plasma filtration in the glomerulus.
• Filtrate passes through and is modified by the nephron.
• Urine volume is regulated by changes in filtrate reabsorption.
• Hormonal regulation adjusts based on body needs.

ADH (Vasopressin)

• ADH (vasopressin) is released from the posterior pituitary gland when osmoreceptors detect high osmolality.
• Causes thirst as a result of excess salt intake or dehydration.
• Stimulates water reabsorption from urine.
• The release of ADH is suppressed by low osmolality.

Aldosterone

• Aldosterone is a steroid hormone secreted by the adrenal cortex.
• Facilitates blood volume and pressure regulation by promoting salt and water reabsorption and retention.
• Release is stimulated by salt deprivation, low blood volume, and low blood pressure.

Renin-Angiotensin-Aldosterone System (RAAS)

• When there's a salt deficit, low blood volume, or low blood pressure, angiotensin II is produced.
• Angio II elevates blood pressure by:
  • Vasoconstriction
  • Stimulating aldosterone secretion
  • Increasing thirst.

Atrial Natriuretic Peptide (ANP)

• ANP is released when blood volume increases, detected by stretch receptors in the left atrium.
• Inhibits aldosterone, promoting salt and water excretion, to lower blood volume.
• Also promotes vasodilation.