Mechanics of Breathing Notes

Mechanics of Breathing

Respiratory Center and Control

• The respiratory center, located in the medulla oblongata, subconsciously governs breathing.

• The medulla oblongata, found in the brain stem atop the spinal cord, houses nerve receptors sensitive to carbon dioxide ($CO_2$) and hydrogen ion ($H^+$) concentrations in blood plasma.

• Elevated levels of $CO_2$ and $H^+$ are toxic byproducts of cell metabolism that require excretion.

• When $CO_2$ and $H^+$ concentrations reach critical levels, the medulla oblongata triggers nerve impulses to the diaphragm and intercostal muscles.

Inhalation

• The nerve impulses from the medulla oblongata causes the diaphragm and the intercostal muscles to contract.

• Diaphragm: When relaxed, it is bowed upwards; contraction flattens it downwards.

• Intercostal Muscles: Their contraction pivots the rib cage upwards and outwards from its dorsal attachments to the vertebral column.

• The combined action of the diaphragm and intercostal muscles increases the volume of the thoracic cavity.

• This expansion creates negative pressure (vacuum effect) within the thoracic cavity.

• Air is drawn into the lungs through the trachea due to this negative pressure.

• Inhalation is an active process, requiring ATP for muscle contractions.

Pleural Membranes

• The serosa lining of the thoracic cavity and lungs consists of thin, delicate coverings called pleural membranes.

• The interpleural space, located between these membranes, has minimal volume and contains a thin fluid layer that acts as a lubricant.

• During inhalation, rib cage movement (upwards and outwards) also moves the outer pleural membrane (lining the rib cage).

• Cohesion between the outer and inner pleural membranes pulls on the lung surface, facilitating lung expansion and aiding inhalation.

• The membrane-fluid-membrane arrangement allows the lung surfaces to slide smoothly against the thoracic cavity walls during breathing.

• The membranes seal off the thoracic cavity, ensuring the trachea as the sole air entry point.

Pneumothorax

• A puncture wound to the chest wall, even without lung damage, can pierce the outer pleural membrane.

• Air entering the interpleural space through the puncture puts pressure on the lung surface, causing it to collapse. This condition is known as pneumothorax.

Alveoli and Stretch Receptors

• Alveoli possess stretch receptors, which are nerve endings sensitive to stretch for monitoring lung inflation.

• During inhalation, as alveoli expand, stretch receptors generate nerve impulses.

• These impulses travel to the medulla oblongata, signaling alveolar expansion (stretching).

• The medulla oblongata responds by halting the impulses that cause diaphragm and intercostal muscle contraction, thus initiating exhalation.

Exhalation

• When the diaphragm relaxes, it returns to its bowed upward position.

• Relaxation of rib muscles allows gravity to pull the rib cage downwards and inwards.

• These actions increase pressure on the thoracic cavity, leading to the outward movement of air.

• Exhalation is a passive process.

Chemoreceptors and Secondary Mechanism

• The aortic arch and carotid arteries contain chemoreceptors, which are nerve receptors sensitive to blood oxygen levels.

• If blood oxygen levels are critically low, the chemoreceptors help initiate the inhalation response.

• This is a secondary mechanism; the primary trigger for inhalation is elevated $CO_2$ and $H^+$levels in plasma.

1. What part of the brain houses the respiratory center, and what is its primary function?

   • The respiratory center is housed in the medulla oblongata, and its primary function is to subconsciously govern breathing.

2. What chemical concentrations do the nerve receptors in the medulla oblongata monitor?

   • The nerve receptors in the medulla oblongata monitor the concentrations of carbon dioxide ($CO_2$) and hydrogen ions ($H^+$) in blood plasma.

3. Describe the action of the diaphragm during inhalation.

   • During inhalation, the diaphragm contracts and flattens downwards, increasing the volume of the thoracic cavity.

4. How do intercostal muscles facilitate the process of breathing?

   • The intercostal muscles contract, causing the rib cage to pivot upwards and outwards, further increasing the volume of the thoracic cavity.

5. What is the role of the pleural membranes in lung expansion?

   • The pleural membranes facilitate lung expansion by creating cohesion between the outer and inner membranes, which pulls on the lung surface during inhalation.

6. What happens during a pneumothorax?

   • In a pneumothorax, air enters the interpleural space through a puncture in the outer pleural membrane, putting pressure on the lung surface and causing it to collapse.

7. What are stretch receptors in alveoli responsible for?

   • Stretch receptors in alveoli are responsible for monitoring lung inflation and generating nerve impulses during inhalation to signal alveolar expansion.

8. Explain the process of exhalation in terms of diaphragm and rib cage movement.

   • During exhalation, the diaphragm relaxes and returns to its upward position, while the relaxation of rib muscles allows gravity to pull the rib cage downwards, increasing pressure in the thoracic cavity and pushing air out.

9. Where are chemoreceptors located, and what do they respond to?

   • Chemoreceptors are located in the aortic arch and carotid arteries, and they respond to blood oxygen levels.

10. What is the primary trigger for inhalation in the respiratory system?

    • The primary trigger for inhalation is elevated levels of carbon dioxide ($CO_2$) and hydrogen ions ($H^+$) in plasma.