Body Fluid Compartments and Acid-Base Balance

Intracellular fluid (ICF)

Fluid within cells; about 40% of body weight and ~25 L.

Extracellular fluid (ECF)

Fluid outside cells; about 20% of body weight and ~15 L.

Interstitial fluid (IF)

The fluid between cells; makes up 80% of ECF (~12 L).

Plasma

The fluid portion of blood; accounts for 20% of ECF (~3 L).

Solutes

Substances dissolved in water; includes electrolytes and nonelectrolytes.

Electrolytes

Chemicals that dissociate into ions in water (e.g., Na+, K+, Cl-); conduct electricity.

Nonelectrolytes

Substances that do not dissociate in water (e.g., glucose, lipids).

Osmosis

Movement of water across a membrane from low solute concentration to high.

Osmotic pressure

Force exerted by solutes to pull water across membranes.

Hydrostatic pressure

The force of fluid pushing against a membrane or vessel wall.

Water balance

The balance between water intake and output; ~2500 mL/day.

Thirst mechanism

Controlled by hypothalamic osmoreceptors in response to ECF osmolality.

Antidiuretic hormone (ADH)

Hormone that increases water reabsorption in kidney collecting ducts.

Obligatory water loss

Unavoidable loss of water via skin, lungs, feces, and urine.

Dehydration

Condition when water loss exceeds intake; cells shrink.

Hypotonic hydration

Excessive water intake; causes cells to swell.

Sodium balance

Regulated mainly by aldosterone, angiotensin II, and ANP.

Aldosterone

Hormone from adrenal cortex; promotes Na+ and water reabsorption in kidneys.

Angiotensin II

Formed in response to low BP; causes vasoconstriction and aldosterone release.

Atrial natriuretic peptide (ANP)

Hormone from atria; inhibits renin, aldosterone, and ADH to reduce BP.

Renin

Enzyme from kidneys that starts the RAAS cascade; promotes angiotensin II formation.

RAAS (Renin-Angiotensin-Aldosterone System)

Hormonal system regulating blood pressure and fluid balance.

Potassium balance

Regulated by aldosterone in distal convoluted tubules and collecting ducts.

Chemical buffer system

Systems that resist pH changes by binding/releasing H+; includes bicarbonate, phosphate, protein buffers.

Bicarbonate buffer system

Primary buffer in extracellular fluid.

Protein buffer system

Major buffer inside cells and in plasma; proteins are amphoteric.

Amphoteric

A substance that can act as both an acid and a base (e.g., proteins).

Respiratory regulation of pH

Alters breathing rate to change CO2 levels and shift H+ concentration.

Hyperventilation

Excess CO2 loss → respiratory alkalosis.

Hypoventilation

CO2 retention → respiratory acidosis.

Renal regulation of pH

Kidneys reabsorb or secrete H+ and HCO3- to regulate blood pH.

Metabolic acidosis

Low blood pH and low HCO3- from causes like diarrhea, renal failure.

Metabolic alkalosis

High blood pH and high HCO3- from vomiting, excess antacids.

Respiratory acidosis

Elevated PCO2 (>45 mm Hg); due to impaired ventilation.

Respiratory alkalosis

Low PCO2 (<35 mm Hg); due to hyperventilation.

Body water percentage in infants

73%

Body water percentage in adult males

60%

Body water percentage in adult females

~50%

Total body water volume and body weight percentage

40 L, 60% of body weight

Intracellular fluid (ICF) volume and body weight percentage

25 L, 40% of body weight

Interstitial fluid (IF) volume and percentage of ECF

12 L, 80% of ECF

Extracellular fluid (ECF) volume and body weight percentage

15 L, 20% of body weight

Plasma volume and ECF percentage

3 L, 20% of ECF

Types of solutes in body fluids

Electrolytes and nonelectrolytes

Forces driving fluid movement among compartments

Osmotic and hydrostatic pressure

Average water intake/output per day

~2500 mL/day

What triggers thirst?

Hypothalamic osmoreceptors

Types of water output

Obligatory (lungs, skin), sensible (urine, sweat, feces)

Hormone regulating water output

Antidiuretic hormone (ADH)

Consequences of water imbalance

Dehydration (cells shrink) and hypotonic hydration (cells swell)

Hormone regulating sodium balance

Aldosterone

Hormones influencing electrolyte balance besides aldosterone

Estrogen, Progesterone, Glucocorticoids, ANP

Triggers release of renin

Stretch in afferent arterioles, NaCl concentration in nephron loop, sympathetic stimulation

Effect of angiotensin II

Vasoconstriction, aldosterone release

Hormone released by adrenal cortex in response to angiotensin II

Aldosterone

Role of ADH

Increases water reabsorption in the collecting ducts

Mechanisms regulating acid-base balance

Chemical buffers, respiratory centers, renal mechanisms

Major chemical buffer systems

Bicarbonate, phosphate, protein buffer systems

Amphoteric molecules

Molecules that can act as both acids and bases

How does the respiratory system regulate H+?

By adjusting CO2 levels (hyperventilation/hypoventilation)

Renal response to acid-base imbalance

Conserve, generate, or excrete bicarbonate; excrete or retain H+

Types of acid-base imbalances

Respiratory acidosis/alkalosis and metabolic acidosis/alkalosis