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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.