Blood PCO2 & Breathing

PCO2 importance:

• Crucial for maintaining blood pH balance.

• Low PCO2, called hypocapnia, arises from hyperventilation, potentially leading to respiratory alkalosis.

• High PCO2, termed hypercapnia, occurs during hypoventilation, increasing blood acidity and possibly causing respiratory acidosis.

Oxygen Partial Pressure (PO2):

• Must decrease to approximately half of normal levels before it begins to significantly affect ventilation rates.

Breathing Regulation

Automatic Breathing:

• Primarily controlled by a rhythmicity center located in the medulla oblongata.

• This rhythmic breathing pattern is influenced by the pons and sensory feedback from the body’s chemoreceptors.

Conscious Breathing:

• Governed by voluntary control from the cerebral cortex, enabling purposeful alterations in breathing rate and depth.

Automatic Breathing Influences

Chemoreceptor Types:

• These receptors monitor both blood and cerebrospinal fluid (CSF) for pH levels, PCO2, and PO2.

• Central Chemoreceptors:

  • Located in the medulla oblongata.

  • Highly sensitive to H+ concentration changes due to rising CO2 levels in CSF.

• Peripheral Chemoreceptors:

  • Situated in the aortic and carotid bodies.

  • Directly respond to H+ changes in blood caused by elevated CO2 levels.

Central Chemoreceptors

PCO2 Increase Response:

• An increase in arterial PCO2 causes an initial rise in CSF PCO2, resulting in a decreased pH.

• This triggers increased ventilation to expel excess CO2.

• The response time is typically within minutes and accounts for 70-80% of the increase in ventilation with rising CO2 levels.

• Note that H+ ions cannot cross the blood-brain barrier.

Peripheral Chemoreceptors

Location:

• Found in the aortic arch and carotid arteries.

Rapid Response to Blood pH Changes:

• Respond quickly to decreased pH levels caused by elevated CO2, facilitating immediate adjustments in ventilation rates.

Blood Oxygen Content

Oxygen and Hemoglobin:

• A significant majority of oxygen (O2) binds to hemoglobin (Hb), which critically increases the blood’s overall O2 carrying capacity.

Oxygen Saturation:

• Determined by PO2 levels: Higher PO2 corresponds to a higher percentage of hemoglobin saturation with O2.

Total Blood O2 Content:

• Total oxygen content is calculated by adding plasma O2 with Hb-bound O2.

Oxyhemoglobin Saturation

Definition:

• Refers to the percentage of hemoglobin in the form of oxyhemoglobin compared to the total hemoglobin available.

Measurement of Lung Function:

• Typical saturation levels are around 97% in arterial blood and approximately 75% in venous blood.

Oxygen Loading & Unloading

Loading:

• Occurs in the lungs where oxygen is absorbed into the bloodstream.

Unloading:

• Takes place in tissues where oxygen is released from hemoglobin for cellular use.

Factors Affecting Affinity:

• High PO2 in the lungs favors increased loading of oxygen onto hemoglobin.

• Low PO2 and higher CO2 concentrations in the tissues favor increased unloading of oxygen.

• The bond strength between Hb and O2 is significant, with carbon monoxide interfering with this binding process.

Bohr Effect

Concept:

• Describes the physiological phenomenon where O2 delivery to muscles is enhanced during periods of physical activity.

Rightward Shift in Curve:

• Indicates decreased Hb-O2 affinity, promoting O2 unloading during conditions of lower pH, increased PCO2, and raised temperature.

Leftward Shift in Curve:

• Reflects increased Hb-O2 affinity, hindering O2 unloading seen in higher pH, lower PCO2, and reduced temperature.

2,3-DPG and Oxygen Transport

Role of 2,3-Diphosphoglycerate (2,3-DPG):

• Produced by red blood cells during glycolysis, especially in states of low oxygen availability.

• Shifts the Hb-O2 dissociation curve to the right, consequently decreasing the affinity of hemoglobin for oxygen.

Clinical Importance:

• Increased levels of 2,3-DPG are commonly associated with conditions such as high altitude adaptations and anemia.

CO2 Transport

Forms of CO2 in Blood:

• Dissolved CO2 accounts for 5-9% of total CO2 in blood.

• Approximately 20% is bound to hemoglobin, while 70% is converted to bicarbonate ions (HCO3-).

Carbonic Anhydrase Function:

• Catalyzes the conversion of carbon dioxide and water into carbonic acid (H2CO3) within red blood cells.

Chloride and Reverse Chloride Shifts

Chloride Shift in Tissues:

• Elevated CO2 levels initiate the formation of H+ and HCO3-, with H+ ions contributing to the Bohr effect.

• The bicarbonate ion (HCO3-) then diffuses out of red blood cells while chloride (Cl-) ions enter.

Reverse Chloride Shift in Lungs:

• As CO2 exits the blood, H+ and HCO3- recombine to form CO2 for expulsion.

• This causes chloride ions to diffuse out of red blood cells back into the plasma.

Clinical Conditions

Emphysema:

• Characterized by the destruction of alveoli leading to decreased gas exchange surface area.

• Often linked to smoking, which exacerbates inflammation and alveolar destruction.

Chronic Obstructive Pulmonary Disease (COPD):

• Encompasses chronic inflammation and the gradual narrowing of airways, resulting in alveolar damage.

• Frequently initiated by smoking, which promotes excess mucus production and provokes inflammation by immune cells.