Body Fluids and Fluid Compartments

• Body fluids can be discussed in terms of their specific fluid compartment, a location that is largely separate from another compartment by some form of a physical barrier.

• The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes.

• Extracellular fluid (ECF) surrounds all cells in the body.

• Extracellular fluid has two primary constituents: the fluid component of the blood (called plasma) and the interstitial fluid (IF) that surrounds all cells not in the blood

• Hydrostatic pressure, the force exerted by a fluid against a wall, causes movement of fluid between compartments.

• Water Balance: Plasma osmolality is thus the ratio of solutes to water in blood plasma.

• Role of ADH: Antidiuretic hormone (ADH), also known as vasopressin, controls the amount of water reabsorbed from the collecting ducts and tubules in the kidney.

Electrolyte Balance

• The body contains a large variety of ions, or electrolytes, which perform a variety of functions.

• Electrolytes in living systems include sodium, potassium, chloride, bicarbonate, calcium, phosphate, magnesium, copper, zinc, iron, manganese, molybdenum, copper, and chromium.

Sodium

• Sodium is the major cation of the extracellular fluid.

• Hyponatremia is a lower-than-normal concentration of sodium, usually associated with excess water accumulation in the body, which dilutes the sodium.

• Hypernatremia is an abnormal increase of blood sodium.

Potassium

• Potassium is the major intracellular cation.

• Hypokalemia is an abnormally low potassium blood level.

• Hyperkalemia, an elevated potassium blood level, also can impair the function of skeletal muscles, the nervous system, and the heart.

Chloride

• Chloride is the predominant extracellular anion.

• Hypochloremia, or lower-than-normal blood chloride levels, can occur because of defective renal tubular absorption.

• Bicarbonate: Bicarbonate is the second most abundant anion in the blood.

Calcium

• Hypocalcemia, or abnormally low calcium blood levels, is seen in hypoparathyroidism, which may follow the removal of the thyroid gland, because the four nodules of the parathyroid gland are embedded in it.

• Hypercalcemia, or abnormally high calcium blood levels, is seen in primary hyperparathyroidism.

Phosphate

• Hypophosphatemia, or abnormally low phosphate blood levels, occurs with heavy use of antacids, during alcohol withdrawal, and during malnourishment.

• Hyperphosphatemia, or abnormally increased levels of phosphates in the blood, occurs if there is decreased renal function or in cases of acute lymphocytic leukemia.

• Angiotensin II: Angiotensin II causes vasoconstriction and an increase in systemic blood pressure.

Acid-Base Balance

Buffer Systems in the Body

• The buffer systems in the human body are extremely efficient, and different systems work at different rates.

  • The buffer systems functioning in blood plasma include plasma proteins, phosphate, and bicarbonate and carbonic acid buffers.

• Hemoglobin as a Buffer: Hemoglobin is the principal protein inside of red blood cells and accounts for one-third of the mass of the cell.

Bicarbonate-Carbonic Acid Buffer

• The bicarbonate-carbonic acid buffer works in a fashion similar to phosphate buffers.

• The bicarbonate is regulated in the blood by sodium, as are the phosphate ions.

Respiratory Regulation of Acid-Base Balance

• The respiratory system contributes to the balance of acids and bases in the body by regulating the blood levels of carbonic acid.

• Hypercapnia, or abnormally elevated blood levels of CO2, occurs in any situation that impairs respiratory functions, including pneumonia and congestive heart failure.

• Hypocapnia, or abnormally low blood levels of CO2, occurs with any cause of hyperventilation that drives off the CO2, such as salicylate toxicity, elevated room temperatures, fever, or hysteria.

Disorders of Acid-Base Balance

• Metabolic acidosis occurs when the blood is too acidic (pH below 7.35) due to too little bicarbonate, a condition called primary bicarbonate deficiency.

• Metabolic alkalosis is the opposite of metabolic acidosis.

• Respiratory acidosis occurs when the blood is overly acidic due to an excess of carbonic acid, resulting from too much CO2 in the blood.

• Respiratory alkalosis occurs when the blood is overly alkaline due to a deficiency in carbonic acid and CO2 levels in the blood.

• Respiratory Compensation: Respiratory compensation for metabolic acidosis increases the respiratory rate to drive off CO2 and readjust the bicarbonate to carbonic acid ratio to the 20:1 level.

• Metabolic Compensation: Metabolic and renal compensation for respiratory diseases that can create acidosis revolves around the conservation of bicarbonate ions.