Respiratory System During Exercise and Recovery

• Breathing rate increases in proportion to exercise intensity up to a maximum of 50-60 breaths per minute.
• Tidal volume increases initially in proportion to exercise intensity at sub-maximal intensities up to approximately 3 litres.
• Minute ventilation increases in line with exercise intensity. During sustained sub-maximal intensity exercise, minute ventilation can plateau as we reach a comfortable steady state.
• The respiratory control centre contains 2 centres: the inspiratory centre and the expiratory centre.
• The inspiratory centre stimulates inspiratory muscles to contract at rest and during exercise.
• The external intercostal muscles are stimulated by the intercostal nerve.
• The diaphragm is stimulated by the phrenic nerve.
• The expiratory centre is inactive at rest, but during exercise will stimulate additional expiratory muscles to contract: internal intercostal muscles, rectus abdominus.
• Neural Control of Inspiration at Rest:

  The inspiratory centre tells the intercostal nerve to stimulate the external intercostals this causes them to contract which pulls the rib cage up and out. The IC also tells the phrenic nerve to stimulate the diaphragm which flattens. This causes the thoracic cavity volume to increase, and the lung air pressure to decrease, causing approx 500ml of air to be sucked into the lungs.
• Neural Control of Expirationa at Rest:

  After about 2 seconds of inspiring, the inspiratory centres stimulations stops and the diaphragm and external intercostals relax. Passive expiration occurs whereby the lungs naturally recoil as the diaphragm and external intercostals relax allowing us to breath out.
• During exercise there is a greater need for O2 at the working muscles, and more waste CO2 removal needed. The sensory nerves send information to the RCC which starts a response from the IC and EC.
• Proprioceptors inform the inspiratory centre in the RCC of increased movement.
• Thermoreceptors inform the IC in the RCC of increased blood temperature.
• Chemoreceptors inform the IC in the RCC of chemical changes in the blood stream, such as increased levels of CO2 and lactic acid, and decreased O2 levels.
• The inspiratory centre responds by increasing the stimulation of the phrenic nerve to the diaphragm and intercostal nerve to the external intercostals. Stimulating additional inspiratory muscles (sternocleidomastoid and pectoralis minor). Which increases the force of contraction and therefore increases depth of breathing.
• Neural control of expiration during exercise:

  Baroreceptors known as stretch receptors, in the lungs, inform and expiratory centre about the extent of lung inflation. They detect an increase in lung inflation and inform the expiratory centre when they become excessively stretched. The inspiratory centre is inhibited and the EC stimulates additional expiratory muscles to contract to cause the forced expiration needed during exercise, these are internal intercostals, and rectus abdominus. This safety mechanism to prevent the lungs overinflating is know as Hering-Breuer reflex.
• Gases always move from an area of high pressure to low pressure.
• The partial pressure of O2 and CO2 in the air coming into the body will always be the same. However, during exercise the amount of O2 left over in the muscles and the amount of CO2 produces will be different than during rest.
• Exchange of oxygen during external respiration at rest: gases move from a high to a low partial pressure, down a diffusion gradient, the partial pressure O2 of the alveolar air is high and the partial pressure O2 of the alveolar capillaries is low, therefore the oxygen will diffuse into the blood stream.
• Exhange of oxygen during external respiration during exercise: the partial pressure O2 of alveolar air is high and the capillary blood has a lower partial pressure O2 than at rest. Gases move from a high to a lower partial pressure, down a steeper diffusion gradient, therefore more oxygen will diffuse into the blood stream faster.
• Exchange of oxygen during internal respiration at rest: gases move from a high to a low partial pressure, down a diffusion gradient, the partial pressure O2 of the blood is high and the partial pressure O2 of the muscle cell is low, therfore the oxygen will diffuse into the muscle cells.
• Exchange of oxygen during internal respiration during exercise: the partial pressure O2 of the blood is high and the partial pressure O2 of the muscle cel is lower than at rest, gases move from a high to a lower partial pressure down a steeper diffusion gradient therefore more oxygen will diffuse into the muscle cells faster.
• The diffusion gradient can increase if: there is an increase in temperature; if there is an increase in the production of carbon dioxide; if there is an increase in acidity.
• Association: the combining of oxygen with haemoglobin to form oxyhaemoglobin.
• Dissociation: the release of oxygen from haemoglobin for gaseous exchange.
• Oxyhaemoglobin dissociation curve: a graph showing the relationship between pO2 and percentage saturation of haemoglobin.
• Haemoglobin is a protein able to carry upto four oxygen molecules. The amount of 02 that associates with haemoglobin is determined by the partial pressure of O2. It readily associates with oxygen when the PPO2 is high to form oxyhaemoglobin (HbO2).
• As the PPO2 decreases, the haemoglobin more readily dissociates with oxygen, releasing O2 to the muscle tissue. The relationship is shown in the oxyhaemoglobin dissociation curve.
• When the blood reaches the resting muscle tissue, the PPO2 lowers to approximately 40mmHg.
• During exercise the muscle tissue: increases in temperature; increases production of CO2 which raises PPCO2; increases production of lactic acid which lowers pH.
• These factors make the curve shift to the right, this is known as the Bohr shift. At any given PPO2 for exercising muscle tissue, the % saturation of oxyhaemoglobin is far lower and therefore dissociation of O2 to respiring muscles is greater, therefore more O2 is available for aerobic energy production.
• As exercise intensity increases, the PPO2 in the muscle cells lowers and more oxygen dissociates from haemoglobin for diffusion into the working muscles.