Ventilation and Respiration During Exercise

Minute Ventilation and Exercise

• During exertion, ventilation increases through adjustments in tidal volume and respiratory rate.
• The body regulates these adjustments based on its needs.

Respiratory Control Centers

• Medulla: Contains ventral (VRG), dorsal (DRG), and lateral respiratory centers that coordinate ventilation.
• Pons: Contains pneumataxic and apneustic centers working with medullary centers.

Voluntary Control

• Cerebral Cortex: Specifically the primary motor cortex near the central sulcus provides voluntary control over breathing.
• Signals from the primary motor cortex travel through the respiratory centers in the medulla to the spinal cord, then to somatic nerves, and finally to the muscles.
• As frequency of Action Potentials increase, breathing frequency and depth also increases.

Muscles for Inspiration

• VRG and DRG stimulate and send messages down through the phrenic nerve.
• It then innervates the external intercostal muscles and the diaphragm which are muscles for inspiration.

Impact on Partial Pressures

• Normal partial pressure of oxygen in the lungs: 100-104 mmHg in alveoli, 40 mmHg in pulmonary arteries.
• Normal partial pressure of carbon dioxide: 40 mmHg in alveoli, 45 mmHg in pulmonary arteries.
• Increased ventilation drives carbon dioxide out of arteries toward the alveoli, facilitating gas exchange.
• The shunt picks up oxygen into red blood cells. The partial pressure rises to 100 mmHg so it is ready to deliver O2 to the muscles/organs that need it.

Hyperpnea

• During exercise, arterial partial pressure of oxygen and carbon dioxide remains stable, a condition called hyperpnea.

Sensory Receptors

• Activated sensory receptors in muscles and tissues:
  • Muscle spindles: In muscles; detect position and stretch.
  • Golgi tendon organs: In tendons; detect changes in stretch.
  • Joint kinesthetic receptors: In joints; detect changes in movement.
• These proprioceptors send action potentials via sensory afferent fibers to the spinal cord (dorsal root ganglion), then to the medulla, thalamus, and somatosensory cortex.
• The messages then simulate VRG which then passes action potentials down the phrenic nerve to the diaphragm and the intercostals muscles. This increases ventilation, hyperpnea, and the cycle continues.

Cardiac Output and Blood Pressure

• Heart rate and cardiac output increase (from ~5 L/min at rest to >30 L/min during exercise) via increased contractility and rate.
• Blood pressure also increases.

Ventilation-Perfusion Ratio

• Lungs don't have uniform perfusion and ventilation.
• Ventilation and perfusion relationships:
  • Highest point: Low perfusion.
  • Middle section: Normal.
  • Lower part: Highest perfusion.
• Increased ventilation expands lungs, making it more uniform; increased cardiac output from the right ventricle normalizes perfusion across all zones and increases oxygenation.

Oxygen and Carbon Dioxide Exchange During Exercise

• As blood travels to tissues, oxygen is released, reducing its availability in arterial blood.
• Oxygen partial pressure goes from 100 to 60 mmHg to drive the exchange of oxygen.
• With moderate exercise, as the amount of oxygen in the muscles lower because of the use of oxygen to create ATP, carbon dioxide production increases.

Chemoreceptors and Lactic Acid

• Increased CO2 combines with water to form carbonic acid, dissociating into lactic acid, which lowers pH.
• This activates chemoreceptors in the carotid bodies and aortic arch.
• These receptors send impulses via cranial nerves to stimulate the VRG and DRG, increasing respiration frequency.

Oxygen Debt

• Anaerobic respiration occurs with an increase in blood pH when switching from aerobic. Lactic acid is broken down in the liver into pyruvate and enters the Krebs cycle.
• Oxygen debt refers to the need for more oxygen during this transition to normalize pH.

Gas Exchange in Alveoli

• Fresh inspired gas mixes with existing air in the alveoli, creating a pressure differential that drives oxygen into the blood and carbon dioxide out.
• The partial pressure difference between the environment and the gas partial pressures drives oxygen into the alveoli which goes into the blood. The CO2 then diffuses out of the blood and back out into the environment.
• This is mainly a passive process based on diffusion principles but is also influenced by structural and physiological features of the pulmonary system.

Fick's Law of Diffusion

• Fick's Law describes gas transfer rate, but is not to be confused with Fick's principle on VO2.
• It says that the rate of gas transfer is proportional to surface area and partial pressure differences and inversly proportional to membrane thickness.
• Rate of gas transfer depends on gas solubility, which is related to molecular weight; carbon dioxide diffuses about 20 times faster than oxygen due to higher solubility.

Lung Surface Area

• The rate of gas transfer is proportional to lung surgace area. The lungs have a surface area of 50-100 m^2 for gas exchange.

Alveolar Capillary Barrier

• Minimal alveolar capillary barrier (0.3 micrometers).
• Only 2 cells between the alveoli and the blood in the capillaries.

Adaptations to Exercise

• The body compensates for reduced transit time of red blood cells by increasing blood volume through pulmonary capillaries (almost triples volume during maximal exercise).
• Alveoli and incappilary partial pressure of oxygen remain remain almost exactly the same as resting values.

Pulmonary Circulation Pressures

• At rest, pulmonary circulation maintains low pressure (~12 mmHg, < 20 mmHg) compared to systemic circulation (~120 mmHg).
• Pulmonary circuit pressures can almost double during exercise.
• There is less surface area and also less resistance within the lungs compared to the rest of the body.
• The lungs must manage the entire cardiac output, maintaining low pressure to prevent fluid buildup in alveoli.

Lung Adaptations

• Low resistance: The lungs have relatively low resistance in pulmonary vasculature, allowing for 5-6 fold increase in blood flow.
• Recruitment of vessels: The lungs recruit previously unused vessels and increase the caliber of existing vessels, which increases the lung surgace area to compensate for the increased blood flow in the lungs.
• Lymphatic drainage: Lymphatic drainage removes excess fluid from the interstitial spaces of the lungs, preventing fluid buildup in alveoli and preserving the partial pressure differential.

Ventilation-Perfusion Matching

• At rest, ventilation and perfusion are well-matched in healthy lungs.
• Ventilation increases slowly from the top to the bottom of the lungs, while flood flow increase more rapidly.
• Some blood bypasses alveoli gas exchange (1-2% shunt), causing a minor drop in arterial oxygen partial pressure.
• During exercise, ventilation-perfusion distribution becomes slightly less uniform, causing higher gradient, leading to increased gas exchange.