The Respiratory System

Functional Anatomy of the Respiratory System

The respiratory system is responsible for gas exchange, supplying oxygen to the body and removing carbon dioxide. It is divided into upper and lower respiratory tracts, each with specialized structures and functions.

• Upper Respiratory System:

  • Nose: Located centrally on the face, the nose filters, warms, and moistens incoming air. It contains hairs and mucus to trap particles.

  • Paranasal Sinuses: Air-filled spaces in the skull that lighten its weight, warm and moisten air, and enhance voice resonance.

  • Pharynx: A muscular tube (throat) that serves as a passageway for air and food; divided into nasopharynx, oropharynx, and laryngopharynx.

• Protective Mechanisms:

  • Mucus and cilia trap and move debris out of the airways.

  • Sneezing and coughing expel irritants.

  • Tonsils and immune cells provide defense against pathogens.

• Lower Respiratory System:

  • Conducting Zone: Includes larynx, trachea, bronchi, and bronchioles; conducts air to the respiratory zone but does not participate in gas exchange.

  • Respiratory Zone: Includes respiratory bronchioles, alveolar ducts, and alveoli; site of gas exchange.

  • Larynx: Voice box; routes air and food, houses vocal cords.

  • Trachea: Windpipe; supported by C-shaped cartilage rings, conducts air to bronchi.

  • Bronchi: Branch into smaller bronchioles, leading to alveoli.

  • Respiratory Membrane: Thin barrier (alveolar and capillary walls) for efficient gas exchange.

  • Respiratory Passageway (Descending Order): Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli.

Respiratory Physiology

Respiratory physiology involves the mechanics of breathing and the principles governing gas movement and exchange.

• Intrapleural Space: The partial vacuum here prevents lung collapse and assists lung expansion during inspiration.

• Boyle's Law: The pressure of a gas is inversely proportional to its volume. During inspiration, lung volume increases and pressure decreases, drawing air in. During expiration, volume decreases and pressure increases, pushing air out.

• Respiratory Muscles and Lung Elasticity: Diaphragm and intercostal muscles contract to increase thoracic volume; lung elasticity aids in passive expiration.

• Physical Factors Influencing Ventilation: Airway resistance, lung compliance, and alveolar surface tension.

Assessing Ventilation

Ventilation is measured by various lung volumes and capacities, which provide information about respiratory health.

• Lung Volumes and Capacities:

  • Tidal Volume (TV): Air moved in/out during normal breathing.

  • Inspiratory Reserve Volume (IRV): Extra air inhaled after normal inspiration.

  • Expiratory Reserve Volume (ERV): Extra air exhaled after normal expiration.

  • Residual Volume (RV): Air remaining after maximal exhalation.

  • Capacities: Combinations of volumes (e.g., Vital Capacity = TV + IRV + ERV).

• Dead Space: Air in conducting passages not involved in gas exchange.

• Pulmonary Function Tests: Assess lung function, diagnose diseases, and monitor progression.

Oxygen and Carbon Dioxide Transport

Oxygen and carbon dioxide are transported in the blood by different mechanisms, influenced by various physiological factors.

• Oxygen Transport:

  • 98.5% bound to hemoglobin, 1.5% dissolved in plasma.

  • Factors affecting loading/unloading: temperature, pH, 2,3-BPG, and partial pressures.

• Carbon Dioxide Transport:

  • Transported as bicarbonate ions (70%), bound to hemoglobin (23%), dissolved in plasma (7%).

Respiratory Controls

Respiratory rate and depth are regulated by neural and chemical mechanisms.

• Neural Controls: Medullary respiratory centers and pons regulate rhythm and depth.

• Chemical Influences: Arterial pH, partial pressures of O2 and CO2 affect chemoreceptors.

• Other Influences: Lung reflexes, voluntary control, and emotions can modify breathing.

Respiratory Diseases

Several diseases affect the respiratory system, each with distinct causes and consequences.

• Chronic Bronchitis: Inflammation of bronchi, excess mucus, chronic cough.

• Emphysema: Destruction of alveolar walls, reduced surface area for gas exchange.

• Asthma: Airway inflammation and constriction, reversible episodes.

• Tuberculosis: Bacterial infection causing granulomas in lungs.

• Lung Cancer: Uncontrolled cell growth, often linked to smoking.

