IB Biology Topic C3.1.15: Feedback Control of Ventilation Rate Notes

Chemoreceptors Adjust Ventilation to Keep Blood pH and Gases in Range

1. Ventilation rate is tightly regulated by feedback to keep oxygen, carbon dioxide, and blood pH at proper levels.
2. Chemoreceptors detect changes in blood chemistry and signal the respiratory centres in the brainstem, which adjust breathing by driving the diaphragm and intercostal muscles.

Carbon Dioxide from Respiration Lowers Blood pH

1. Carbon dioxide and pH
   1. Cellular respiration produces carbon dioxide as a waste product.
   2. In blood, CO2 forms carbonic acid, which releases hydrogen ions, lowering pH and making blood more acidic.
2. Exercise and increased carbon dioxide
   1. During exercise, muscles work harder and produce more CO2.
   2. This raises acidity and can push blood outside its optimal pH range.

Where the Chemoreceptors Are

1. Central chemoreceptors (major role)
   1. Located in the medulla oblongata of the brainstem.
   2. Detect pH changes by sensing CO2 in the cerebrospinal fluid.
2. Peripheral chemoreceptors (secondary role)
   1. Located in the carotid bodies and aortic bodies.
   2. Detect oxygen, CO2, and pH in arterial blood.

The Diaphragm and Intercostal Muscles Do the Breathing

1. Diaphragm: contracts to expand the chest cavity, drawing air into the lungs.
2. External intercostal muscles lift the ribcage during inhalation.
3. Internal intercostal muscles help with forced exhalation during intense activity.

The Feedback Loop: Rising CO2 Speeds Breathing Until pH Recovers

1. Detection
   1. Increased CO2 lowers blood pH.
   2. Chemoreceptors detect this and signal the respiratory centres in the brainstem.
2. Response
   1. The brainstem signals the diaphragm and intercostal muscles.
   2. They contract more often, raising the ventilation rate.
3. Correction
   1. Faster ventilation expels more CO2, reducing acidity and restoring normal pH.
4. Negative feedback
   1. Once pH returns to normal, chemoreceptors reduce their signals and breathing slows.

Exhalation Becomes Active During Exercise

Low Oxygen Provides a Backup Drive to Breathe

1. CO2 is the main driver of ventilation, but low oxygen (hypoxia) can also raise breathing rate.
2. Peripheral chemoreceptors in the carotid arteries detect low oxygen and can override CO2 signals when needed.

Ventilation Control Protects pH for Enzymes, Exercise, and Survival

1. Homeostasis: stable blood pH is critical for enzyme function and cell health.
2. Adaptability: the system lets the body meet changing demands, such as exercise or high altitude.
3. Survival: without it, blood pH could fall dangerously, causing acidosis and organ dysfunction.