Regulation of Breathing

Regulation of Breathing

• Breathing is regulated by the CNS
• Voluntary breathing uses the cerebral cortex.
• Involuntary breathing uses the respiratory control centers of the Medulla Oblongata & Pons.

Respiratory Centers

• There are four respiratory centers: Motor Neurons, Medulla Oblongata, Pons, and Chemoreceptors.

Motor Neurons

• Diaphragm innervation occurs via the phrenic nerve.
• Other breathing muscles' nerves come from the thoracolumbar region of the spinal cord.

Medulla Oblongata

• The rhythmicity center of the brainstem controls breathing patterns.
• It is made of different groups of neurons.
• The Dorsal Respiratory Group (DRG) has inspiratory neurons which stimulate the phrenic nerve, initiating inhalation via I neurons.
• The Ventral Respiratory Group (VRG) has inspiratory neurons that help with inhalation through other muscles.
• VRG also has expiratory neurons (E neurons) that inhibit the phrenic nerve and diaphragm contraction, controlling exhalation.
• Activation of I and E neurons alternates to create a regular breathing rhythm.

Pons

• The pons influences medulla oblongata activity.
• The apneustic center promotes inspiration.
• The pneumotaxic center inhibits inspiration.

Chemoreceptors

• Automatic breathing is regulated by feedback chemoreceptors.
• Chemoreceptors are special sensors in the body that monitor brain fluid pH, blood pH, PCO2, & PO2.
• These sensors provide feedback to the brain, adjusting breathing to keep homeostasis.
• Central chemoreceptors are located in the medulla oblongata.
• Peripheral chemoreceptors are located in the Aortic & Carotid Bodies.

Breathing Regulation Summary

• The medulla oblongata receives chemoreceptor & neural input from aortic & carotid bodies, in addition to the apneustic & pneumotaxic centers of the Pons.
• Voluntary breathing uses direct connections from the cerebral cortex to the spinal cord via motor neurons.

Ventilation and pH

• CO2 directly affects blood pH, which is tightly regulated.
• Hypoventilation (breathing too slowly or shallowly, not expelling enough CO2) increases blood CO2 and decreases pH, resulting in more acidic blood.
• Hyperventilation (breathing too rapidly or deeply) decreases blood CO2 and increases pH, resulting in more basic blood.
• Oxygen levels do not change rapidly because of hemoglobin O2 reserves.
• Ventilation is regulated to maintain CO2 levels, and oxygen levels naturally align.

Oxygen in the Blood

• Oxygen is present in the blood in two forms.
• Oxygen can bind to hemoglobin.
• Oxygen can be a dissolved gas in plasma.

Hemoglobin

• Most blood oxygen is bound to hemoglobin, forming oxyhemoglobin.
• Each hemoglobin can carry 4 molecules of O2 with 1 per heme group.
• Hemoglobin makes up about 33% of the total content of a red blood cell.
• Each red blood cell carries over a billion O2 molecules.

Types of Hemoglobin

• Oxyhemoglobin/deoxyhemoglobin is a form of hemoglobin.
• Methemoglobin cannot bind to O2.
• Methemoglobin is an abnormal form also caused by some drugs.
• Methemoglobin cannot carry oxygen, so an increase in methemoglobin levels in the blood can reduce oxygen delivery to tissues.
• Carboxyhemoglobin is bound to carbon monoxide (CO) instead of oxygen.
• Carbon monoxide has a much higher affinity for hemoglobin than oxygen, so inhaled carbon monoxide binds to hemoglobin, leading to carbon monoxide poisoning.

Muscle Myoglobin

• Myoglobin stores oxygen in skeletal and cardiac muscles.
• Myoglobin is similar to hemoglobin, but with only 1 heme group, carrying 1 oxygen molecule.
• Myoglobin has a higher affinity for oxygen than hemoglobin and releases oxygen only when PO2 is very low.

Carbon Dioxide in the Blood

• CO2 is carried in the blood in three forms; 10% dissolved in plasma, 20% as carbaminohemoglobin, and 70% as bicarbonate ions.
• CO2 is more soluble in plasma than O2.
• Carbaminohemoglobin is CO2 attached to hemoglobin.
• Bicarbonate ions are the primary source of CO2 in the blood.

Carbonic Anhydrase

• Carbonic anhydrase is an enzyme that catalyzes the combination of CO2 & H2O into carbonic acid (H2CO3).
• Carbonic anhydrase is located inside RBCs.
• Carbonic acid builds up inside RBCs.

Acid-Base Balance of Blood

• Healthy blood pH is between 7.35-7.45.
• Lungs/ventilation regulate CO2 levels.
• Kidneys help remove excess H+ in urine to regenerate HCO3- for buffering.

Blood pH Pathology: Acidosis

• With acidosis, blood pH is below 7.35.
• Respiratory acidosis is caused by excess CO2 in the blood due to inadequate ventilation
• Reduced ventilation causes Co2 + H2O to form carbonic acid, which consequently releases H+, lowering the pH.
• Metabolic acidosis is caused by excessive acids in the body or a loss of bicarbonate (HCO3-), which helps neutralize acid.
• Metabolic acidosis results in a decreased pH.

Blood pH Pathology: Alkalosis

• With alkalosis, blood pH is above 7.45.
• Respiratory alkalosis is caused by excessive ventilation producing too little CO2 in the blood, which results in decreased carbonic acid and increased pH.
• Metabolic alkalosis occurs with an excess loss of acid or an excessive gain of bicarbonate.

Partial Compensation

• Respiratory & Metabolic blood pH pathologies like acidosis and alkalosis can have partial compensation.
• A person with metabolic acidosis will hyperventilate to "blow off" CO2, decreasing H+ and increasing pH.
• A person with metabolic alkalosis will hypoventilate or slow respiration and the build up of CO2 increases H+ and lowers pH.
• The original condition of acidosis or alkalosis persists, but with less severity than without compensatory mechanisms.