Chapter 21: The Respiratory System – Structure, Function, and Gas Exchange

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Overview of the Respiratory System

Major Organs and Anatomical Divisions

The respiratory system is responsible for the exchange of oxygen and carbon dioxide between the body and the environment. It consists of organs located in the head, neck, and thoracic cavity, including the nasal cavity, pharynx, larynx, trachea, bronchial tree, and lungs. The system is divided into the upper and lower respiratory tracts, as well as conducting and respiratory zones.

Upper Respiratory Tract: Passageways from the nasal cavity to the larynx.
Lower Respiratory Tract: Passageways from the trachea to the alveoli, including the lungs.
Conducting Zone: Tubes that transport air into and out of the body, where air is filtered, warmed, and moistened.
Respiratory Zone: Structures containing alveoli where gas exchange occurs.

Anatomy of the respiratory system, showing nasal cavity, pharynx, larynx, trachea, bronchi, lungs, and diaphragm

Basic Functions of the Respiratory System

Pulmonary Ventilation: Movement of air in and out of the lungs.
Pulmonary Gas Exchange: Exchange of gases between alveoli and blood.
Gas Transport: Movement of gases through the blood.
Tissue Gas Exchange: Exchange of gases between blood and tissues.
Other Functions: Speech, olfaction, blood pH regulation, blood pressure regulation (angiotensin-II), and assisting in venous return and abdominal pressure changes.

Pulmonary Ventilation

Mechanics of Breathing

Pulmonary ventilation consists of two phases: inspiration (inhalation) and expiration (exhalation). The movement of air is driven by pressure gradients created by changes in lung volume, as described by Boyle's Law.

Boyle’s Law: At constant temperature, the pressure of a gas is inversely proportional to its volume:
As lung volume increases, intrapulmonary pressure decreases, drawing air in. As lung volume decreases, intrapulmonary pressure increases, pushing air out.

Person inflating a balloon, demonstrating inspiration Balloon flying away, demonstrating expiration

Pressure Relationships in the Thoracic Cavity

Three main pressures influence pulmonary ventilation:

Atmospheric Pressure: Pressure exerted by air outside the body (typically 760 mm Hg at sea level).
Intrapulmonary (Alveolar) Pressure: Pressure within the alveoli; equalizes with atmospheric pressure between breaths.
Intrapleural Pressure: Pressure within the pleural cavity; always slightly less than intrapulmonary pressure, creating a suction effect that keeps the lungs inflated.

Diagram showing atmospheric, intrapleural, and intra-alveolar pressures

Pressure Changes During Breathing

During quiet breathing, the following sequence occurs:

Between Breaths: Intrapulmonary pressure equals atmospheric pressure; no air movement.
Inspiration: Diaphragm and external intercostals contract, increasing thoracic volume and decreasing intrapulmonary pressure below atmospheric, causing air to flow in.
Between Inspiration and Expiration: Pressures equalize; no air movement.
Expiration: Muscles relax, thoracic volume decreases, intrapulmonary pressure rises above atmospheric, and air flows out.

Animation of lung expansion and contraction during breathing Pressure changes during inspiration Pressure changes between inspiration and expiration

Pathological Changes: Pneumothorax

If intrapleural pressure rises to atmospheric or above, the suction effect is lost, and the lung collapses. Causes include trauma, pleural effusion, pneumothorax (air), or hemothorax (blood).

Normal lung vs. collapsed lung due to pneumothorax

Physical Factors Influencing Ventilation

Airway Resistance

Airway resistance is anything that impedes airflow through the respiratory tract. It is primarily influenced by the diameter of the bronchioles: larger diameter means less resistance, and smaller diameter means more resistance.

Airway resistance increases as smooth muscle contracts and airway diameter decreases

Alveolar Surface Tension

Alveoli are lined with a thin film of water, creating surface tension that can cause alveoli to collapse. Pulmonary surfactant reduces this surface tension, allowing alveoli to remain inflated and facilitating gas exchange.

