Chapter 25: Fluids, Electrolytes & Acid–Base Balance – Exam Review

Sixty question-and-answer flashcards covering body-fluid distribution, hormonal water balance, electrolytes, and respiratory acid–base disturbances for Chapter 25 exam preparation.

What four main factors determine total body water content?

Age, sex, percentage of body fat, and overall muscle mass.

Why do adult males usually have a higher body-water percentage than females?

Males have more skeletal muscle (water-rich) and less adipose tissue (water-poor) than females.

How does body-water percentage change from infancy to old age?

It is highest in newborns (~75%), decreases to about 60% in adult males and 50% in adult females, and falls to ~45% in the elderly.

Roughly what fraction of total body water is intracellular fluid (ICF)?

About two-thirds (≈25 L in the average adult).

What are the two subdivisions of extracellular fluid (ECF)?

Plasma and interstitial fluid.

Which ion is the chief cation of ECF?

Sodium (Na⁺).

Which ion is the chief anion of ECF?

Chloride (Cl⁻).

Which ion is the chief cation of ICF?

Potassium (K⁺).

Name the predominant anions inside cells.

Phosphate (HPO₄²⁻) and negatively charged proteins.

Why do electrolytes exert greater osmotic pressure than nonelectrolytes?

Electrolytes dissociate into multiple ions, effectively multiplying the number of solute particles in solution.

What two pressures primarily govern fluid exchange between plasma and interstitial fluid?

Hydrostatic pressure and colloid osmotic pressure.

What drives water movement between the ICF and ECF across cell membranes?

Osmotic gradients created by solute (mainly electrolyte) concentrations on the two sides.

List three major routes of water intake.

Liquids we drink, water in foods, and metabolic (oxidation) water.

List four major routes of water output.

Urine, insensible losses via lungs/skin, perspiration (sweat), and feces.

What triggers the hypothalamic thirst center?

Increased plasma osmolarity, dry mouth, decreased blood volume/pressure, and angiotensin II.

Differentiate obligatory and facultative water losses.

Obligatory losses are unavoidable (insensible losses + minimum urine needed to excrete wastes); facultative losses are urine water reabsorption controlled by hormones, mainly ADH.

Give two common causes of dehydration.

Excessive fluid loss (e.g., hemorrhage, severe burns, vomiting) or inadequate fluid intake.

State two serious consequences of untreated dehydration.

Cell shrinkage leading to neurological impairment and hypovolemic shock from low blood volume.

What is hypotonic hydration (water intoxication)?

Excess water dilutes ECF sodium (hyponatremia), causing water to move into cells and potentially produce cerebral edema.

How are fluid intake, blood volume, blood pressure, and blood osmolarity interrelated?

Increased intake raises plasma volume → ↑ blood pressure and ↓ blood osmolarity; the opposite occurs with fluid loss.

Outline the conversion pathway from angiotensinogen to angiotensin II.

Renin converts angiotensinogen to angiotensin I; angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II.

Where and how is low blood pressure detected to begin the renin–angiotensin pathway?

Juxtaglomerular (granular) cells in kidney afferent arterioles sense low stretch/pressure or receive sympathetic stimulation.

What effect does angiotensin II have on systemic arterioles?

It causes widespread vasoconstriction, raising blood pressure.

Which hormone inserts aquaporin-2 channels in collecting-duct cells?

Antidiuretic hormone (ADH).

Describe urine when ADH levels rise.

Low in volume, highly concentrated, and dark yellow.

Describe urine when ADH levels fall.

High in volume, very dilute, and pale/clear.

Where is aldosterone produced, and what triggers its release?

Zona glomerulosa of the adrenal cortex; stimulated by angiotensin II, elevated plasma K⁺, or ACTH (minor).

Describe urine characteristics with high aldosterone.

Lower volume with greater Na⁺ and water reabsorption, relatively concentrated and darker.

Describe urine characteristics with low aldosterone.

Higher volume, more Na⁺ lost, relatively dilute and lighter in color.

Where is atrial natriuretic peptide (ANP) released from?

Cardiac atrial myocytes when they are stretched by increased blood volume.

