Fluid, Electrolyte, and Acid-Base Balance: Study Notes for Anatomy & Physiology

스터디 가이드 - 스마트 노트

자료에 맞춘 맞춤형 노트, 핵심 정의, 예시, 맥락을 확장해 제공합니다.

Fluid, Electrolyte, and Acid-Base Balance

Overview of Body Fluid Compartments

The human body maintains homeostasis through the regulation of water, electrolytes, and acid-base balance. Understanding the distribution and composition of body fluids is essential for comprehending physiological processes.

Water Content: Water constitutes approximately 99% of the fluid volume outside cells.
Extracellular Fluid (ECF):
- Includes interstitial fluid (between cells) and plasma (in blood).
- Minor components: lymph, cerebrospinal fluid (CSF), synovial fluid, serous fluids (pleural, pericardial, peritoneal), aqueous humor, perilymph, and endolymph.
- Major ions: sodium (Na+), chloride (Cl-), and bicarbonate (HCO3-).
Intracellular Fluid (ICF):
- Consists of cytosol inside cells.
- Major ions: potassium (K+), magnesium (Mg2+), phosphate (PO43-), and negatively charged proteins.
- All cellular operations depend on water as a diffusion medium for gases, nutrients, and wastes.
Water Gains and Losses:
- Gains: Primarily from the digestive system (drinking and eating) and metabolic generation within cells.
- Losses: Mainly through the urinary system, but also via feces, perspiration, and respiration.

Electrolyte Function and Balance

Electrolytes are ions released through the dissociation of inorganic compounds. They are essential for conducting electrical currents in solution and are vital for nerve and muscle function.

Electrolyte Balance: Achieved when the gains and losses of each electrolyte are equal.
Primarily regulated by absorption in the digestive tract and loss via the kidneys.

Acid-Base Balance

The body must precisely regulate the production and loss of hydrogen ions (H+) to maintain a stable pH.

Acids are generated during normal metabolism, which can lower pH.
Kidneys: Secrete H+ into urine and generate buffers for the bloodstream.
Lungs: Regulate pH by eliminating carbon dioxide (CO2).

Hormonal Regulation of Fluid and Electrolyte Balance

Several hormones play key roles in maintaining fluid and electrolyte homeostasis:

Antidiuretic Hormone (ADH):
- Stimulates water conservation at the kidneys, reducing urinary water loss and concentrating urine.
- Activates the hypothalamic thirst center to promote fluid intake.
Aldosterone:
- Secreted by the adrenal cortex in response to rising K+ or falling Na+ levels.
- Part of the renin-angiotensin-aldosterone system.
- Increases Na+ reabsorption and K+ loss in the kidneys.
- High levels cause kidneys to conserve Na+ and retain water ("water follows salt").
Natriuretic Peptides (ANP & BNP):
- Released by cardiac muscle cells in response to stretching of heart walls.
- Reduce thirst, block ADH and aldosterone release, promote diuresis, and lower blood pressure and plasma volume.

Fluid Balance: Gains and Losses

Maintaining fluid balance is crucial for homeostasis. The body must match water intake with water loss.

Daily Water Gains (approx. 2500 mL):
- Drinking: 1200 mL
- Eating: 1000 mL
- Metabolic generation: 300 mL (from oxidative phosphorylation in mitochondria)
Daily Water Losses (approx. 2500 mL):
- Urine
- Feces
- Insensible perspiration (evaporation from skin and lungs)
- Sensible perspiration (sweat), which can be significant during activity (up to 4 L/hr)
- Fever increases water loss

Disorders of Fluid Balance

Several conditions can disrupt normal fluid distribution and balance:

Edema: Abnormal accumulation of fluid in interstitial spaces, causing swelling.
Lymphedema: Edema due to blockage of lymphatic drainage.
Dehydration: Occurs when water losses exceed gains.
- If electrolytes are retained, ECF osmotic concentration rises, causing water to move from ICF to ECF.
- Severe water loss may result from excessive sweating, inadequate intake, vomiting, or diarrhea.
- Homeostatic responses include increased ADH and renin secretion, and increased fluid intake.
Hyperhydration (Water Excess):
- Occurs when water is gained without electrolytes.
- ECF becomes hypotonic, causing water to shift into cells (ICF), which can distort cells and disrupt function.
- Signs include hyponatremia (low Na+), and CNS effects (water intoxication).

Main Ions and Their Functions

Electrolyte concentrations directly affect water balance and cell function. Imbalances can have significant physiological consequences.

Ion	Main Location	Functions	Disorders
Sodium (Na+)	ECF (dominant cation)	Major contributor to ECF osmotic concentration Essential for nerve impulse transmission	Hyponatremia (low Na+, often from hyperhydration) Hypernatremia (high Na+, often from dehydration)
Potassium (K+)	ICF (dominant cation)	Maintains resting membrane potential Required for muscle and nerve function	Hypokalemia (low K+) Hyperkalemia (high K+)
Calcium (Ca2+)	99% in skeleton	Muscle contraction Neural activity Blood clotting Cofactor for enzymes Second messenger	Hypercalcemia (high Ca2+, often from hyperparathyroidism) Hypocalcemia (low Ca2+, from hypoparathyroidism, vitamin D deficiency, or renal failure)

Additional info: Most electrolyte imbalances involve sodium; potassium imbalances are less common but more dangerous.

Acid-Base Disorders and Key Terms

The body must maintain blood pH within a narrow range (7.35–7.45) for proper physiological function.

Acidosis: Physiological state with abnormally low blood pH (< 7.35). Acidemia refers to the actual low pH condition.
Alkalosis: Physiological state with abnormally high blood pH (> 7.45). Alkalemia refers to the actual high pH condition.
CO2 and pH:
- Most CO2 in solution forms carbonic acid, which dissociates into H+ and HCO3-.
- The partial pressure of CO2 (PCO2) is the most important factor affecting blood pH.
Respiratory Acidosis:
- Occurs when the respiratory system cannot eliminate all CO2 produced by tissues.
- Primary sign: low blood pH due to hypercapnia (high PCO2).
- Primary cause: hypoventilation (abnormally low respiratory rate).

Key Equations

Formation of Carbonic Acid:
Normal Blood pH Range: