Control of Ventilation

Question 1

Explain how respiratory motor movements are affected by the central nervous system.

Answer 1

The brainstem (specifically the medulla oblongata and pons) is responsible for controlling respiratory motor movements.

The medulla contains the respiratory centers that regulate the basic rhythm and rate of breathing. The pons helps modulate the breathing pattern, coordinating inspiration and expiration.

Question 2

What are the respiratory centers located in the medulla oblongata?

Answer 2

The medulla oblongata contains two primary respiratory centers:

The dorsal respiratory group (DRG): Mainly controls inspiration by stimulating the diaphragm and external intercostal muscles.

The ventral respiratory group (VRG): Controls both inspiration and expiration, especially during forceful breathing (e.g., exercise, coughing).

Question 3

What is the role of the pons in respiratory control?

Answer 3

The pons contains two groups of neurons that modulate the respiratory rhythm:
The pontine respiratory group (PRG):

Includes the pneumotaxic center and the apneustic center.

The pneumotaxic center limits the duration of inhalation, helping regulate breathing rate.

The apneustic center stimulates prolonged inhalation, affecting the depth of breath and overall respiratory rhythm.

Question 4

How do chemoreceptors influence respiratory motor movements?

Answer 4

Chemoreceptors in the medulla and in the carotid and aortic bodies monitor the levels of oxygen (O₂), carbon dioxide (CO₂), and pH in the blood and cerebrospinal fluid.

High CO₂ levels (hypercapnia) or low pH (acidosis) stimulate the respiratory centers to increase the rate and depth of breathing to expel CO₂ and restore normal pH balance.

Question 5

How does the cerebral cortex influence respiratory motor movements?

Answer 5

The cerebral cortex can consciously control breathing, allowing for voluntary changes in breathing patterns, such as holding breath, speaking, or breathing rapidly during exercise.

However, the brainstem maintains the involuntary control of basic breathing when voluntary control is overridden.

Question 6

What is the role of the phrenic nerve in respiratory motor movements?

Answer 6

The phrenic nerve carries motor signals from the medulla oblongata to the diaphragm, causing diaphragmatic contraction during inspiration.

The phrenic nerve is essential for normal breathing and is directly influenced by the brainstem’s respiratory centers.

Question 7

How do stretch receptors in the lungs influence respiratory motor movements?

Answer 7

Stretch receptors in the lungs (located in the smooth muscles of the airways) detect lung inflation and send signals to the brainstem to prevent over-inflation.

These receptors help initiate the Hering-Breuer reflex, which inhibits further inspiration when the lungs are sufficiently inflated, contributing to the regulation of tidal volume.

Question 8

How do baroreceptors influence respiratory motor movements?

Answer 8

Baroreceptors, which detect changes in blood pressure, indirectly influence breathing.

When blood pressure decreases, such as during hypotension, respiratory rate may increase to maintain oxygenation and help stabilize blood pressure.
Conversely, increased blood pressure can slow the respiratory rate.

Question 9

What is the role of the cough reflex in respiratory motor movements?

Answer 9

The cough reflex is mediated by sensory receptors in the airways that detect irritants, such as mucus or foreign particles.

These receptors send signals to the medulla, which coordinates a rapid exhalation effort through the muscles of the abdomen and diaphragm to expel irritants from the airways.

Question 10

How does the sympathetic and parasympathetic nervous systems affect respiratory motor movements?

Answer 10

The sympathetic nervous system causes bronchodilation (via beta-2 adrenergic receptors), increasing airflow to the lungs during stress or exercise.

The parasympathetic nervous system, through the vagus nerve, causes bronchoconstriction (via muscarinic receptors), reducing airflow during rest or relaxation. These systems help modulate airway resistance in response to body demands.

Question 11

How does exercise affect respiratory motor movements and CNS control?

Answer 11

During exercise, the central nervous system (CNS) increases the rate and depth of breathing to meet the body’s demand for oxygen and to expel carbon dioxide.

The motor cortex sends signals to increase respiratory drive, and chemoreceptors respond to increased CO₂ levels, further stimulating the respiratory centers to increase ventilation.

Question 12

Describe the location of the two classes of chemoreceptors and identify the stimuli which activate them.

Answer 12

Central:

• medulla
• These respond directly to H+ (directly reflects PCO2)
• Primary ventilatory drive

Peripheral:

• carotid and aortic bodies
• respond primarily to PO2 and plasma [H+] (less so to PCO2)
• secondary ventilatory drive

Question 13

What is respiratory drive?

Answer 13

Respiratory drive refers to the neurological and chemical stimuli that regulate the rate and depth of breathing.
It is primarily controlled by the medullary respiratory centers and modulated by factors like blood gas levels (e.g., O₂ and CO₂) and pH.

Question 14

How do chemoreceptors influence respiratory drive?

