Human Biology & Histology: Respiratory and Urinary Systems

Larynx

Structure and Functions

The larynx is a crucial organ in the upper respiratory tract, located between the pharynx and trachea. It plays essential roles in protecting the airway, facilitating breathing, and producing sound.

• Prevention of food entry: The larynx closes during swallowing to prevent food and liquids from entering the airway.

• Airway closure: The larynx can close off the airway, protecting the lower respiratory tract.

• Sound generation: The vocal cords within the larynx vibrate to produce voice.

Anatomy of the Larynx

• Skeleton: Stabilized by hyaline cartilage, including the thyroid, cricoid, and arytenoid cartilages.

• Epiglottis: A flap of cartilage that covers the glottis during swallowing.

• Glottis: The space between the vocal cords, which can be opened or closed to regulate airflow and sound production.

Clinical Application

• Laryngoscopy: A procedure to visualize the larynx and glottis, useful for diagnosing voice and airway disorders.

Trachea

Structure and Histology

The trachea is a tubular structure that connects the larynx to the bronchi, allowing air passage to the lungs. It is supported by C-shaped rings of hyaline cartilage.

• Diameter: Approximately 15 mm.

• Location: Anterior to the esophagus.

• Cartilage: 16-20 C-shaped rings maintain patency; open ends are connected by smooth muscle.

• Wall layers:

  • Tunica mucosa: Lined by respiratory epithelium (ciliated, pseudostratified columnar cells).

  • Submucosa: Contains seromucous glands and connective tissue.

  • Tunica fibro-musculo-cartilaginea: Cartilage and smooth muscle.

  • Adventitia: Outermost connective tissue layer.

Histological Features

• Respiratory epithelium: Contains ciliated cells, goblet cells (mucus-secreting), basal cells (stem cells), brush cells (receptor), and small granule cells (endocrine).

• Mucociliary clearance: Mucus traps inhaled particles; cilia transport mucus toward the pharynx for removal.

Clinical Relevance

• Impairment of mucociliary clearance: Conditions such as bronchitis, primary ciliary dyskinesia (Kartagener Syndrome), and cystic fibrosis can disrupt ciliary function or mucus properties, leading to respiratory complications.

Lungs

Gross Anatomy

The lungs are paired organs responsible for gas exchange. They are divided into lobes and segments for functional and anatomical organization.

• Right lung: 3 lobes, subdivided into 10 segments.

• Left lung: 2 lobes, subdivided into 9 segments (smaller due to heart position).

Conductive System Branching

• Trachea → Main bronchi → Segmental bronchi → Bronchioles → Respiratory bronchioles → Alveolar ducts → Alveolar sacs → Alveoli

• Branching: Bronchi branch dichotomously; bronchioles branch further, leading to alveoli where gas exchange occurs.

Blood Circulation in the Lung

• Pulmonary arteries: Carry deoxygenated blood from the heart to the lungs.

• Pulmonary veins: Return oxygenated blood to the heart.

• Bronchial arteries: Supply lung tissue with oxygenated blood from the aorta.

Histology of the Lung

• Bronchi: Lined by ciliated pseudostratified epithelium, with cartilage plates and glands.

• Bronchioles: Lined by ciliated cuboidal epithelium, few goblet cells, no cartilage or glands.

• Terminal bronchioles: Lead to alveolar ducts and sacs.

• Alveoli: Sites of gas exchange, lined by type I and type II pneumocytes.

Gas Exchange in the Alveoli

Mechanism of Diffusion

Gas exchange in the alveoli is driven by diffusion across the alveolar-capillary barrier.

• Fick's Law of Diffusion:

$V = K \cdot F_a \cdot \frac{\Delta p}{a}$

• V: Diffusion flux

• K: Diffusion constant

• Fa: Diffusion area

• a: Diffusion distance

• Δp: Pressure difference

Alveolar Septum Structure

• Barriers to diffusion:

  1. Alveolar epithelium (type I pneumocytes)

  2. Basement membrane

  3. Capillary endothelium

• Surface area: ~100 m2

• Thickness: Can be below 1 μm

Cell Types in Alveoli

• Type I pneumocytes: Thin cells facilitating gas exchange.

