What is the primary function of the respiratory system?

The primary function of the respiratory system is gas exchange, which involves uptake of oxygen into the blood and elimination of carbon dioxide from the body. It also plays a crucial role in acid–base balance, phonation, defense mechanisms, and metabolic functions.

What is the difference between ventilation and respiration?

Ventilation refers to the mechanical movement of air into and out of the lungs, while respiration includes ventilation, diffusion of gases across the alveolar membrane, transport of gases in blood, and cellular utilization of oxygen.

What determines alveolar ventilation?

Alveolar ventilation is determined by tidal volume, respiratory rate, and dead space. It is calculated as (tidal volume minus dead space) multiplied by respiratory rate and directly influences arterial carbon dioxide levels.

Why is surfactant important in lung physiology?

Pulmonary surfactant reduces surface tension within alveoli, preventing alveolar collapse during expiration, increasing lung compliance, and reducing the work of breathing. It is produced by type II pneumocytes.

What is the ventilation perfusion ratio and why is it important?

The ventilation perfusion ratio is the ratio of alveolar ventilation to pulmonary blood flow. A normal value is about 0.8. Matching of ventilation and perfusion is essential for efficient gas exchange, and mismatch leads to hypoxemia.

What causes hypoxemia that does not improve with oxygen therapy?

Physiological shunt is the main cause of hypoxemia that does not correct with oxygen therapy, as blood bypasses ventilated alveoli, commonly seen in pneumonia and pulmonary edema.

What factors shift the oxyhemoglobin dissociation curve to the right?

Increased carbon dioxide, increased hydrogen ion concentration, increased temperature, and increased 2,3-BPG shift the curve to the right, facilitating oxygen unloading to tissues.

Which chemoreceptors are most sensitive to carbon dioxide changes?

Central chemoreceptors located in the medulla are most sensitive to changes in carbon dioxide via changes in cerebrospinal fluid pH and provide the strongest drive for respiration.

What is the physiological basis of respiratory acidosis?

Respiratory acidosis occurs due to hypoventilation leading to carbon dioxide retention, increased PaCO₂, and a fall in blood pH. Renal compensation occurs by increased bicarbonate reabsorption.

Why does emphysema cause difficulty in expiration?

Emphysema causes destruction of elastic tissue in the lungs, leading to loss of elastic recoil. This results in airway collapse during expiration and air trapping.

Respiratory System Physiology

Primary Functions

Gas exchange – O₂ uptake and CO₂ elimination
Acid–base regulation – via control of PaCO₂
Phonation – airflow for speech
Defense – filtration, mucociliary clearance, immune cells
Metabolic functions – ACE production, inactivation of vasoactive substances
Thermoregulation & water balance

Anatomical–Functional Divisions

Conducting zone

* Nose → pharynx → larynx → trachea → bronchi → terminal bronchioles

* No gas exchange

* Functions: warming, humidifying, filtering air

Respiratory zone

* Respiratory bronchioles → alveolar ducts → alveoli

* Site of gas exchange

Mechanics of Breathing (Pulmonary Ventilation)

Inspiration (Active)

Diaphragm contracts → moves downward
External intercostals elevate ribs
Thoracic volume ↑ → intrapleural pressure becomes more negative
Alveolar pressure ↓ (≈ –1 cm H₂O) → air flows in

Expiration

Quiet expiration: passive (elastic recoil)
Forced expiration: active (internal intercostals + abdominal muscles)

Pressures Involved

| Pressure | Normal Value |

| ----------------------- | ------------ |

| Atmospheric pressure | 760 mmHg |

| Intrapleural pressure | –5 cm H₂O |

| Alveolar pressure | 0 cm H₂O |

| Transpulmonary pressure | +5 cm H₂O |

Lung Compliance

Definition: ΔVolume / ΔPressure
High compliance: emphysema
Low compliance: fibrosis, ARDS
Determined by:

* Elastic tissue

* Alveolar surface tension

Surfactant

Secreted by Type II pneumocytes
Reduces surface tension
Prevents alveolar collapse (atelectasis)
Increases lung compliance
Deficiency → neonatal respiratory distress syndrome

Airway Resistance

Greatest in medium-sized bronchi
Influenced by:

* Airway radius (most important)

* Lung volume

* Smooth muscle tone

Bronchodilation: sympathetic (β₂)
Bronchoconstriction: parasympathetic (M₃)

Lung Volumes and Capacities

Volumes

Tidal Volume (TV) ≈ 500 mL
Inspiratory Reserve Volume (IRV)
Expiratory Reserve Volume (ERV)
Residual Volume (RV)

Capacities

Vital Capacity (VC = TV + IRV + ERV)
Total Lung Capacity (TLC = VC + RV)
Functional Residual Capacity (FRC = ERV + RV)

Alveolar Ventilation

[

V_A = (V_T – V_D) × f

]

Dead space ≈ 150 mL
Alveolar ventilation determines PaCO₂

Diffusion of Gases

Governed by Fick’s law
Factors:

* Surface area

* Thickness of membrane

* Partial pressure gradient

* Diffusion coefficient

Partial Pressures (mmHg)

| Location | O₂ | CO₂ |

| --------------- | --- | --- |

| Atmospheric air | 160 | 0.3 |

| Alveoli | 104 | 40 |

| Arterial blood | 95 | 40 |

| Venous blood | 40 | 46 |

Ventilation–Perfusion (V/Q) Ratio

Normal ≈ 0.8
High V/Q → dead space (pulmonary embolism)
Low V/Q → shunt (pneumonia, asthma)

Transport of Oxygen

Bound to hemoglobin (98%)
Dissolved in plasma (2%)

Oxyhemoglobin Dissociation Curve

Sigmoid shape
Right shift (↓ affinity):

* ↑ CO₂

* ↑ H⁺ (↓ pH)

* ↑ Temperature

* ↑ 2,3-BPG

(Bohr effect)

Transport of Carbon Dioxide

Bicarbonate (70%)
Carbaminohemoglobin (23%)
Dissolved CO₂ (7%)

Chloride Shift

Exchange of HCO₃⁻ and Cl⁻ in RBCs

Control of Respiration

Respiratory Centers

Medulla

* Dorsal respiratory group (inspiration)

* Ventral respiratory group (forced breathing)

Pons

* Pneumotaxic center (rate control)

* Apneustic center (depth)

Chemoreceptors

Central Chemoreceptors

Located in medulla
Respond to ↑ CO₂ / ↓ pH
Most powerful stimulus

Peripheral Chemoreceptors

Carotid & aortic bodies
Respond to:

* ↓ PaO₂ (< 60 mmHg)

* ↑ PaCO₂

* ↓ pH

Reflexes

Hering–Breuer reflex – prevents overinflation
Cough reflex – airway protection
Sneezing reflex – nasal clearance
J-receptor reflex – pulmonary congestion → dyspnea

Non-Respiratory Functions

ACE converts angiotensin I → II
Filtration of micro-emboli
Immune defense (macrophages, IgA)

Key Clinical Correlations

Hypoventilation → respiratory acidosis
Hyperventilation → respiratory alkalosis
COPD → ↓ PaO₂, ↑ PaCO₂
Fibrosis → ↓ compliance
Emphysema → ↑ compliance, ↓ elastic recoil

Respiratory System Physiology Complete Guide for Medical Students

Respiratory System Physiology

Primary Functions

Anatomical–Functional Divisions