Regulation of Respiration

Regulation of Respiration

• Respiration is controlled involuntarily by the brainstem (medulla oblongata and pons).
• Voluntary control from the cerebral cortex can override this involuntary control, as seen when speaking or holding one's breath.

Medullary Respiratory Centers

• The medulla oblongata contains two key neuronal groups:
  • Dorsal Respiratory Group (DRG): Controls the basic rhythm of breathing by prompting diaphragm and external intercostal muscles to contract, thus initiating inspiration. It mainly responds to signals from chemoreceptors and mechanoreceptors.
  • Ventral Respiratory Group (VRG): Activated during forced breathing (e.g., exercise). Controls both inhalation and exhalation by stimulating additional respiratory muscles.

Pontine Respiratory Centers

• Located in the pons, these centers work alongside medullary centers to regulate breathing rhythm.
  • Pneumotaxic Center: Limits inspiration and manages the transition from inspiration to expiration to prevent lung overinflation.
  • Apneustic Center: Enhances inspiration duration and depth by stimulating the DRG.

Chemical Regulation

• The most important regulators of breathing are chemoreceptors.
• Chemoreceptors monitor levels of carbon dioxide (CO2), oxygen (O2), and pH in the blood and cerebrospinal fluid (CSF).
• They adjust breathing rate and depth to maintain appropriate gas levels.

Central Chemoreceptors

• Located in the medulla oblongata.
• Sensitive to the pH of cerebrospinal fluid (influenced by blood CO2 levels).
• High CO2 levels lead to increased H+ concentration (lower pH) which stimulates the central chemoreceptors, increasing breathing rate and depth (hyperventilation) to eliminate CO2.
• Low CO2 levels lead to decreased ventilation (hypoventilation).

Peripheral Chemoreceptors

• Located in the carotid bodies and aortic bodies.
• Primarily detect blood oxygen levels (PaO2) and, to a lesser extent, CO2 and pH.
• Significant drops in oxygen levels (hypoxemia) trigger signals to the brainstem, increasing breathing rate and depth.
• Crucial in situations with low oxygen levels, such as high altitudes or lung disease.

Mechanical Control

• Pulmonary Stretch Receptors (Hering-Breuer Reflex):

  • Located in airway smooth muscles.
  • Prevent overinflation of the lungs by detecting lung stretch.
  • Send inhibitory signals to the brain stem to halt inspiration (inhibit DRG) and stimulate expiration when lungs are overstretched during deep inhalation.

• Proprioceptors:

  • Located in the muscles and joints.
  • Detect body movement, signalling an increased need for oxygen during activities such as exercise.
  • Contribute to the rise in breathing rate.

Voluntary Control

• The cerebral cortex allows for voluntary control over breathing (important for activities like speaking and singing or breath-holding).
• However the brainstem will eventually override voluntary control when CO2 levels become too high.

Chemical Feedback

• Carbon Dioxide (CO2): The most important driver of respiration; even small increases in CO2 trigger increased breathing to expel excess CO2.
• Oxygen (O2): Less important than CO2 but plays a role, and when levels fall below a critical threshold, peripheral chemoreceptors initiate increased ventilation.
• pH (Hydrogen Ion Concentration): Changes in blood pH (acidosis or alkalosis) influence breathing, e.g., increased H+ levels stimulus increased respiratory rate, blowing off CO2, and raising pH in the blood.

Other Factors Affecting Respiratory Control

• Emotions: The limbic system and hypothalamus influence breathing during emotional responses.
• Temperature: Increased body temperature, such as during fever or exercise, increases breathing rate.
• Medications: Drugs (e.g., opioids or sedatives) can depress the respiratory centers.
• Exercise: During physical activity, the breathing rate and depth increase to satisfy the high oxygen demand and remove excess CO2 produced by working muscles.

Hypoxia

• Hypoxia is a condition with insufficient oxygen supply to tissues.

Hypoxic Hypoxia

• Reduced blood oxygen pressure (PaO2) due to insufficient oxygen in the environment or poor lung function.
• Low oxygen tension (high altitudes), lung diseases (COPD, pneumonia, ARDS), and airway obstruction cause it.

Anemic Hypoxia

• Reduced blood's oxygen-carrying capacity despite normal lung function and oxygen in the environment.
  • Anemia due to reduced red blood cell count or hemoglobin levels (e.g., iron deficiency anemia, vitamin B12 deficiency), chronic diseases, or hemorrhage cause it.
  • Carbon monoxide poisoning prevents proper oxygen binding to hemoglobin.
  • Hemoglobin abnormalities (e.g., sickle cell disease, thalassemia).
  • Reduced erythropoietin (hormone stimulating red blood cell production) in chronic kidney disease.

Stagnant (Ischemic) Hypoxia

• Reduced blood flow to tissues despite normal blood oxygen levels.
  • Heart failure reduces blood pumping effectiveness, limiting circulation and oxygen delivery.
  • Shock (septic, hypovolemic, or cardiogenic shock) severely reduces blood flow and oxygen transport.
  • Blood clots or embolisms block blood vessels, causing local hypoxia.
  • Prolonged bed rest or immobility reduces circulation in certain body areas, leading to stagnation and inadequate oxygen supply.

Histotoxic Hypoxia

• Tissues are unable to use available oxygen due to cellular poisoning or damage that disrupts cellular respiration
  • Cyanide poisoning inhibits crucial cellular enzymes (e.g., cytochrome oxidase), blocking ATP production.
  • Alcohol poisoning can interfere with cell metabolism, hindering oxygen use.
  • Drugs or chemicals, severe infections (sepsis), and carbon monoxide.

Symptoms of Hypoxia

• Shortness of breath (dyspnea)
• Rapid breathing (tachypnea)
• Dizziness or lightheadedness
• Cyanosis (bluish discoloration of skin/lips)
• Confusion or impaired mental function
• Fatigue and weakness
• Loss of consciousness in severe cases

Cyanosis

• Bluish discoloration of skin and mucous membranes due to increased levels of reduced hemoglobin (above 5%).
• Central Cyanosis: involves lips, tongue and central body parts, usually caused by a systemic lack of oxygen in the blood (e.g., respiratory disorders, congenital heart disease, severe hypoxia).
• Peripheral Cyanosis: affects extremities (e.g., fingers, toes) often due to poor circulation or reduced oxygen delivery to those areas (e.g., cold exposure, peripheral vascular disease, heart failure).

Explore the mechanisms behind the regulation of respiration, focusing on the roles of the brainstem, including the medulla oblongata and pons. This quiz covers both involuntary and voluntary control of breathing as well as the functions of various respiratory centers within the brain.