The document describes the mechanisms and structures involved in human respiration. During inhalation, the diaphragm contracts and flattens while the intercostal muscles raise the rib cage, both actions serving to expand the thoracic cavity and decrease internal pressure. This pressure difference causes air to flow into the lungs. During exhalation, the muscles relax and the thoracic cavity size decreases, raising pressure and forcing air out of the lungs. Key gas laws including Boyle's law and Charles' law are also discussed in relation to lung volume and pressure changes during breathing.

1. MECHANISM OF
BREATHING
Circle of Willis

2. 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

3. 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

4. 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
•

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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