Answer 14

Central chemoreceptors in the medulla oblongata monitor the pH of cerebrospinal fluid (CSF), which reflects CO₂ levels in the blood.

Peripheral chemoreceptors in the carotid bodies and aortic bodies detect changes in O₂, CO₂, and pH in the blood.

High CO₂ (hypercapnia) or low O₂ (hypoxia) stimulate both central and peripheral chemoreceptors to increase the rate and depth of breathing to restore normal gas levels.

Question 15

How does hypoxia affect respiratory drive?

Answer 15

Hypoxia (low oxygen levels in the blood) stimulates peripheral chemoreceptors in the carotid and aortic bodies, which in turn increase respiratory drive to enhance oxygen intake.

This mechanism is particularly important when oxygen levels are dangerously low, such as in high altitudes or respiratory disorders.

Question 16

How does hypercapnia (high CO₂ levels) influence respiratory drive?

Answer 16

Hypercapnia is detected by central chemoreceptors in the medulla and peripheral chemoreceptors in the carotid bodies.

It triggers an increase in the rate and depth of breathing to expel more CO₂ and restore the body’s acid-base balance.

This is the most important stimulus for regulating breathing under normal conditions.

Question 17

How does pH (acid-base balance) affect respiratory drive?

Answer 17

A decrease in blood pH (acidosis) is often caused by elevated CO₂ or the buildup of lactic acid.

The chemoreceptors respond to acidosis by stimulating the respiratory centers to increase ventilation, helping to expel CO₂ and restore normal blood pH.

Conversely, alkalosis (increased pH) can reduce respiratory drive.

Question 18

How does physical activity affect respiratory drive?

Answer 18

During exercise, the body requires more oxygen and needs to expel more CO₂.

Proprioceptors in muscles and joints send signals to the brainstem, which increases the rate and depth of breathing to meet the increased demand for gas exchange.

This is an example of voluntary and involuntary modulation of respiratory drive.

Question 19

How does pain affect respiratory drive?

Answer 19

Pain, especially acute or severe pain, can stimulate the sympathetic nervous system and affect the medullary respiratory centers.

Pain can lead to rapid shallow breathing (tachypnea), and in some cases, it can inhibit normal breathing patterns, especially with chronic pain or conditions like pleurisy.

Question 20

How do emotions (such as anxiety or stress) influence respiratory drive?

Answer 20

Emotions can affect respiratory drive through the autonomic nervous system.

Anxiety and stress often trigger sympathetic activation, leading to rapid, shallow breathing (tachypnea).

In contrast, relaxation or meditative states can slow breathing and increase its depth, often through parasympathetic activation.

Question 21

How does body position influence respiratory drive?

Answer 21

Body position can affect respiratory mechanics, including lung expansion and diaphragm movement.

For example, in the supine position, diaphragmatic movement can be restricted, and ventilation may decrease, leading to increased respiratory rate.

Conversely, upright positions promote better lung expansion and can enhance ventilation, decreasing the need for increased respiratory drive.

Question 22

How does temperature influence respiratory drive?

Answer 22

Increased body temperature (fever) typically leads to increased metabolic demands, which stimulates an increase in respiratory rate and depth to meet the body’s oxygen needs.

Cold temperatures can slow breathing, as sympathetic nervous activity decreases and the body’s demand for oxygen decreases.

Question 23

How does lung stretch receptors affect respiratory drive?

Answer 23

Stretch receptors in the lungs detect the degree of lung inflation.

When the lungs are inflated too much, these receptors activate the Hering-Breuer reflex, which inhibits further inspiration and prevents over-inflation, contributing to normal respiratory rhythm.

This feedback helps regulate the depth of breathing and prevents damage to lung tissues.

Question 24

How does blood pressure influence respiratory drive?

Answer 24

Baroreceptors, which monitor blood pressure, can influence respiratory drive.

Low blood pressure (hypotension) often leads to an increase in respiratory rate to help improve oxygen delivery to tissues and stabilize blood pressure.

High blood pressure may reduce respiratory drive or cause slower breathing rates.

Question 25

Where are the central chemoreceptors located, and what do they monitor?

Answer 25

The central chemoreceptors are located in the medulla oblongata of the brainstem. These receptors monitor the pH of the cerebrospinal fluid (CSF), which reflects the levels of carbon dioxide (PCO2) in the blood.

Question 26

How does CO₂ in the blood affect the pH of the cerebrospinal fluid (CSF)?

Answer 26

CO₂ from the blood diffuses across the blood-brain barrier into the cerebrospinal fluid (CSF). Once in the CSF, CO₂ reacts with water (H₂O) to form carbonic acid (H₂CO₃), which quickly dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻). The increase in hydrogen ions lowers the pH of the CSF (acidic condition), which is detected by the central chemoreceptors.