What is the net renal effect of ANP?

Increases excretion of Na⁺ and water, thereby lowering blood volume and pressure.

Describe urine when ANP levels are high.

Very large volume, dilute, and pale.

Describe urine when ANP levels are low.

Lower volume and somewhat more concentrated.

Differentiate fixed acids and volatile acids.

Fixed acids (non-volatile) cannot be exhaled and must be buffered/excreted by kidneys; volatile acids (carbonic acid from CO₂) can be removed by the lungs.

List three metabolic sources of fixed acids.

Lactic acid from anaerobic metabolism, ketoacids from fat metabolism, phosphoric acid from nucleic acid/protein catabolism.

How do kidneys respond to an excess of fixed acids?

They secrete H⁺ into the filtrate and generate/reabsorb new HCO₃⁻ to raise blood pH.

What is the source of the main volatile acid in the body?

Carbon dioxide produced by cellular respiration, which forms carbonic acid in blood.

How does the respiratory system regulate volatile acid levels?

By altering ventilation rate to change CO₂ elimination—hyperventilation lowers, hypoventilation raises arterial CO₂.

Distinguish the physiological buffering systems from chemical buffers.

Physiological buffers (lungs and kidneys) remove acids/bases from the body; chemical buffers (bicarbonate, phosphate, protein systems) temporarily bind or release H⁺.

Write the bicarbonate buffer equilibrium equation.

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻.

What primary disturbance causes respiratory acidosis?

Hypoventilation or impaired gas exchange, leading to CO₂ accumulation.

What primary disturbance causes respiratory alkalosis?

Hyperventilation, which removes CO₂ faster than it is produced.

Predict pH, PCO₂, and HCO₃⁻ levels in respiratory acidosis before renal compensation.

Low pH, high PCO₂, and normal or slightly increased HCO₃⁻.

Predict pH, PCO₂, and HCO₃⁻ levels in respiratory alkalosis before renal compensation.

High pH, low PCO₂, and normal or slightly decreased HCO₃⁻.

What average percentage of body weight is water in an adult male?

Approximately 60%.

What average percentage of body weight is water in an adult female?

Approximately 50%.

Give the approximate body-water percentage in a newborn.

About 75% of body weight.

Give the approximate body-water percentage in a typical elderly individual.

Around 45% of body weight.

Define metabolic water.

Water produced during aerobic cellular respiration and other metabolic reactions.

Name two sites of insensible water loss.

Skin (evaporation) and lungs (humidified air).

Where does facultative water reabsorption occur?

In the collecting ducts, under the control of ADH.

Which hormone increases Na⁺ reabsorption and K⁺ secretion in the distal nephron?

Aldosterone.

What stimulates renin release besides low BP?

Sympathetic nervous activity and decreased NaCl sensed by macula densa cells.

Explain why glucose is considered a nonelectrolyte.

It dissolves in water but does not dissociate into ions.

What pathophysiological condition exemplifies dehydration caused by endocrine dysfunction?

Diabetes insipidus (lack of ADH action).

Which aquaporin channel is regulated by ADH in principal cells?

Aquaporin-2 (AQP2).

What is a life-threatening consequence of severe hyponatremia during hypotonic hydration?

Cerebral edema leading to seizures, coma, and possible death.

Explain how ANP counters the actions of the renin–angiotensin–aldosterone system.

ANP inhibits renin, aldosterone, and ADH release, dilates afferent arterioles, and increases GFR, promoting natriuresis and diuresis.

What minimum daily urine output is required for obligatory water loss?

About 500 mL to excrete metabolic wastes.

How does aldosterone ultimately affect blood pressure?

By increasing Na⁺ and water reabsorption, it expands plasma volume and raises blood pressure.

Which hormone’s overproduction would most likely produce very concentrated, low-volume urine and elevated blood pressure?

Antidiuretic hormone (ADH).

Which two buffers provide the majority of chemical buffering in the ICF?

Protein buffer system and phosphate buffer system.

How do kidneys compensate for respiratory acidosis?

They increase H⁺ secretion and reabsorb/generate more HCO₃⁻ to raise pH.