Functional Anatomy of the Lungs and Physiology of Breathing

Introduction

1. Functional anatomy of lungs

A. Location, shape, and coverings

• The lungs lie on either side of the mediastinum within the thoracic cavity.
• Each lung is enclosed by a double-layered pleura:
  • Visceral pleura covers the lung surface.
  • Parietal pleura lines the thoracic wall, diaphragm, and mediastinum.
• The thin pleural fluid layer permits smooth movement of lungs during breathing and helps maintain lung expansion through negative intrapleural pressure.

B. Lobes and fissures

C. Bronchial tree

• The right main bronchus is wider, shorter and more vertical. Therefore, inhaled foreign bodies commonly enter the right bronchus.
• Each bronchopulmonary segment has its own segmental bronchus and branch of pulmonary artery. This allows surgical removal of a segment if necessary.

D. Conducting and respiratory zones

1. Conducting zone
   • Extends from nose to terminal bronchioles.
   • Conducts, warms, humidifies and filters inspired air.
   • Does not take part in gas exchange, hence called anatomical dead space.
2. Respiratory zone
   • Includes respiratory bronchioles, alveolar ducts and alveoli.
   • It is the site of gaseous exchange.

E. Alveoli

• Alveoli are thin-walled air sacs providing a very large surface area for gas exchange.
• Main cell types:
  • Type I pneumocytes: thin squamous cells forming most of the alveolar surface; permit diffusion of gases.
  • Type II pneumocytes: secrete pulmonary surfactant.
  • Alveolar macrophages: remove dust, microbes and particulate matter.

F. Blood supply

• Pulmonary circulation: pulmonary arteries carry deoxygenated blood from the right ventricle to alveolar capillaries; pulmonary veins carry oxygenated blood to the left atrium.
• Bronchial circulation: supplies oxygenated blood to lung tissues and bronchi.

2. Physiology of breathing

1. Pulmonary ventilation
2. Diffusion of gases
3. Perfusion and ventilation-perfusion matching
4. Transport of gases in blood
5. Regulation of respiration

A. Pulmonary ventilation

Inspiration

• Normally an active process.
• The diaphragm contracts and descends.
• External intercostal muscles contract and elevate the ribs.
• Thoracic volume increases.
• Intrapleural pressure becomes more negative.
• Alveolar pressure falls below atmospheric pressure.
• Air flows into the lungs.

Expiration

• During quiet breathing, expiration is mainly passive.
• Diaphragm and external intercostal muscles relax.
• Elastic recoil of lungs and chest wall reduces thoracic volume.
• Alveolar pressure becomes greater than atmospheric pressure.
• Air flows out.

B. Pressures responsible for lung expansion

• Atmospheric pressure: pressure of air outside the body.
• Alveolar pressure: pressure inside alveoli.
• Intrapleural pressure: pressure in pleural cavity; normally negative.
• Transpulmonary pressure: difference between alveolar and intrapleural pressures. It keeps the lungs expanded.

C. Lung volumes

• Tidal volume (TV): air inspired or expired in a normal breath, approximately 500 mL.
• Inspiratory reserve volume (IRV): extra air inspired after normal inspiration.
• Expiratory reserve volume (ERV): extra air expired after normal expiration.
• Residual volume (RV): air remaining after maximum forced expiration.
• Vital capacity (VC): maximum air expired after maximum inspiration.
• Total lung capacity (TLC): total air in lungs after maximum inspiration.

D. Alveolar gas exchange

• Alveolar epithelium
• Fused basement membranes
• Capillary endothelium
• Oxygen diffuses from alveoli, where its partial pressure is high, into pulmonary capillary blood.
• Carbon dioxide diffuses from blood, where its partial pressure is relatively high, into alveoli for expiration.
• Carbon dioxide diffuses much more readily than oxygen because it is more soluble.

1. Adequate alveolar ventilation
2. Adequate pulmonary capillary perfusion
3. A thin diffusion barrier
4. Proper matching of ventilation and perfusion, called the V/Q ratio

E. Transport of oxygen and carbon dioxide

• About 98% of oxygen is carried bound to haemoglobin as oxyhaemoglobin.
• A small amount is dissolved in plasma.
• Carbon dioxide is transported:
  • Mainly as bicarbonate ions
  • Partly bound to haemoglobin as carbaminohaemoglobin
  • Partly dissolved in plasma

F. Control of breathing

• Medulla: produces the basic respiratory rhythm.
• Pons: modifies rate and depth of breathing.

• Central chemoreceptors respond mainly to increased carbon dioxide and hydrogen ion concentration in cerebrospinal fluid.
• Peripheral chemoreceptors in carotid and aortic bodies respond to low arterial oxygen, increased carbon dioxide and acidosis.

Conclusion