Anatomy and Physiology - Chapter 21

Fluid Movement

Water moves out of cells when concentration of electrolytes in tissue fluid rises. Water moves into cells when concentration of electrolytes in tissue fluid falls.

Fluid Flow Determinants

The amount and direction of fluid that flows between body compartments is determined by the concentration of solutes.

Balancing Gains and Losses

Amount of water gained and lost during one day should be equal.

Regulation of Intake and Output

Water loss varies with activity and temperature. To maintain balance, the body uses mechanisms to adjust intake and output.

Mechanisms to Increase Intake

When total body water declines: 1. Physical changes stimulate the thirst center in the hypothalamus. 2. Salivation decreases, causing dry mouth and thirst. 3. Water consumption occurs.

Mechanisms to Decrease Output

At the same time: 1. Physical changes stimulate the hypothalamus. 2. This stimulates the posterior pituitary to secrete antidiuretic hormone (A D H). 3. A D H prompts the kidneys to reabsorb water and produce less urine. 4. Fluid loss slows until water is ingested.

Effect of Antidiuretic Hormone (ADH)

ADH prompts the kidneys to reabsorb more water and produce less urine.

Disorders of Water Balance

Can result from an abnormality in fluid volume, fluid concentration, or distribution of fluid between compartments.

Fluid Deficiency

Volume depletion results from blood loss or the loss of both water and sodium. Dehydration results when the body eliminates more water than sodium.

Fluid Excess

Kidneys usually compensate by producing more urine. A specific type includes water intoxication.

Fluid Accumulation

Involves the accumulation of fluid between compartments.

Edema

Fluid accumulation in interstitial spaces, causing tissue swelling.

Electrolyte Balance

Are substances that break up into electrically charged particles called ions when dissolved in water. A balance is crucial for proper body functioning.

Major Cations

N a+, K+, C a+, H+.

Major Anions

C l−, H C O3−, Pi.

Sodium

Main electrolyte in extracellular fluid. Determines the volume of total body water. Influences how body water is distributed. Plays a key role in depolarization.

Sodium Regulation (Low)

water level too high; Aldosterone prompts renal tubules to reabsorb Na+. Antidiuretic hormone (ADH) is suppressed → kidneys secrete water

Sodium Regulation (High)

Water level too low; Antidiuretic hormone (ADH) causes kidneys to reabsorb water. ADH stimulates thirst

Hypernatremia

Indicates greater sodium than water; usually self-corrects by triggering thirst.

Hyponatremia

Results from excess body water lower than normal ; usually corrected by excretion of excess water.

Potassium

Chief cation of intracellular fluid; works hand-in-hand with sodium; crucial for proper nerve and muscle function.

Hypokalemia

Low levels of Potassium; may result from diuretics, vomiting, or chronic diarrhea; makes cells less excitable.

Hyperkalemia

High levels of Potassium; makes nerve and muscle cells irritable; imbalances can cause life-threatening cardiac arrhythmias.

Hypercalcemia

High levels of Calcium; may result from hyperparathyroidism, hypothyroidism, alkalosis; inhibits depolarization.

Hypocalcemia

Low levels of Calcium; may result from hypoparathyroidism, hyperthyroidism, acidosis, diarrhea; increases excitation of nerves and muscles.

Main cation of extracellular fluid

Sodium

pH of blood

Ranges from 7.35 to 7.45

Chemical Buffers

Include bicarbonate, phosphate, and protein buffer systems; use weak base to bind H+ ions and weak acid to release them.

Physiological Buffers

Include respiratory and urinary systems; lungs expel CO2 to lower pH; kidneys expel H+ ions to lower pH.

Respiratory Control of pH

Central chemoreceptors in the brainstem detect a decline in pH from an accumulation of CO2; they signal the respiratory centers to increase the rate and depth of breathing.

Renal Control of pH

Kidneys expel H+ ions and reabsorb bicarbonate; this is the most powerful buffer system and is also the slowest to respond.

Compensation for Acid-Base Imbalances

Respiratory system responds to metabolic disturbances by adjusting ventilation; renal system responds by adjusting the rate of H+ ion excretion.

Acid-Base Imbalances

Respiratory imbalances result from an excess or deficiency of CO2; metabolic imbalances result from an excess or deficiency of bicarbonate.

Electrolyte disturbance from acidosis

Hyperkalemia

First reaction when pH rises above normal

Hydrogen would bind with bicarbonate.