Fluid, Electrolyte, and Acid-Base Homeostasis Practice Flashcards

Total Body Fluid Distribution and Homeostasis

Body fluid distribution varies based on gender and is categorized into distinct compartments within the human body. Homeostasis focuses on maintaining these fluid levels through rigorous biological mechanisms.

• Total Body Fluid (By Gender):

  • Females: Approximately $55\%$ of the total body fluid is found in females. This lower percentage compared to males is generally due to a higher proportion of adipose tissue, which contains less water than muscle.
  • Males: Approximately $60\%$ of the total body fluid is found in males, reflecting a higher lean muscle mass.

• Fluid Compartments:

  • Intracellular Fluid (ICF): This fluid exists inside the cells and accounts for $2/3$ of the total body fluid.
  • Extracellular Fluid (ECF): This fluid exists outside the cells and accounts for $1/3$ of the total body fluid.

• Subdivisions of Extracellular Fluid (ECF):

  • Interstitial Fluid: This component makes up approximately $80\%$ of the extracellular fluid. It is the fluid found in the spaces between cells.
  • Blood Plasma: This component makes up approximately $20\%$ of the extracellular fluid. It is the liquid portion of the blood.

Mechanisms of Fluid Gain, Loss, and Hormonal Control

Water balance is maintained by equalizing the amount of water gained with the amount of water lost, a process regulated by the central nervous system and various hormones.

• Sources of Water Gain:

  • Includes ingested liquid, food, and metabolic water (water produced as a byproduct of cellular metabolism).

• Sources of Water Loss:

  • Fluid is primarily lost through urine, sweat, the exhalation of water (vapor) from the lungs, and through the gastrointestinal (GI) tract.

• Regulation of Intake and Output:

  • Hypothalamus: This region of the brain serves as the location of the thirst center, which initiates the urge to drink when body fluids are low or osmolarity is high.
  • Antidiuretic Hormone (ADH): This hormone reduces water loss in the urine. It works by causing the collecting ducts in the kidneys to become more permeable to water, allowing for increased water reabsorption into the bloodstream.
  • Aldosterone: This hormone reduces water loss in the urine by increasing sodium reabsorption in the kidneys. As sodium is reabsorbed, water follows osmotically back into the blood.
  • Atrial Natriuretic Peptide (ANP): This hormone increases the excretion of sodium into the urine. This process promotes water loss (diuresis) to lower blood volume and pressure.

Electrolyte Distribution and Regulation in Body Fluids

Electrolytes are crucial for maintaining osmotic balance and nerve/muscle function. Their concentrations are specific to whether they reside in extracellular or intracellular compartments.

• Sodium ($Na^+$):

  • Sodium is the most abundant cation in the extracellular fluid.
  • Its levels are regulated by the hormones aldosterone and atrial natriuretic peptide (ANP).

• Potassium ($K^+$):

  • Potassium is the most abundant cation in the intracellular fluid.
  • Blood levels of potassium are controlled by aldosterone, which promotes its secretion into the urine in exchange for sodium.

• Calcium ($Ca^{2+}$):

  • Calcium is the most abundant mineral in the entire human body.
  • Blood levels of calcium are tightly regulated by parathyroid hormone (PTH) and calcitonin.

• Chloride ($Cl^-$):

  • Chloride is the most abundant anion in the extracellular fluid.

Acid-Base Homeostasis and Clinical Imbalances

The body maintains a strict blood pH range between $7.35$ and $7.45$. Deviations from this range result in acidosis or alkalosis, categorized as either respiratory or metabolic in origin.

• Respiratory Acidosis:

  • Clinical Presentation: Increased $pCO_2$ (above $45\,mmHg$) and a decreased pH (below $7.35$).
  • Common Causes: Chronic Obstructive Pulmonary Disease (COPD), Pulmonary edema, and asthma.

• Respiratory Alkalosis:

  • Clinical Presentation: Decreased $pCO_2$ (below $35\,mmHg$) and an increased pH (above $7.45$).
  • Common Causes: Hyperventilation resulting from stress or pain.

• Metabolic Acidosis:

  • Clinical Presentation: Decreased bicarbonate ($HCO_3^-$) levels and a decreased pH (below $7.35$).
  • Common Causes: Persistent diarrhea, excessive alcohol consumption, and kidney failure.

• Metabolic Alkalosis:

  • Clinical Presentation: Increased bicarbonate ($HCO_3^-$) levels and an increased pH (above $7.45$).
  • Common Causes: Persistent vomiting (loss of gastric acid) and excessive intake of antacids.