Pulmonary Mechanics: Fundamentals of Gas Movement

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Questions and Answers

What is the fundamental mechanism that drives air movement into the lungs during inspiration?

Generating a pressure differential by lowering lung pressure below atmospheric pressure

What happens to the pressure inside the lungs when lung volume increases?

The pressure inside the lungs decreases

Which muscle is responsible for generating the greatest change in thoracic volume during breathing?

The diaphragm

What is the role of the accessory muscles of breathing during expiration?

They help to increase thoracic pressure and push air out of the lungs Signup and view all the answers

What is the relationship between the pleural membranes and the thoracic cage during inspiration?

The visceral pleura adheres to the lung, and the parietal pleura adheres to the thoracic cage, allowing the lung to expand Signup and view all the answers

How does the diaphragm's shape change during inspiration?

The diaphragm flattens, allowing the thoracic volume to increase Signup and view all the answers

What is the primary mechanism of expiration?

Elastic recoil of the lung and thoracic cage Signup and view all the answers

What is the effect of increasing thoracic volume on the pressure inside the lungs?

The pressure inside the lungs decreases Signup and view all the answers

What type of muscle is the diaphragm, and what characteristics does it possess?

The diaphragm is a skeletal muscle, and it has the force-generation characteristics of skeletal muscle. Signup and view all the answers

What is the role of the phrenic nerve in respiration, and what happens to the diaphragm when it is stimulated?

The phrenic nerve stimulates the diaphragm, causing it to contract and flatten, which increases thoracic volume and allows air to enter the lungs. Signup and view all the answers

How does the diaphragm's movement affect the abdominal contents during inspiration?

During inspiration, the diaphragm's descent compresses the abdominal contents, reducing abdominal volume. Signup and view all the answers

What is the role of the external intercostal muscles in expanding the thoracic cage during inspiration?

The external intercostal muscles, controlled by the intercostal nerve, contract to rise the rib cage upward and outward, increasing thoracic volume. Signup and view all the answers

What happens to the diaphragm when phrenic nerve activity stops, and how does this affect thoracic pressure?

When phrenic nerve activity stops, the diaphragm relaxes and returns to its resting dome-like position, reducing thoracic volume and increasing thoracic pressure. Signup and view all the answers

What is the mechanism by which air exits the lungs during expiration?

Air exits the lungs during expiration due to the reversal of the pressure gradient, as thoracic pressure increases above atmospheric pressure. Signup and view all the answers

What role do the scalenus muscles play in the expansion of the thoracic cage during inspiration?

The scalenus muscles stabilize the sternum and upper ribs, allowing the external intercostal muscles to expand the thoracic cage. Signup and view all the answers

What is the net result of the diaphragm's descent during inspiration, and how does this affect lung expansion?

The diaphragm's descent increases thoracic volume, reducing thoracic pressure below atmospheric pressure, which allows air to enter the lungs. Signup and view all the answers

What is the significance of recognizing the use of accessory muscles during rest in a patient?

It is highly indicative of a raised respiratory effort to cope with an underlying and probably significant problem. Signup and view all the answers

How does the recoil of the elastic tissue of the lung contribute to expiration?

It reduces lung volume and increases lung pressure above atmospheric pressure, causing air to exit the lung. Signup and view all the answers

What is the role of the internal intercostal muscles during increased ventilation?

They draw the rib cage downward to reduce thoracic volume. Signup and view all the answers

What is the relationship between the thoracic wall and the lungs?

The lungs are adhered to the inside of the thorax, so when the thoracic wall moves, the lungs follow. Signup and view all the answers

How do the accessory muscles of breathing contribute to thoracic cage expansion?

They allow for a greater thoracic expansion and thus a greater lung volume. Signup and view all the answers

What is the primary mechanism of expiration during quiet resting breathing?

Passive recoil of the elastic tissue of the lung. Signup and view all the answers

What is the role of the abdominal muscles during forced expiration?

They increase abdominal pressure and help push the diaphragm upward. Signup and view all the answers

What is the function of the pleural membranes and space?

They allow the lungs to adhere to the inside of the thorax, enabling the lungs to follow the movement of the thoracic wall. Signup and view all the answers

Study Notes

Diaphragm and Respiratory Muscles

The diaphragm is a skeletal muscle that controls a visceral organ and performs a homeostatic function.
The diaphragm can be controlled voluntarily, such as during speech.
The phrenic nerve stimulates the diaphragm, generating inspiration.
When the diaphragm contracts, it flattens out and descends toward the abdomen, increasing thoracic volume and decreasing thoracic pressure.
This decrease in thoracic pressure causes air to enter the lung.

Inspiration

The diaphragm may descend as much as 10 cm, but a descent of 1 cm is sufficient to provide tidal breathing.
The external intercostal muscles, controlled by the intercostal nerve, also increase thoracic volume by causing the rib cage to rise upward and outward.
The scalenus muscles stabilize the sternum and upper ribs during inspiration.
The fundamental mechanism of inspiration is to generate a pressure differential, where the pressure inside the lungs is lower than atmospheric pressure.

Fundamentals of Gas Movement

To get air to move into the lungs, a pressure differential must be generated, where the pressure inside the lungs is lower than atmospheric pressure.
Increasing lung volume decreases the number of molecules in the same space, resulting in lower pressure (Boyle's law).
The basis of inspiration is to lower lung pressure below atmospheric pressure, allowing air to enter the lungs.

Changing Thoracic Volume

The diaphragm generates the greatest change in thoracic volume and is the primary muscle responsible for breathing.
The diaphragm is a sheet-like muscle that separates the thoracic and abdominal cavities and forms a dome shape in the relaxed state.
The diaphragm has three sections: anterior, posterior, and central.
During periods of high ventilatory need, other muscles, such as the sternocleidomastoids, scalenes, and pectoralis minor, can contribute to expansion of the rib cage.

Expiration

Expiration is generally simpler than inspiration and relies on the recoil of the expanded lung tissue.
The elastic tissue of the lung has been expanded during inspiration, and when the inspiratory muscles relax, the lungs recoil, reducing lung volume and increasing lung pressure.
Depending on the final lung volume achieved during inspiration, recoil of the chest wall may also contribute to expiration.
During quiet resting breathing, expiration is passive, but when ventilation needs to be increased, active help is needed to increase the rate of breathing.

How the Lungs Move with the Chest Wall

The lungs and the inside of the thorax are adhered to each other, so when the thoracic wall moves, the lungs follow.
The pleural membranes and space play a crucial role in this relationship.

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Description

This chapter explains the mechanisms of air movement into and out of the lungs, covering the basics of pressure, volume, and flow.