Fluid, Electrolyte, and Acid-Base Balance; The Respiratory and Urinary Systems

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Fluid, Electrolyte, and Acid-Base Balance

Overview of Body Fluid Compartments

The human body contains several fluid compartments, each with distinct water and electrolyte content. Understanding these compartments is essential for grasping fluid and electrolyte balance.

Intracellular fluid (ICF): Fluid within cells; accounts for about two-thirds of total body water.
Extracellular fluid (ECF): Fluid outside cells, including interstitial fluid, plasma, and transcellular fluids.
Electrolyte content: Sodium is the major cation in ECF; potassium is the major cation in ICF.
Water content: Varies by compartment and is regulated by intake and output.

Movement of Fluids Between Compartments

Fluid movement is governed by osmotic and hydrostatic pressures.

Osmotic pressure: The force exerted by solutes drawing water across a membrane.
Hydrostatic pressure: The force exerted by fluid pressing against a wall, such as blood pressure in capillaries.
Equation: (Starling equation for capillary exchange)

Electrolytes vs. Non-Electrolytes

Electrolytes are substances that dissociate into ions in solution, while non-electrolytes do not.

Electrolytes: Sodium, potassium, calcium, chloride, etc.
Non-electrolytes: Glucose, urea, etc.

Regulation of Fluid Intake and Output

Fluid balance is maintained by matching intake (drinking, food) with output (urine, sweat, feces).

Thirst mechanism: Controlled by the hypothalamus, triggered by increased plasma osmolality or decreased blood volume.

Hormonal Regulation of Fluid and Electrolyte Balance

Several hormones regulate fluid and electrolyte balance:

Antidiuretic hormone (ADH): Promotes water reabsorption in kidneys.
Renin-Angiotensin-Aldosterone System (RAAS): Regulates sodium and water balance, blood pressure, and blood volume.
Aldosterone: Increases sodium reabsorption and potassium excretion.

Major Electrolytes and Their Regulation

Sodium (Na+): Regulated by aldosterone; affects ECF volume and osmolarity.
Potassium (K+): Regulated by aldosterone; critical for nerve and muscle function.
Calcium (Ca2+): Regulated by parathyroid hormone and calcitonin; important for bone, muscle, and nerve function.
Chloride (Cl-): Follows sodium; helps maintain osmotic pressure.

Acid-Base Balance

Maintaining acid-base balance is vital for normal cellular function.

Buffer systems: Bicarbonate, phosphate, and protein buffers help resist changes in pH.
Respiratory regulation: CO2 exhalation affects blood pH.
Renal regulation: Kidneys excrete H+ and reabsorb HCO3-.
Equation:

Homeostatic Imbalances

Imbalances can result in dehydration, overhydration, electrolyte disturbances, acidosis, or alkalosis.

Examples: Hyponatremia (low sodium), hyperkalemia (high potassium), metabolic acidosis.

The Respiratory System

Functions and Major Organs

The respiratory system is responsible for gas exchange, supplying oxygen, and removing carbon dioxide from the body.

Main organs: Nose, pharynx, larynx, trachea, bronchi, lungs.
Functions: Air conduction, gas exchange, voice production, olfaction.

Conducting vs. Respiratory Zone Structures

The respiratory tract is divided into conducting and respiratory zones.

Conducting zone: Nose to terminal bronchioles; transports air.
Respiratory zone: Respiratory bronchioles, alveolar ducts, alveoli; site of gas exchange.

Structure and Function of Key Components

Nose: Filters, warms, and moistens air.
Pharynx: Passageway for air and food; aids in speech.
Larynx: Contains vocal cords; produces sound.
Trachea: Windpipe; supported by cartilage rings.
Bronchial tree: Branching system of bronchi and bronchioles.
Alveoli: Site of gas exchange; surrounded by capillaries.

Lungs and Pleura

Lungs: Right lung has three lobes; left lung has two lobes.
Pleura: Double-layered membrane; visceral pleura covers lungs, parietal pleura lines thoracic cavity.

Mechanics of Breathing

Breathing involves changes in thoracic volume and pressure.

Inspiration: Diaphragm contracts, thoracic volume increases, pressure decreases, air flows in.
Expiration: Diaphragm relaxes, thoracic volume decreases, pressure increases, air flows out.
Boyle's Law: (Pressure and volume are inversely related)

Alveolar Surface Tension and Lung Compliance

Surfactant: Reduces surface tension, preventing alveolar collapse.
Lung compliance: Measure of lung expandability.

Gas Exchange and Transport

External respiration: Gas exchange between alveoli and blood.
Internal respiration: Gas exchange between blood and tissues.
Oxygen transport: Mostly bound to hemoglobin; small amount dissolved in plasma.
Carbon dioxide transport: Dissolved in plasma, bound to hemoglobin, or as bicarbonate ions.

Control of Respiration

Medullary respiratory centers: Control rate and depth of breathing.
Chemoreceptors: Detect changes in CO2, O2, and pH.

Homeostatic Imbalances

Examples: Asthma, chronic obstructive pulmonary disease (COPD), respiratory acidosis.

The Urinary System

Functions and Major Organs

The urinary system maintains homeostasis by regulating water, electrolytes, and removing wastes.

Main organs: Kidneys, ureters, bladder, urethra.
Functions: Filtration of blood, formation of urine, regulation of blood volume and pressure.

Kidney Structure and Function

Nephron: Functional unit of the kidney; responsible for filtration, reabsorption, secretion.
Renal corpuscle: Includes glomerulus and Bowman's capsule.
Renal tubule: Proximal convoluted tubule, loop of Henle, distal convoluted tubule.
Collecting duct: Final site for water reabsorption and urine concentration.

Mechanisms of Urine Formation

Filtration: Movement of water and solutes from blood into Bowman's capsule.
Reabsorption: Return of water and solutes from filtrate to blood.
Secretion: Addition of substances from blood to filtrate.
Equation:

Countercurrent Mechanism

The countercurrent mechanism in the nephron allows for concentration of urine.

Countercurrent multiplier: Loop of Henle creates a gradient for water reabsorption.
Countercurrent exchanger: Vasa recta preserves the gradient.

Hormonal Regulation

ADH: Increases water reabsorption in collecting ducts.
Aldosterone: Increases sodium reabsorption.
ANP (Atrial Natriuretic Peptide): Reduces sodium reabsorption.

Homeostatic Imbalances

Examples: Diabetes insipidus, renal failure, urinary tract infections.

Summary Table: Comparison of Fluid Compartments

Compartment	Water Content (%)	Major Electrolytes
Intracellular Fluid (ICF)	~66%	K+, Mg2+, PO43-
Extracellular Fluid (ECF)	~33%	Na+, Cl-, HCO3-
Plasma (subset of ECF)	~7%	Na+, Cl-, proteins
Interstitial Fluid (subset of ECF)	~26%	Na+, Cl-