Respiratory System: Structure, Function, and Gas Exchange

Respiratory System Overview

Structure and Function

The respiratory system is responsible for the exchange of gases (oxygen and carbon dioxide) between the body and the environment. It consists of several anatomical structures that work together to ensure efficient gas exchange and maintain homeostasis.

• Nasal Cavity: Warms and humidifies incoming air; cools and dries outgoing air. The nasal conchae increase surface area and help condition the air.

• Pharynx and Larynx: The larynx is located between the pharynx and the trachea. It is responsible for opening and closing the airway, aided by the epiglottis during swallowing.

• Trachea and Bronchial Tree: The trachea and bronchi direct air to the respiratory zone. The conducting zone consists of all structures that direct air but do not participate in gas exchange.

• Alveoli: The primary site of gas exchange. Alveolar walls are thin and closely associated with capillaries, allowing for efficient diffusion of gases.

Gas Exchange Mechanisms

Gas exchange occurs in the alveoli, where oxygen diffuses into the blood and carbon dioxide diffuses out. This process is driven by differences in partial pressures of the gases.

• Partial Pressure: The pressure exerted by a single gas in a mixture. Gases move from areas of higher partial pressure to lower partial pressure.

• Oxygen Transport: About 98.5% of oxygen is transported bound to hemoglobin; the remaining 1.5% is dissolved in plasma.

• Carbon Dioxide Transport: CO2 is transported in three ways:

  • Dissolved in plasma (about 7%)

  • As bicarbonate ion (HCO3-) (about 70-90%)

  • Bound to hemoglobin (about 20-23%)

• Diffusion: A thicker membrane slows diffusion, while a thinner membrane speeds it up. Greater differences in partial pressure increase the rate of diffusion.

Respiratory Volumes and Capacities

Respiratory volumes and capacities are measurements used to assess lung function.

• Tidal Volume (TV): The amount of air inhaled or exhaled during normal breathing.

• Inspiratory Reserve Volume (IRV): The additional air that can be inhaled after a normal inhalation.

• Expiratory Reserve Volume (ERV): The additional air that can be exhaled after a normal exhalation.

• Residual Volume (RV): The air remaining in the lungs after maximal exhalation.

• Vital Capacity (VC): The total amount of air that can be exhaled after a maximal inhalation. Formula: $VC = IRV + TV + ERV$

• Inspiratory Capacity (IC): The maximum amount of air that can be inspired. Formula: $IC = TV + IRV$

• Functional Residual Capacity (FRC): The volume of air remaining in the lungs after a normal exhalation. Formula: $FRC = ERV + RV$

• Total Lung Capacity (TLC): The total volume of the lungs. Formula: $TLC = IRV + RV + ERV + TV$

Alveolar Cells and Surfactant

The alveoli contain two main types of cells that are essential for lung function.

• Type I Alveolar Cells: Responsible for gas exchange due to their thin structure.

• Type II Alveolar Cells: Produce surfactant, a substance that reduces surface tension and prevents alveolar collapse.

• Surfactant: Lowers the surface tension of water in the alveoli, making it easier for the lungs to expand during inhalation.

Mechanics of Breathing

Breathing involves changes in pressure within the thoracic cavity, allowing air to move in and out of the lungs.

• Inspiration: Occurs when intrapulmonary (alveolar) pressure drops below atmospheric pressure, causing air to flow into the lungs.

• Expiration: Occurs when intrapulmonary pressure rises above atmospheric pressure, causing air to flow out of the lungs.

• Low pH, high CO2 partial pressure: Increases ventilation to expel more CO2 and raise pH.

• High pH, low CO2 partial pressure: Decreases ventilation.

Control of Respiration

Respiratory rate and depth are regulated by chemoreceptors and the respiratory centers in the brainstem.

• Central Chemoreceptors: Located in the brainstem; respond to changes in CO2 and pH in cerebrospinal fluid.

• Peripheral Chemoreceptors: Located in the carotid and aortic bodies; respond to significant drops in O2 levels.

• Hyperventilation: Rapid or deep breathing that decreases CO2 levels and increases blood pH (respiratory alkalosis).

• Hypoventilation: Slow or shallow breathing that increases CO2 levels and decreases blood pH (respiratory acidosis).

Summary Table: Respiratory Volumes and Capacities

Term	Definition	Formula
Tidal Volume (TV)	Normal breath in or out	-
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	-
Vital Capacity (VC)	Total air exhaled after maximal inhalation	$VC = IRV + TV + ERV$
Inspiratory Capacity (IC)	Maximum air inspired	$IC = TV + IRV$
Functional Residual Capacity (FRC)	Air remaining after normal exhalation	$FRC = ERV + RV$
Total Lung Capacity (TLC)	Total volume of the lungs	$TLC = IRV + RV + ERV + TV$

Additional info:

• Respiratory acidosis and alkalosis refer to imbalances in blood pH due to changes in CO2 levels.

• Surface tension in the alveoli is a critical factor in lung compliance and is regulated by surfactant.

• Gas exchange efficiency is influenced by membrane thickness, surface area, and partial pressure gradients.