Question 27

What happens when the central chemoreceptors detect a decrease in pH (increase in H⁺) in the CSF?

Answer 27

When the central chemoreceptors detect a decrease in pH (indicating an increase in PCO2 in the blood), they send signals to the respiratory centers in the medulla to increase ventilation. This increase in ventilation helps to expel CO₂ from the body, lowering the PCO2 levels in the blood and restoring normal pH in the CSF.

Question 28

How does increasing ventilation help regulate arterial PCO2 and pH?

Answer 28

Increased ventilation results in faster exhalation of CO₂. This decreases arterial PCO2, which reduces the amount of CO₂ entering the CSF, thus preventing further acidification and bringing the pH of the CSF back to normal. As a result, the central chemoreceptors detect the corrected pH and reduce the stimulation for increased ventilation.

Question 29

Why do central chemoreceptors primarily respond to PCO2 rather than O₂?

Answer 29

CO₂ is more soluble in blood and CSF compared to O₂, and it has a stronger influence on pH regulation. O₂ levels are sensed mainly by peripheral chemoreceptors in the carotid bodies and aortic bodies, whereas the central chemoreceptors are much more sensitive to changes in PCO2 because of its direct effect on pH.

Question 30

How does the central chemoreceptor response to PCO2 affect breathing rate and depth?

Answer 30

When PCO2 increases (leading to a drop in pH), the central chemoreceptors signal for increased respiratory drive, which results in faster and deeper breathing. This process expels more CO₂, which lowers arterial PCO2 and returns the pH to normal. Conversely, when PCO2 decreases (and pH rises), the chemoreceptors reduce respiratory drive, slowing the rate and depth of breathing to retain CO₂.

Question 31

What is the significance of the blood-brain barrier in the regulation of PCO2 by central chemoreceptors?

Answer 31

The blood-brain barrier is relatively impermeable to bicarbonate ions (HCO₃⁻), but it is permeable to CO₂. This allows CO₂ from the blood to diffuse freely into the cerebrospinal fluid (CSF), where it can interact with water to form carbonic acid and alter the pH. The central chemoreceptors detect this pH change and regulate breathing accordingly to adjust arterial PCO2.

Question 32

How quickly do central chemoreceptors respond to changes in PCO2?

Answer 32

The central chemoreceptors respond to changes in PCO2 relatively quickly, with a response time of seconds to minutes. However, the magnitude of the response may be more pronounced with chronic changes in CO₂ (e.g., in patients with chronic obstructive pulmonary disease), where chemoreceptors become less sensitive to CO₂ over time.

Question 33

Where are peripheral chemoreceptors located?

Answer 33

Peripheral chemoreceptors are located in the carotid bodies (at the bifurcation of the carotid arteries) and the aortic bodies (in the aortic arch). They monitor arterial blood for changes in oxygen (O₂), carbon dioxide (CO₂), and pH.

Question 34

How do peripheral chemoreceptors respond to hypoxia (low O₂)?

Answer 34

Peripheral chemoreceptors primarily detect low oxygen levels (hypoxia) in the arterial blood. When O₂ levels drop below a certain threshold (typically below 60 mmHg), these chemoreceptors signal the medullary respiratory centers to increase the rate and depth of breathing to improve oxygen uptake. This response helps restore normal oxygen levels in the blood.

Question 35

How do peripheral chemoreceptors respond to hypercapnia (high CO₂) and acidosis (low pH)?

Answer 35

Peripheral chemoreceptors also detect high CO₂ levels (hypercapnia) and low pH (acidosis) in the blood. When PCO2 rises or pH drops (indicating acidosis), the chemoreceptors stimulate the respiratory centers to increase ventilation. This helps expel excess CO₂, restoring normal blood pH and reducing acidity.

Question 36

How do peripheral chemoreceptors complement the role of central chemoreceptors in regulating breathing?

Answer 36

Central chemoreceptors primarily respond to changes in PCO2 and pH in the cerebrospinal fluid, which reflects blood CO₂ levels. Peripheral chemoreceptors, however, are more sensitive to O₂ levels in the blood and are especially important when hypoxia occurs. Together, they coordinate the body's response to CO₂ retention, O₂ deprivation, and acid-base imbalances, ensuring optimal gas exchange and homeostasis.

Question 37

Why do peripheral chemoreceptors become more important during chronic hypoxia?

Answer 37

In conditions like chronic obstructive pulmonary disease (COPD), where blood oxygen levels are persistently low, the central chemoreceptors become less sensitive to CO₂ over time. As a result, the peripheral chemoreceptors become more important in detecting hypoxia and triggering the ventilatory response to increase oxygen intake. This compensatory mechanism helps maintain oxygenation when the central chemoreceptor response is impaired.