Effect of surfactant on alveolar surface tension

Pulmonary Compliance

Pulmonary compliance refers to the ability of the lungs and chest wall to stretch and expand. It depends on alveolar surface tension, the distensibility of the chest wall, and the ability of respiratory muscles to move the chest wall.

Compliance of lungs

Pulmonary Volumes and Capacities

Measuring Pulmonary Function

Pulmonary volumes are measured using a spirogram and include:

Tidal Volume (TV): Air exchanged during quiet breathing (~500 mL).
Inspiratory Reserve Volume (IRV): Maximum air forcibly inhaled after normal inhalation.
Expiratory Reserve Volume (ERV): Maximum air forcibly exhaled after normal exhalation.
Residual Volume (RV): Air remaining in lungs after forced exhalation.

Pulmonary capacities are combinations of these volumes:

Inspiratory Capacity (IC):
Functional Residual Capacity (FRC):
Vital Capacity (VC):
Total Lung Capacity (TLC):

Spirogram showing lung volumes and capacities Graph of lung volumes and capacities over time

Types of Lung Diseases

Restrictive Diseases: Reduced lung expansion (e.g., pulmonary fibrosis).
Obstructive Diseases: Blocked airways, difficulty exhaling (e.g., asthma, emphysema).

Spirograms for normal, asthma, emphysema, and fibrosis lungs

Gas Exchange and Transport

Gas Laws and Exchange Mechanisms

Gas exchange in the lungs and tissues is governed by Dalton’s Law and Henry’s Law:

Dalton’s Law: Total pressure of a gas mixture is the sum of the partial pressures of each gas.
Henry’s Law: The amount of gas dissolved in a liquid is proportional to its partial pressure and solubility.

Pulmonary Gas Exchange (External Respiration)

Oxygen diffuses from alveoli (high PO2) to blood (low PO2), while carbon dioxide diffuses from blood (high PCO2) to alveoli (low PCO2).

Pulmonary gas exchange at the alveolus

Tissue Gas Exchange (Internal Respiration)

Oxygen diffuses from blood to tissues, and carbon dioxide diffuses from tissues to blood, driven by partial pressure gradients.

Tissue gas exchange between blood and tissues

Gas Transport in the Blood

Oxygen Transport

Oxygen is poorly soluble in plasma; most is transported bound to hemoglobin (Hb) in red blood cells. Each Hb molecule can carry four oxygen molecules.

Hemoglobin structure and oxygen binding

Loading: Oxygen binds to Hb in the lungs (forming oxyhemoglobin).
Unloading: Oxygen is released from Hb in tissues (forming deoxyhemoglobin).

The oxygen-hemoglobin dissociation curve shows how Hb saturation varies with PO2. Factors such as temperature, pH, and PCO2 shift the curve and affect oxygen unloading.

Oxygen-hemoglobin dissociation curve Effect of temperature on oxygen unloading Effect of pH on oxygen unloading Effect of temperature, acidity, and PCO2 on oxygen unloading

Carbon Dioxide Transport

Carbon dioxide is transported in three forms:

Dissolved in plasma (7-10%)
Bound to hemoglobin as carbaminohemoglobin (20%)
As bicarbonate ions (HCO3-) in plasma (70%)

The bicarbonate buffer system is crucial for maintaining blood pH:

Bicarbonate buffer equation

Effect of Ventilation on Blood pH

Normal Ventilation: Maintains stable pH.
Hyperventilation: Excess CO2 exhaled, pH rises (alkalosis).
Hypoventilation: CO2 retained, pH drops (acidosis).

Summary of Respiratory Processes

Pulmonary Ventilation: Air moves between atmosphere and alveoli.
Pulmonary Gas Exchange: O2 enters blood, CO2 exits at alveoli.
Gas Transport: O2 and CO2 carried by blood (O2 by Hb, CO2 as bicarbonate).
Tissue Gas Exchange: O2 delivered to tissues, CO2 picked up from tissues.