• Type II pneumocytes: Stem cells for regeneration; produce surfactant.

• Macrophages: Phagocytose particles and pathogens.

Surfactant

• Definition: Surface-active agent produced by type II pneumocytes.

• Function: Reduces surface tension, prevents alveolar collapse, aids lung expansion after birth, and provides some defense against microorganisms.

Pleura

Structure and Function

The pleura is a double-layered membrane covering the lungs and lining the thoracic cavity.

• Pulmonary (visceral) pleura: Covers the lung surface.

• Parietal pleura: Lines the inner thoracic wall.

• Pleural cavity: Space between pleural layers, contains lubricating fluid and maintains a vacuum for lung expansion.

Mechanics of Respiration

Breathing Movements

• Abdominal breathing: Driven by diaphragm contraction.

• Chest breathing: Involves muscles that lift the ribs.

• Elastic fibers: Lung tissue contains elastic fibers; passive expiration is mainly due to their recoil.

Urinary System

Components and Functions

The urinary system maintains homeostasis by excreting waste, regulating water and electrolytes, and producing hormones.

• Organs: Kidney, ureter, urinary bladder, urethra.

• Kidney functions:

  • Excretion of waste (metabolites, xenobiotics)

  • Homeostasis of water and electrolytes

  • Regulation of blood pH

  • Regulation of arterial blood pressure

  • Endocrine functions: erythropoietin (EPO) production, vitamin D3 activation

Kidney Anatomy

• Location: Retroperitoneal, right kidney slightly lower due to liver.

• Structure: Divided into cortex and medulla; contains renal lobes, arteries, and pelvis.

• Unilobar vs. multilobar: Human kidneys are multilobar, with multiple pyramids and lobes.

Nephron Structure

• Nephron: Functional unit of the kidney, consisting of renal corpuscle and tubule system.

• Segments:

  • Proximal convoluted tubule

  • Loop of Henle (descending and ascending limbs)

  • Distal convoluted tubule

  • Collecting duct

Renal Blood Supply

• Renal artery: Branches from abdominal aorta, supplies ~20% of cardiac output.

• Branching: Renal artery → segmental arteries → interlobar arteries → arcuate arteries → interlobular arteries.

Urine Production

• Three steps:

  1. Filtration (in renal corpuscle)

  2. Reabsorption (in tubules and collecting duct)

  3. Secretion (in tubules and collecting duct)

Renal Corpuscle and Filtration Barrier

• Renal corpuscle: Composed of glomerulus (capillary tuft) and Bowman's capsule.

• Filtration barrier:

  1. Fenestrated capillary endothelium

  2. Glomerular basement membrane (GBM)

  3. Filtration slits between podocyte foot processes, covered by a diaphragm

• Mesangial cells: Provide structural support, phagocytose debris, and produce signaling molecules.

Filtration Properties

• Permits: Water, salts, sugars, amino acids, urea (small molecules)

• Restricts: Cells and most proteins

• Filtrate rate: ~120 mL/min (170 L/day)

• Primary urine: Similar solute concentration to plasma, but lacks cells and proteins

Table: Filtration Barrier Layers

Layer	Thickness	Function
Capillary Endothelium	50-100 nm	Fenestrated, allows passage of small molecules
Basement Membrane	~300 nm	Physical and charge barrier
Podocyte Filtration Slit	~180 nm	Size-selective barrier, covered by diaphragm

Table: What is Filtrated?

Substance	Size	Filtration
Water	~0.3 nm	Yes
Glucose	~0.9 nm	Yes
Albumin	~7 nm	No
Red blood cells	~8,000 nm	No

Additional info: Some details on cell types, branching generations, and clinical syndromes were inferred for completeness.