Question 38

How do peripheral chemoreceptors respond to alkalosis (high pH)?

Answer 38

During alkalosis, where blood pH is high, the peripheral chemoreceptors sense the increase in pH. In response, the peripheral chemoreceptors may reduce respiratory drive, leading to slower, more shallow breathing to retain CO₂, which helps to lower the pH back to normal.

Question 39

How do peripheral chemoreceptors contribute to the body's response to acid-base imbalances?

Answer 39

Acidosis (low pH) stimulates the peripheral chemoreceptors to increase ventilation to expel CO₂, which helps neutralize excess acid in the blood. In alkalosis (high pH), the chemoreceptors decrease ventilation to retain CO₂, which generates more carbonic acid and lowers the pH back toward normal.

Question 40

What is the role of peripheral chemoreceptors in conditions like sleep apnea or high altitude?

Answer 40

In sleep apnea, hypoxia during apneic episodes triggers peripheral chemoreceptors to signal the brain to increase breathing once normal respiration resumes. At high altitudes, where O₂ levels are lower, the peripheral chemoreceptors become more sensitive to the decrease in oxygen and trigger an increase in breathing rate to enhance oxygen uptake.

Question 41

How do peripheral chemoreceptors affect breathing in response to exercise?

Answer 41

During exercise, the body produces more CO₂ (from increased metabolism) and requires more O₂. The peripheral chemoreceptors detect the increase in CO₂ and the decrease in O₂ and stimulate an increase in the rate and depth of breathing to meet the body’s metabolic demands.

Question 42

Outline the role of the respiratory system in acid-base disturbances.

Answer 42

The **respiratory system** helps regulate **acid-base balance** by controlling **CO₂** levels, which affect blood pH. 1. **CO₂ and pH Regulation**: - CO₂ forms **carbonic acid**, which dissociates into **H⁺** and **bicarbonate (HCO₃⁻)**, affecting blood pH. - Increased CO₂ lowers pH (acidosis), while decreased CO₂ raises pH (alkalosis). 2. **Acidosis**: - **Respiratory acidosis** occurs with **CO₂ retention** (e.g., in lung diseases), leading to **increased H⁺** and lower pH. - The body compensates by increasing **ventilation** to expel CO₂. 3. **Alkalosis**: - **Respiratory alkalosis** occurs with **excessive CO₂ loss** (e.g., hyperventilation), raising pH. - The body compensates by reducing **ventilation** to retain CO₂. 4. **Compensation**: - The lungs adjust ventilation to restore pH balance, while kidneys help in the long term by adjusting **bicarbonate** levels. In summary, the respiratory system regulates pH by controlling **CO₂**, helping to correct **acidosis** and **alkalosis** through changes in ventilation.

Question 43

Explain how CO2 affects acid-base balance

Answer 43

1. **CO₂ and pH Relationship**: - **CO₂** in the blood reacts with **water (H₂O)** to form **carbonic acid**:  - Carbonic acid dissociates into **hydrogen ions (H⁺)** and **bicarbonate ions (HCO₃⁻)**. - **H⁺** ions lower the **pH**, making the blood more acidic. 2. **Impact on Acid-Base Balance**: - **Increased CO₂** (e.g., from hypoventilation or lung disease) leads to **more H⁺ ions** and **acidosis** (low pH). - **Decreased CO₂** (e.g., from hyperventilation) results in fewer **H⁺ ions**, raising the pH and causing **alkalosis** (high pH). 3. **Respiratory Regulation**: - The body adjusts **breathing** to control CO₂ levels: - **In acidosis**, the body increases ventilation to expel excess CO₂ and raise pH. - **In alkalosis**, the body decreases ventilation to retain CO₂ and lower pH. Summary: - **CO₂** affects **acid-base balance** by altering the concentration of **H⁺** in the blood. High CO₂ causes **acidosis**, while low CO₂ causes **alkalosis**. The respiratory system helps regulate CO₂ to maintain normal pH.

Question 44

Outline how the respiratory system can both create, and compensate for, acid-base disturbances.

Answer 44

1. **Creating Acid-Base Disturbances**: - **Respiratory Acidosis**: Caused by **CO₂ retention** (e.g., hypoventilation, lung diseases), increasing **H⁺ ions** and lowering pH. - **Respiratory Alkalosis**: Caused by **CO₂ loss** (e.g., hyperventilation), decreasing **H⁺ ions** and raising pH. 2. **Compensating for Disturbances**: - **Respiratory Acidosis**: The **kidneys** excrete **H⁺** and retain **bicarbonate** to neutralize excess H⁺. The respiratory system may increase ventilation to expel CO₂. - **Respiratory Alkalosis**: The **kidneys** retain **H⁺** and excrete **bicarbonate** to lower pH. The respiratory system may reduce ventilation to retain CO₂.