Control of Ventilation and Respiration

Control of Ventilation

Matching Ventilation with Metabolic Demands

• Ventilation increases dramatically (15-20 fold) from rest to exercise.
• Minute ventilation can be increased by:
  • Breathing more frequently (increased respiratory rate).
  • Increasing the depth of each breath (tidal volume).
  • A combination of both.
• Ventilation increases in direct proportion to metabolic needs.
• At lower exercise intensities, increased minute ventilation is primarily achieved by increasing tidal volume (depth of breath).
• Around 60% of forced vital capacity, further increases in minute ventilation are achieved by increasing respiratory rate.

Control Mechanisms

• Many interrelated complex mechanisms exquisitely adjust breathing rate and depth to match metabolic demand.
• Control mechanisms include:
  • Chemoreceptors.
  • Stretch receptors.
  • Proprioceptors (movement receptors).
  • Changes in core temperature.
  • Changes in the chemical state of the blood.
  • Role of the motor cortex (higher brain).
• Information is collected, processed, and used to manipulate breathing rate and depth.
• All of this happens within the medulla in specialized areas called respiratory control centers.

Monitoring the Internal Environment

• The body constantly monitors its internal environment for changes.
• Intricate neural circuits gather and relay information from:
  • The brain.
  • The lungs.
  • Other sensors around the body.
• The body monitors the gaseous and chemical state of the blood, specifically:
  • Partial pressure of oxygen (PO_2).
  • Partial pressure of carbon dioxide (PCO_2).
  • Acidity of the blood (pH).

Neural and Humoral Factors

• Two main systems control ventilation adjustments:
  • Neural factors: transported by the nervous system (quick firing response).
  • Humoral factors: transported by the blood.
• Matching ventilation and oxygen delivery requires coordination of the respiratory and cardiovascular systems.

Brain Control

• The brain is divided into the cerebrum and the cerebellum.
• The cerebellum consists of the pons and the medulla.
• The medulla oblongata connects to the spinal cord.
• Within the medulla are specialized respiratory centers:
  • Inspiratory control center.
  • Expiratory control center.
• These centers coordinate to trigger inspiration, stop inspiration, and start expiration.

Afferent and Efferent Signals

• The body senses chemicals, stretches, etc., and sends signals to the brain centers.
• If the brain detects imbalances, it sends messages through:
  • Afferent nerves (sensory): Signals sent to the brain.
  • Efferent nerves (motor): Signals sent from the brain to target organs (lungs, muscles, etc.).

Sensors and Information Collection

• Numerous sensors around the body send information to the respiratory control centers.

Central Command

• The motor cortex (higher brain) can override involuntary control of respiratory centers.
• This is called the cortical feedforward loop, allowing some voluntary control (e.g., holding breath).

Pulmonary Stretch Receptors

• Located within airway smooth muscles in the lungs.
• Detect changes in lung stretch and send messages to the medulla.
• Efferent messages are sent back to slow down breathing if the lungs are stretching too much.

Proprioceptors

• Stretch receptors sensitive to movement within joints or muscles.
• Increased firing increases ventilation (e.g., during movement).

Baroreceptors

• Pressure receptors that detect changes in blood pressure.
• A drop in blood pressure can cause hypoventilation.

Respiratory Centers and Efferent Messages

• All collected information is sent to respiratory centers to control rate and depth of breathing.
• Efferent messages (motor impulses) are sent to:
  • Inspiratory center: activates inspiratory muscles (external intercostals and diaphragm).
  • Expiratory center: activates expiratory muscles (internal intercostals and abdominal muscles).

Inspiration

• Inspiratory neurons activate the diaphragm and external intercostal muscles.
• Contraction expands the thorax, increasing volume.
• According to Boyle's law, P \propto \frac{1}{V}, increased volume leads to decreased pressure, drawing air in.
• Inflation of the lungs initiates stretch receptors.

Expiration

• Stretch receptors in bronchioles send afferent signals to stop inspiration and activate the expiratory center.
• Expiratory centers send motor messages to internal intercostals and abdominal muscles.
• Expiration forces air out.

Humoral Factors

• Messages passed through the bloodstream, working in synergy with neural factors.
• Slower to activate but act as fine-tuning and a backup system.
• Monitor the chemical system in the blood and cerebrospinal fluid.

Central Chemoreceptors

• Located in the medulla, within the brain.
• Detect changes in the cerebrospinal fluid.
• Stimulated by changes in pH (acidity); increased hydrogen ion (H^+) concentration stimulates increased ventilation.
• Increased carbon dioxide (CO_2) leads to increased hydrogen ions through the buffering system:
  • CO2 + H2O \rightleftharpoons H2CO3 \rightleftharpoons H^+ + HCO_3^-
• The body is very sensitive to carbon dioxide levels.

Peripheral Chemoreceptors

• Located outside the brain, in the main arteries:
  • Carotid bodies (feeding the brain).
  • Aortic arch (blood leaving the heart).
• Monitor gases and pH in the blood.
• Increased partial pressure of carbon dioxide (PCO_2) stimulates increased ventilation.
• Increased acidity (hydrogen ion concentration) stimulates ventilation.
• Also sensitive to the partial pressure of oxygen (PO_2), but requires significant drops (less than 60 mmHg) to activate changes.
• Normal arterial PO_2 is around 100 mmHg.

Coordination

• Peripheral and central chemoreceptors, along with neural mechanisms, work together to adjust ventilation.
• Adjustments are made by changing the rate or depth of breathing.
• Ensures ventilation matches metabolic demand as closely as possible.