Respiratory System Lecture Notes

Respiratory System

Overview

• The respiratory system's primary goal is to supply oxygen to the body's tissues and remove carbon dioxide.
• This process involves four key functions:
  1. Pulmonary ventilation: The movement of air between the atmosphere and the lung alveoli.
  2. Diffusion: The exchange of oxygen and carbon dioxide between the alveoli and the blood.
  3. Transport: The transport of oxygen and carbon dioxide to and from the body's tissue cells.
  4. Regulation of respiration: Controlling the overall respiratory process.

Functions of the Respiratory System

• Maintain constant blood gas levels.
• Aid in water loss and heat elimination through humidification and warming of inspired air.
• Enhance venous return.
• Control acid-base balance within the body.
• Enable speech, singing, and other vocalizations.
• Defend against foreign substances.
• Modify, activate, or inactivate certain substances.
• Facilitate the sense of smell.

Internal vs. External Respiration

• Internal (cellular) respiration: Refers to the metabolic processes within mitochondria where oxygen is used and carbon dioxide is produced during energy derivation from nutrient molecules.
• External respiration: The entire sequence of events involved in oxygen and carbon dioxide exchange between the external environment and the body's cells.
  1. Ventilation
  2. Gas exchange between alveoli and blood
  3. Gas transport
  4. Gas exchange between tissues and blood

Ventilation

• Boyle's Law: P1V1 = P2V2 - This underpins the mechanics of breathing.
• Atmospheric (barometric) pressure: Pressure exerted by the weight of air in the atmosphere. At sea level, it is approximately 760 mm Hg.
• Intra-alveolar pressure: Pressure within the alveoli.
• Intrapleural pressure: Pressure within the pleural sac, which is typically less than atmospheric pressure, averaging 756 mm Hg at rest.

Mechanics of Breathing

• Inspiration (Inhalation): Always an active process, requiring skeletal muscle contraction.
• Expiration (Exhalation): Can be passive or active.
• Respiratory Muscles:
  • Diaphragm: Primary muscle for inspiration.
  • External intercostal muscles: Elevate the rib cage during inspiration.
  • Accessory respiratory muscles: Used during forceful or increased respiration (e.g., sternocleidomastoid, scalenus).

Inspiration Process

• Diaphragm contracts, increasing the vertical dimension of the thoracic cavity.
• External intercostal muscles contract, elevating the ribs and widening the thoracic cavity.

Expiration Process

• Passive expiration involves relaxation of inspiratory muscles.
• Active expiration involves contraction of internal intercostal and abdominal muscles to further reduce the size of the thoracic cavity.

Gas Exchange Between Alveoli and Blood

• Alveoli are thin-walled air sacs surrounded by pulmonary capillaries, facilitating gas exchange.
• Type I alveolar cells: Form the thin walls of the alveoli.
• Type II alveolar cells: Secrete pulmonary surfactant, which aids in lung expansion by reducing surface tension.

Role of Surfactant

• Alveolar walls are coated with a thin film of water, creating surface tension due to the attraction between water molecules.
• Surfactant, a complex mixture of lipids and proteins, intersperses between water molecules to lower alveolar surface tension, reducing the tendency for alveoli to collapse.

Fick's Law of Diffusion

• The rate of diffusion is inversely proportional to the distance through which diffusion occurs. Therefore, the short distance in the lungs optimizes gas exchange.

Transport of Oxygen

• Approximately 97% of oxygen is transported in combination with hemoglobin (oxyhemoglobin, Hb-O_2).
• The remaining 3% is dissolved in the plasma.
• Each hemoglobin molecule can bind up to four oxygen molecules.

Transport of Carbon Dioxide

1. Dissolved form (7%)
2. Bound to hemoglobin (Carbaminohemoglobin) (23%)
3. In the Bicarbonate ion form (HCO_3 -) (70%)

• Reaction: CO2 + H2O \rightleftharpoons H2CO3 (carbonic acid) \rightleftharpoons H^+ + HCO_3^- (catalyzed by carbonic anhydrase in red blood cells)

Control of Respiration

• Respiratory centers in the brainstem establish a rhythmic breathing pattern.
• This rhythm is generated by cyclic neural activity to the respiratory muscles, originating in the brain, not the lungs or respiratory muscles themselves.

Respiratory Control System

• Central Controller: Pons, medulla, and other brain regions.
• Sensors: Chemoreceptors, lung receptors, and other receptors.
• Effectors: Respiratory muscles.

Brainstem Centers

1. Medulla Oblongata:
   • Dorsal respiratory group (DRG): Primarily associated with inspiration.
   • Ventral respiratory group (VRG): Primarily associated with expiration.
   • Pre-Botzinger Complex: Thought to be the pattern generator (ventral medulla oblongata).
2. Pons:
   • Apneustic center: Excitatory function, prolongs inspiration.
   • Pneumotaxic center: Inhibits inspiration.

Other Brain Regions

1. Cortex: Allows for voluntary control of breathing.
2. Limbic system and hypothalamus: Influence respiration based on emotional states.

Medullary Respiratory Center

• Located in the brainstem, it generates the rhythmic pattern of breathing.
• Consists of neuronal cell bodies that provide output to the respiratory muscles.

Aspects of Ventilation Control

• Rhythmic cycling between inspiration and expiration.
• Regulation of ventilation magnitude (respiratory rate and tidal volume).
• The pre-Bötzinger complex drives inspiratory neurons in the dorsal respiratory group (DRG).

Pulmonary Capacities

• Tidal volume (resting): Amount of air moved in/out of lungs during a single respiratory cycle under resting conditions.
• Inspiratory reserve volume (IRV): Additional air that can be inhaled beyond tidal volume.
• Expiratory reserve volume (ERV): Additional air that can be exhaled after a normal respiratory cycle.
• Residual volume: Air remaining in lungs after maximal exhalation (approximately 1200 ml in males; 1100 ml in females).
• Minimal volume: Air left if the lungs collapsed.
• Inspiratory capacity: Tidal volume + inspiratory reserve volume.
• Vital capacity: Maximum amount of air that can be moved in/out of lungs during forced exhalation and inhalation.
• Total lung capacity: Total volume of lungs = vital capacity + residual volume (approximately 6000 ml in males; 4200 ml in females).