RGI_1_Respiratory_System_Respiratory_Pathways_Mechanism_of_ventilation_25-6

Learning Objectives

• Outline the respiratory requirements of homeotherm animals.

• Discuss the respiratory pathways of external respiration.

• Describe alveolar ultrastructure and its functional significance.

• Briefly discuss respiratory distress syndrome in neonates.

• Describe the structure of the thoracic cavity.

• Differentiate between alveolar and intrapleural pressures.

• Discuss the mechanism of ventilation.

• Describe the respiratory centre and its control of respiration.

• Discuss the Hering-Breuer reflex and humoral regulation.

Respiratory System Overview

Function of the Respiratory System

• Key functions:

  • Supply oxygen to the body.

  • Remove carbon dioxide from the body.

Definition of Respiration

• Exchange of gases between an organism and its environment.

Aerobic Energy Production

• Aerobic respiration: Highly effective in producing ATP from glucose but requires a steady supply of O2.

• Homeotherms (warm-blooded animals) have high O2 requirements.

• Continuous supply of oxygen is essential for cellular respiration.

• Oxygen requirement is an index of metabolic activity.

Functions of the Respiratory System

• Enables inhalation and exhalation.

• Facilitates speaking and smelling.

• Warms and moistens inhaled air to optimal levels.

• Protects airways from harmful substances.

• Delivers oxygen to body cells.

• Removes waste gases, including carbon dioxide (gas exchange).

Stages of Gas Exchange

1. External respiration: Transfer of O2 from the atmosphere into the blood.

2. Gas transport: Transport of O2 and CO2 in the blood.

3. Internal respiration: Cellular respiration, involving O2 uptake and CO2 production within cells.

Respiratory Pathways - External Respiration

Airflow Pathway

• Sequence of structures:

  • Nostrils → Nasal cavities → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli.

Conducting Passages

• Upper Respiratory Tract: Nasal Cavity, Pharynx, Larynx.

• Lower Respiratory Tract: Trachea, Primary Bronchi.

Respiratory Pathways - Upper Respiratory Tract

• Includes nasal passages, Eustachian tubes, middle ear and mastoid cavities.

• Pharynx connects with the larynx, which contains vocal cords and is supported by cartilage.

• Epiglottis: Flap that prevents food and liquid from entering the larynx during swallowing.

Respiratory Tract Infections

• Common form of infection (often viral but can be bacterial).

• Symptoms vary from mild to severe, especially in vulnerable populations (young children, elderly, immunosuppressed).

• Common infections include the common cold, sore throat, earache, and blocked sinuses.

Respiratory Pathways - Lower Respiratory Tract

• Trachea: Single tube branching into left and right bronchi; reinforced by rings of hyaline cartilage.

• Further branches into secondary bronchi and bronchioles; ciliated epithelium lines these passages.

• Mucociliary Escalator: Cilia beat upwards to push mucus secreted by goblet cells, aided by the cough reflex.

Alveolar Structure and Function

• Alveoli are sites for oxygen and carbon dioxide exchange, lined by thin epithelial cells.

• Gases diffuse freely through the walls into surrounding capillaries.

Alveolar Ultrastructure

• Composed of thin epithelial layers for gas diffusion.

Surfactant Function

• Surfactant reduces surface tension to prevent alveoli collapse, produced by type II pneumocytes.

Respiratory Distress Syndrome in Premature Infants

• Premature infants (under 32 weeks gestation) may not secrete adequate surfactant.

• Insufficient type II pneumocytes lead to difficulties in lung expansion and atelectasis.

• Management includes mechanical ventilation, oxygen therapy, and surfactant administration.

Thoracic Cavity and Lung Structure

• Lungs are large, elastic organs in the thoracic cavity; right lung has 3 lobes, left lung has 2 (smaller for heart space).

• Pleura: Two layers (visceral covering lungs, parietal lining thoracic cavity) with fluid for lubrication.

• Intrapleural pressure: Negative pressure helps keep lungs inflated.

Breathing and Pressure Regulation

• Key pressures:

  1. Atmospheric pressure.

  2. Intra-alveolar pressure (equalizes with atmospheric pressure).

  3. Intra-pleural pressure (always negative).

• Air flows from high to low pressure.

Mechanism of Ventilation

Inhalation

• Active process; thoracic volume increases via diaphragm and intercostal muscles.

• Diaphragm contracts, flattening and lowering.

• Intrapulmonary pressure falls, causing air inflow.

Exhalation

• Passive process under resting conditions, mainly due to elastic recoil.

• Diaphragm and intercostal muscles relax; chest volume decreases, raising pressure to force air out.

Respiratory Center Control

Neuronal Groups in Brain Stem

• Medulla: Dorsal respiratory group (inhalation stimulation), Ventral respiratory group (exhalation stimulation).

• Pons: Pneumotaxic center (controls breath length), Apneustic center (stimulates inhalation).

Chemical Regulation of Respiration

• Controlled by medulla and pons responding to CO2 and O2 levels.

• Increased CO2 and H+ levels excite the inspiratory area, enhancing respiratory rate.

Hering-Breuer Reflex

• Prevents lung over-inflation via stretch receptors in lung tissue that send inhibitory signals to the brain.