MECHANISM OF BREATHING

Circle of Willis

STRUCTURES INVOLVED IN RESPIRATION

• DIAPHRAGM – Accomplishes normal quiet breathing

• ABDOMINAL MUSCLES – Accomplishes deep heavy breathing

• Raising and lowering the rib cage cause lung to contract and expand

STRUCTURES INVOLVED IN RESPIRATION

• Muscles that depress the rib cage are muscles of expiration

• Internal intercostals

• Abdominal recti

• Muscles that raise the rib cage are muscle of inspiration: external intercostals, sternocleidomastoid muscles, serratia anterior and scaleni muscles

GAS LAWS

• Dalton's law of partial pressures states that the total pressure is the sum of the pressures that each of the gases would exert if it were present on its own in the same volume

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GAS LAWS

• Boyle's law states that the pressure exerted by a gas is inversely proportional to its volume so that

• Charles' law states that the volume occupied by a gas is directly related to the absolute temperature T

• These two laws are combined with Avogadro's law in the ideal gas law

GAS LAWS

• Where n is the number of moles of gas and R is the gas constant (8.31 J K-1 mol-1). Each mole of gas occupies 22.4 liters at standard temperature and pressure

Mechanism of breathing

• When we inhale the intercostal muscle and diaphragm contract to expand the thoracic cavity

• The diaphragm flattens and moves downward and the intercostal muscles moves the rib cage upwards and outwards

• This increase in size decreases the internal air pressure and so air from the outside (now at a higher pressure than that of the thorax ) rushes into the lungs to equilise the pressure

Mechanism of breathing

• When we exhale the diaphragm and intercostal muscles return to their resting positionthis reduces the size of the thoracic cavity.

• Thereby increasing the pressure and forcing air out of the lungs

Mechanism of breathing

• Pleural pressure -5 cm H2O

• In inspiration -7.5 cm H20

• Alveolar pressure

• Inspiration -1 cm H2O

• Expiration +1 cm H2O

• SURFACTANT reduces the work of breathing by decreasing alveolar surface tension

questions

1. The mechanism behind breathing involves

• Change the pressure in the thoracic cavity to change volume

• Using the intercostal muscles to push air into the lungs

• filling the diaphragm with air• Changing the volume of the thoracic cavity

to change the pressure

questions

• When the volume of the thoracic cavity increases, the lung expands, and air enters. What stops this process?

1. The intercostal muscles contract

2. The pressure inside the lung and outside the body reach equilibrium

3. The diaphragm relaxes

4. The ribs descend