The Respiratory System

Overview and Major Functions

The respiratory system is essential for supplying the body with oxygen and removing carbon dioxide, a waste product of metabolism. It accomplishes this through a series of coordinated processes collectively known as respiration.

• Pulmonary ventilation: Movement of air into and out of the lungs (breathing).

• External respiration: Gas exchange between the lungs and the blood (pulmonary gas exchange).

• Transport of respiratory gases: Movement of oxygen and carbon dioxide between the lungs and tissues via the blood.

• Internal respiration: Gas exchange between systemic blood vessels and tissues (tissue gas exchange).

Organs and Passageways of the Respiratory System

Major Organs (in Descending Order)

• Nose

• Nasal cavity/Paranasal sinuses

• Pharynx (nasopharynx, oropharynx, laryngopharynx)

• Larynx

• Trachea

• Bronchi and bronchial tree

• Alveoli

• Respiratory membrane

Upper and Lower Respiratory System

Conducting and Respiratory Zones

The respiratory system is divided into conducting and respiratory zones:

• Conducting zone: Passageways that transport air to the respiratory zone (nose, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles).

• Respiratory zone: Site of gas exchange (respiratory bronchioles, alveolar ducts, alveoli).

Microscopic Structure and Gas Exchange

Respiratory Zone and Alveoli

The respiratory zone begins where terminal bronchioles feed into respiratory bronchioles, which lead to alveolar ducts and alveolar sacs. Alveoli are the primary sites of gas exchange, with approximately 300 million in the lungs, providing a large surface area.

Respiratory Membrane

• Composed of alveolar and capillary walls with fused basal laminas.

• Type I alveolar cells: Simple squamous epithelium for gas diffusion.

• Type II alveolar cells: Secrete surfactant to reduce surface tension.

• Alveolar macrophages: Remove debris and pathogens.

Mechanics of Breathing

Pressure Relationships in the Thoracic Cavity

• Atmospheric pressure (Patm): Pressure exerted by air around the body (760 mmHg at sea level).

• Intrapulmonary pressure (Ppul): Pressure within alveoli; equalizes with atmospheric pressure during breathing cycles.

• Intrapleural pressure (Pip): Pressure within pleural cavity; always negative relative to Patm and Ppul.

• Transpulmonary pressure: Difference between Ppul and Pip; keeps lungs inflated.

Boyle’s Law and Pulmonary Ventilation

Boyle’s Law describes the inverse relationship between pressure and volume of a gas:

• As thoracic volume increases, intrapulmonary pressure decreases, causing air to flow into the lungs (inspiration).

• As thoracic volume decreases, intrapulmonary pressure increases, causing air to flow out (expiration).

Gas Exchange and Transport

External Respiration (Pulmonary Gas Exchange)

• Oxygen diffuses from alveoli (PO2 = 104 mmHg) into pulmonary capillaries (PO2 = 40 mmHg).

• Carbon dioxide diffuses from blood (PCO2 = 45 mmHg) into alveoli (PCO2 = 40 mmHg).

• CO2 is 20 times more soluble than O2, so it diffuses efficiently despite a smaller gradient.

Ventilation-Perfusion Coupling

Efficient gas exchange requires matching of alveolar ventilation (airflow) and pulmonary perfusion (blood flow). Local changes in CO2 and O2 concentrations regulate bronchiolar and arteriolar diameter to optimize exchange.

Internal Respiration (Tissue Gas Exchange)

• Oxygen diffuses from systemic capillaries into tissues (where PO2 is lower).

• CO2 diffuses from tissues into blood (where PCO2 is lower).

Oxygen Transport

• 98.5% of O2 is carried bound to hemoglobin (Hb) as oxyhemoglobin (HbO2).

• Each Hb molecule binds up to four O2 molecules.

• O2 binding and release are influenced by PO2, temperature, pH, PCO2, and BPG.

Carbon Dioxide Transport

• 7–10% dissolved in plasma

• 20% bound to Hb as carbaminohemoglobin

• ~70% as bicarbonate ion (HCO3–) in plasma

CO2 + H2O H2CO3 $\rightarrow$ H+ + HCO3–

The chloride shift maintains ionic balance as HCO3– exits RBCs and Cl– enters.

Control of Respiration

Neural Regulation

• Medullary respiratory centers (VRG and DRG) set basic rhythm and integrate sensory input.

• Pontine centers smooth transitions between inspiration and expiration.

Chemical Factors

• Central chemoreceptors (brainstem) and peripheral chemoreceptors (aortic arch, carotid arteries) monitor CO2, O2, and pH.

• CO2 is the most potent stimulus for breathing; increased CO2 (hypercapnia) increases ventilation.

• Low O2 (hypoxemia) stimulates breathing only when very low.

• pH changes (acidosis/alkalosis) also affect respiratory rate.

Respiratory Pathophysiology

Chronic Obstructive Pulmonary Disease (COPD)

• Includes chronic bronchitis and emphysema.

• Characterized by airway obstruction, dyspnea, frequent infections, and abnormal ventilation-perfusion ratios.

• Most common cause: smoking.

Asthma

• Characterized by reversible airway inflammation, bronchospasm, and increased mucus production.

• Triggered by immune responses and environmental factors.

Emphysema

• Permanently enlarged alveoli and destruction of alveolar walls.

• Loss of lung elasticity, air trapping, and increased work of breathing.

Chronic Bronchitis

• Chronic inflammation and excessive mucus production in lower airways.

• Obstructs airflow and increases risk of infection.

Tuberculosis

• Infectious disease caused by Mycobacterium tuberculosis.

• Symptoms: fever, night sweats, weight loss, cough.

• Treatment: long-term antibiotics.

Lung Cancer

• Leading cause of cancer death; strongly linked to smoking.

• Types: squamous cell carcinoma, adenocarcinoma, small cell carcinoma.