The Urinary System

Anatomy of the Urinary System

The urinary system maintains homeostasis by filtering blood, removing wastes, and regulating fluid and electrolyte balance.

• Kidney Anatomy: Bean-shaped organs with a cortex, medulla, and renal pelvis; surrounded by protective coverings (renal capsule, adipose capsule, renal fascia).

• Blood Supply: Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arterioles → glomerulus → efferent arterioles → peritubular capillaries/vasa recta → veins.

• Nephron Anatomy: Functional unit; consists of renal corpuscle (glomerulus + Bowman's capsule) and renal tubule (proximal tubule, loop of Henle, distal tubule, collecting duct).

Urinary Physiology

Urine formation involves filtration, reabsorption, and secretion, regulated by various pressures and controls.

• Steps in Urine Formation:

  1. Filtration: Blood pressure forces water and solutes from glomerulus into Bowman's capsule.

  2. Reabsorption: Useful substances (e.g., glucose, ions, water) reabsorbed into peritubular capillaries.

  3. Secretion: Additional wastes and ions secreted into tubule from blood.

• Forces Affecting Filtration: Glomerular hydrostatic pressure, blood colloid osmotic pressure, capsular hydrostatic pressure.

• Intrinsic Controls: Renal autoregulation (myogenic and tubuloglomerular feedback).

• Extrinsic Controls: Neural and hormonal mechanisms (e.g., sympathetic nervous system, renin-angiotensin system).

• Water and Solute Reabsorption: Occurs mainly in proximal tubule; regulated in distal tubule and collecting duct by hormones (e.g., aldosterone, ADH).

• Tubular Secretion: Removes substances like H+, K+, creatinine, drugs.

• Medullary Osmotic Gradient: Established by countercurrent mechanisms; allows concentration of urine.

• Formation of Dilute/Concentrated Urine: Regulated by ADH and medullary gradient.

• Renal Clearance: Volume of plasma cleared of a substance per unit time; summarizes renal handling.

  • Where C = clearance, U = urine concentration, V = urine flow rate, P = plasma concentration.

Urine Characteristics

• Normal Properties: Clear to pale yellow, slightly acidic (pH ~6), specific gravity 1.001–1.035, contains water, urea, creatinine, ions.

• Abnormal Components and Conditions:

  Component	Condition
  Glucose	Glycosuria (diabetes mellitus)
  Proteins	Proteinuria (renal disease)
  Blood	Hematuria (trauma, infection)
  Bilirubin	Bilirubinuria (liver disease)
  Ketones	Ketonuria (starvation, diabetes)

Acid-Base Balance and Fluid/Electrolyte Regulation

Fluid Shifts and Water Balance

Body fluids are distributed between intracellular and extracellular compartments, with movement governed by osmotic and hydrostatic pressures.

• Routes of Water Entry: Ingestion (food and drink), metabolic water production.

• Routes of Water Loss: Urine, sweat, feces, insensible loss (lungs, skin).

• Feedback Mechanisms: Thirst center in hypothalamus regulates intake; ADH and aldosterone control output.

• Obligatory Water Losses: Minimum urine output required to excrete wastes.

• Dehydration: Water loss exceeds intake; leads to cell shrinkage, low blood pressure.

• Hypotonic Hydration: Excess water intake; cells swell, risk of hyponatremia.

Electrolyte Balance

• Sodium: Main extracellular cation; regulates fluid volume and blood pressure. Controlled by aldosterone, ANP, and ADH.

• Potassium: Main intracellular cation; affects membrane potential. Regulated by aldosterone.

• Calcium: Important for muscle contraction, blood clotting. Regulated by parathyroid hormone and calcitonin.

• Anion Balance: Chloride and bicarbonate are major anions; help maintain osmotic balance and pH.

Acid-Base Balance

The body maintains pH within a narrow range (7.35–7.45) through buffer systems, respiratory, and renal mechanisms.

• Kidney Regulation: Kidneys excrete or reabsorb H+ and HCO3- to maintain pH.

• Acidosis: Blood pH below 7.35; can be respiratory (CO2 retention) or metabolic (acid accumulation/loss of HCO3-).

• Alkalosis: Blood pH above 7.45; can be respiratory (CO2 loss) or metabolic (loss of acid/gain of HCO3-).

• Compensation: Respiratory system alters CO2 exhalation; kidneys adjust H+ and HCO3- excretion.