Respiratory System Overview

• Respiration involves the exchange of gases between the atmosphere and the body's cells.
• Four key processes involved include pulmonary ventilation, alveolar gas exchange, gas transport, and systemic gas exchange.

Pulmonary Ventilation

• Pulmonary ventilation is the movement of gases between the atmosphere and the alveoli.

Alveolar Gas Exchange (External Respiration)

• Alveolar gas exchange involves the exchange of gases between the alveoli and the blood.

Gas Transport

• Gas transport is the movement of gases in the blood between the lungs and systemic cells.

Systemic Gas Exchange (Internal Respiration)

• Systemic gas exchange is the exchange of respiratory gases between the blood and the systemic cells.
• Autonomic nuclei within the brain coordinate breathing.
• Normal breathing is a rhythmic, involuntary act.

Respiratory Center

• The respiratory center of the brainstem controls breathing.
• The medullary respiratory center contains two groups.

Ventral Respiratory Group (VRG)

• The VRG is located in the anterior medulla.
• It becomes active when more forceful breathing is required.

Dorsal Respiratory Group (DRG)

• The DRG is located in the posterior medulla.
• It is responsible for the basic rhythm of breathing.

Pontine Respiratory Center

• The pontine respiratory center is located in the pons (part of the brainstem).
• It influences and modifies the activity of the medullary centers.
• The pontine respiratory group (PRG) continuously inhibits the inspiration center, smoothing out inspiration and expiration transitions.
• Chemoreceptors monitor changes in the concentrations of H+, PCO₂, and PO₂.

Central Chemoreceptors

• Central chemoreceptors located in the medulla monitor the pH of the cerebrospinal fluid (CSF).
• CSF pH changes are caused by changes in blood PCO₂.
• CO₂ diffuses from the blood to CSF where carbonic anhydrase is present.
• Carbonic anhydrase builds carbonic acid from CO₂ and water.

Peripheral Chemoreceptors

• Peripheral chemoreceptors are located in the aortic and carotid bodies.
• These are stimulated by changes in H+ or respiratory gases in the blood.
• Peripheral chemoreceptors respond to H+ produced independently of CO₂, e.g., H+ from ketoacidosis.
• Carotid chemoreceptors send signals to the respiratory center via the glossopharyngeal nerve.
• Aortic chemoreceptors send signals to the respiratory center via the vagus nerve.
• Other receptors also influence respiration.
• Irritant receptors in air passageways are stimulated by particulate matter.
• Baroreceptors in the pleurae and bronchioles respond to stretch.
• Proprioceptors of muscles and joints are stimulated by body movements.
• Blood PCO₂ is the most important stimulus affecting breathing.
• Raising blood PCO₂ by 5 mm Hg doubles the breathing rate.
• CO₂ fluctuations influence sensitive central chemoreceptors.
• CO₂ combines with water to form carbonic acid in CSF.
• CSF lacks protein buffers, so its pH change triggers reflexes.
• Blood PO₂ is not a sensitive regulator of breathing.
• Arterial oxygen must decrease from 95 to 60 mm Hg to have a major effect independent of PCO₂.
• When PO₂ drops, it causes peripheral chemoreceptors to be more sensitive to blood PCO₂.

Hering-Breuer Reflex (Inflation Reflex)

• Stretch receptors in the pleurae and airways are stimulated by lung inflation.
• Inhibitory signals to the medullary respiratory centers end inhalation and allow expiration to occur.
• Overall, this reflex acts more as a protective response.

Apnea

• Apnea is the absence of breathing.
• Apnea can occur voluntarily, such as during swallowing or holding your breath.
• Apnea can be drug-induced or result from neurological disease or trauma.
• Sleep apnea is a temporary cessation of breathing during sleep.
• Alveolar surface tension is caused by the attraction of liquid molecules to one another at a liquid-gas interface.
• The liquid coating the alveolar surface acts to reduce the alveoli to the smallest possible size.
• Surfactant, a detergent-like complex, reduces surface tension and helps prevent the alveoli from collapsing.
• Compliance is the measure of the ease with which the lungs and thorax expand.
• The greater the compliance, the easier it is for a change in pressure to cause expansion.
• Lower-than-normal compliance means the lungs and thorax are harder to expand.

Conditions that Decrease Compliance:

• Pulmonary fibrosis: deposition of inelastic fibers in the lung.
• Scar tissue or fibrosis reduces the natural resilience of the lungs.
• Pulmonary edema: Blockage of the smaller respiratory passages with mucus or fluid.
• Respiratory distress syndrome: Reduced production of surfactant
• Increased resistance to airflow, caused by airway obstructions (asthma, bronchitis, lung cancer).
• Deformities of the thoracic wall (kyphosis, scoliosis), decreasing the flexibility of the thoracic cage.

Respiratory Volumes Include:

• Tidal volume: amount of air inspired or expired with each breath; ~500 mL at rest.
• Inspiratory reserve volume: the amount that can be forcefully inspired after inspiration of the tidal volume.
• Expiratory reserve volume: the amount that can be forcefully expired after the expiration of the tidal volume.
• Residual volume: the volume of air still remaining in the respiratory passages and lungs after most forceful expiration.

Respiratory Capacities Include:

• Inspiratory capacity: tidal volume plus inspiratory reserve volume.
• Functional residual capacity: expiratory reserve volume plus residual volume.
• Vital capacity: sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume.
• Total lung capacity: sum of inspiratory and expiratory reserve volumes plus tidal volume and residual volume.
• Blood's ability to transport oxygen depends on the solubility coefficient of oxygen.
• Blood's ability to transport oxygen depends on the presence of hemoglobin.
• The iron of hemoglobin attaches oxygen.
• About 98% of O₂ in the blood is bound to hemoglobin.
• HbO₂ is oxyhemoglobin
• HHb is deoxyhemoglobin
• Carbon dioxide has three means of transport:
• As CO₂ dissolved in plasma (7%).
• As CO₂ attached to the amine group of the globin portion of hemoglobin (23%).
• As bicarbonate dissolved in plasma (70%).
• At a given temperature, the solubility of a gas in liquid depends upon:
• Partial pressure of the gas in the air.
• Solubility coefficient of the gas in the liquid.
• Gases vary in their solubility in water as follows:
  • Carbon dioxide is more soluble than oxygen.
  • Nitrogen is less soluble.
  • Gases with low solubility require larger pressure gradients to “push” the gas into the liquid.

Alveolar And Systemic Gas Exchange

• Alveolar and Systemic Gas exchange involves he gas solubility and Henry's law.
• Henry's law states at a given temperature
  • Solubility depends upon parital pressure and the gas in the air
• Gases vary in their solubility in water
  • CO2 is very soluble, N2 is half as soluble
  • Gases with low soluble require pressure to facilitate gas exchange

Hypoventilation

• Is when breathing is too slow or shalow
• O2 decreases and CO2 increases
• Can cause lethargy, cyanosis, and headache.

Hyperventilation

• Is when breathing is too high or too deep
• It increases o2 and decreases co2
• Vessels constrict, lowering blood and O2, leading to fainting, dizziness, LOC and Coma