Respiratory System: Structure, Function, and Physiology Study Guide

Respiratory System

Overview

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It plays a critical role in maintaining homeostasis and supporting cellular metabolism.

Definitions and Key Concepts

• Respiration: The process by which oxygen is taken in and carbon dioxide is expelled from the body. It includes both external (lung-level) and cellular (tissue-level) respiration.

• Cellular Respiration: The metabolic process in which cells use oxygen to produce energy (ATP) from food molecules, releasing carbon dioxide and water as byproducts.

• Homeostasis Contribution: The respiratory system maintains blood pH, regulates gas concentrations, and supports metabolic processes by ensuring adequate oxygen delivery and carbon dioxide removal.

• Respiratory Quotient (RQ): The ratio of carbon dioxide produced to oxygen consumed during metabolism. It varies with the type of macromolecule metabolized.

Respiratory Quotient Table

The respiratory quotient helps determine which macromolecules are being metabolized for energy.

Macromolecule	Respiratory Quotient
Carbohydrate	1.0
Fat	0.7
Protein	0.8
Typical American Diet	~0.8

External and Cellular Respiration

External respiration refers to gas exchange between the alveoli and blood, while cellular respiration occurs within tissue cells.

• External Respiration: Oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli.

• Cellular Respiration: Oxygen is used by cells to produce ATP; carbon dioxide is generated as a waste product.

• Equation for Cellular Respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Anatomy of the Respiratory System

• Epiglottis: Flap that prevents food from entering the trachea during swallowing.

• Bronchi: Airways branching from the trachea into the lungs (primary, secondary, tertiary bronchi).

• Terminal Bronchiole: The smallest airway before the alveoli.

• Diaphragm: Major muscle of respiration, contracts to allow lung expansion.

• Alveoli: Tiny air sacs where gas exchange occurs.

Alveolar Cells

• Type 1 Alveolar Cell: Thin cells that form the structure of the alveolar wall and facilitate gas exchange.

• Type 2 Alveolar Cell: Cells that secrete surfactant, reducing surface tension and preventing alveolar collapse.

Relationship Between Lungs, Pleural Sacs, and Thoracic Wall

• Pleural Sacs: Double-layered membranes surrounding each lung, containing pleural fluid to reduce friction.

• Thoracic Wall: The chest wall that protects and supports the lungs.

• Diaphragm: Separates the thoracic cavity from the abdominal cavity and aids in breathing.

Pressures Important to Ventilation

• Atmospheric Pressure: The pressure exerted by air outside the body, typically 760 mm Hg at sea level.

• Intrapleural Pressure: The pressure within the pleural cavity, usually slightly less than atmospheric pressure (sub-atmospheric), helping keep lungs inflated.

• Alveolar Pressure: The pressure within the alveoli, which changes during breathing to allow air movement.

Ventilation and Pressure Changes

• Inspiration: Diaphragm contracts, thoracic volume increases, alveolar pressure drops below atmospheric, air flows in.

• Expiration: Diaphragm relaxes, thoracic volume decreases, alveolar pressure rises above atmospheric, air flows out.

• Forced Expiration: Active process using abdominal and internal intercostal muscles to expel air rapidly.

Accessory Muscles of Respiration

• Accessory Muscles of Inspiration: Sternocleidomastoid, scalene muscles, and external intercostals assist in deep or labored breathing.

• Muscles of Active Expiration: Internal intercostals and abdominal muscles.

• No Major Muscles of Expiration: Normal expiration is passive due to elastic recoil of lungs and thoracic wall.

Pneumothorax

• Pneumothorax: A condition where air enters the pleural cavity, causing lung collapse due to loss of negative intrapleural pressure.

Key Equations and Concepts

• Gas Exchange Equation: $\text{O}_2 + \text{Glucose} \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{ATP}$

• Pressure Relationships: $\text{Air Flow} \propto (\text{Atmospheric Pressure} - \text{Alveolar Pressure})$

Summary Table: Pressures in Ventilation

Pressure Type	Typical Value (mm Hg)	Role
Atmospheric	760	Reference for air movement
Alveolar	Varies (759-761)	Drives inspiration/expiration
Intrapleural	~754	Keeps lungs expanded

Additional info:

• Figures in the original file illustrate the flow of gases, anatomical structures, and pressure relationships. These diagrams are essential for visualizing the mechanics of breathing and gas exchange.

• Students should be familiar with the sequence of air flow: atmosphere → alveoli → blood → tissues, and the reverse for carbon dioxide.

• Understanding pressure gradients is crucial for grasping the mechanics of ventilation.