Respiratory System

Question 1

describe the main pressures in the lungs at rest

Answer 1

intrapulmonary pressure
- the pressure in the alveoli
- at rest it is 0 (in respect to barometric pressure)

intrapleural pressure
- the pressure in the pleural cavity
- at rest it is negative (in respect to barometric pressure)
- always less than the intrapulmonary pressure

intrapleural must always be negative, this is due to three things
- elasticity of the lungs - lungs want to be smallest size possible so when they recoil this pulls the visceral pleura away from the parietal pleura, increasing pleural cavity volume
- surface tension - allows the alveoli to recoil
- elasticity of the chest wall - tries to pull the lungs outward, so this means the parietal pleura is being pulled outward, increasing pleural cavity volume
- increased volume decreases pressure

barometric pressure
- pressure in the atmosphere
- 760mmHg at sea level

transpulmonary pressure
- intrapulmonary minus intrapleural pressure
- positive pressure at rest - inflating pressure

transthoracic pressure
- intrapleural minus the barometric pressure
- negative pressure at rest - deflating pressure

transrespiratory pressure
- intrapulmonary minus barometric pressure
- at rest this is 0, so because there is no pressure difference there is no air flow.

Question 2

describe the structure of the respiratory system in the chest cavity

Answer 2

each lung is made up of alveoli and surrounded by a thin layer of epithelial tissue and some areolar connective tissue. this layer is the visceral pleura.

moving out, there is a hollow cavity of POTENTIAL space called the pleural cavity.

moving further out, lining the actual chest wall is the parietal pleura.

visceral and parietal are tethered together, and will rub against each other. there is fluid to prevent friction.

Question 3

describe how gravity impacts pressures in the lungs

Answer 3

when gravity is acting downward, this base of the lung is being pulled on. this pulls the base toward the diaphragm, and pulls the apex away from neck.

so at the apex, the visceral and parietal are being pulled away from each other, increasing volume and decreasing pressure

at the base, they are being brought toward each other, decreasing volume and increasing pressure.

so intrapleural pressure is more negative at the apex than at the base at rest

Question 4

describe inspiration

Answer 4

beginning of inspiration
- external intercostal muscle and diaphragm contract
- this pulls ribs and sternum out
- thoracic cavity volume increases
- intrapleural pressure decreases
- parietal and visceral pleura are tethered, so the visceral pleura will try to move out to meet parietal, this expands the lungs.
- increased lung volume decreases intrapulmonary pressure
- this makes the transrespiratory pressure negative, so air will flow from the atmosphere into the alveoli until there is pressure equilibrium again

forced inspiration
- this means there is more force being used to increase the thoracic cavity and lung volume, to make the pressures more negative and therefore lead to a greater air flow
- sternocleidomastoid contracts to pull the sternum out
- scalenes are connected to the ribs and aid in pulling them out
- pectoralis minor can aid in pulling the ribs out as well

Question 5

describe expiration

Answer 5

quiet expiration
- no muscles involved, passive process
- depends on the elastic recoil of the lungs instead
- decreased volume increases elasticity
- when we inspire, stretch receptors in the lungs and bronchi send signals to the medulla to inhibit the nerve signals that make the external intercostals and the diaphragm contract
- relaxation means the thoracic cavity volume decreases and the intrapleural pressure increases
- intrapulmonary pressure becomes positive, meaning the air will flow from the alveoli into the atmosphere until there is gas equilibrium

forced expiration
- abdominal wall muscles contract to make the thoracic cavity volume even smaller and the pressures more positive so that more air is flowing out of the lungs
- internal and external oblique, rectus abdominus and transversus abdominus contract and push the diaphragm upward