BMS2031 Lecture Notes - Lecture 21: Extracellular Fluid, Alveolar Cells, Edema
31 May 2018
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Week 8. Respiratory 2&3 Pulmonary ventilation and
lung mechanics and, Principles of gas exchange
PULMONARY VENTILATION AND LUNG MECHANICS
• The hilus is the point of attachment for lungs which hang in the thoracic cavity
• The lungs have 2 pleural membranes:
o Visceral pleural sac: membrane that surrounds each lung
o Parietal pleura – membrane that lines the inside of the thoracic wall and diaphragm
The 2 membranes are separated by a thin layer of fluid which allows easy movement
• Intrapleural pressure = always negative to keep lungs held up in the chest wall
-if >0 lugs ill fall aay fro the hest all ad lugs ill ollapse a’t take i air
• Air flow depends on the volume of chest and whether diaphragm/muscles are contracting
-contraction = chest cage out and air moves in
• Increase in volume of chest = decrease in alveolar pressure compared to room pressure = air
moves from room into lungs stimulate contraction of diaphragm and intercostal muscles, once
equilibrates -> point at which pressure inside is greater inside than outside -> air moves out
-once muscles stop contracting -> chest cage goes back to normal
• How the lung behaves during ventilation:
o Boyles law
o Air always moves from high to low and is dependent on pressure differences
find more resources at oneclass.com
find more resources at oneclass.com
• Factors affecting pulmonary ventilation:
-how lugs inflate and how air gets into lungs
Airway radius
o If decreases = makes air harder to get through
o Radius decreases as you move down the pulmonary tree
o Poiseuille’s la:
Resistance =
o Double the length = 2x resistance
o Half radius = 16x resistance – much greater effect
o The longer the tube the greater the resistance
o Control of airway radius:
Vagal efferent nerves – Ach = bronchoconstriction
Sympathetic nerve – NA = bronchodilation
Catecholamines – B receptors = bronchodilation (adrenaline)
Inhaled stimuli eg. smoke/dust = reflex bronchoconstriction
o Bronchoconstriction = increased resistance = increased air flow
resistance
o Diameter of bronchioles is maintained by nerves
Lung and chest
compliance
=volume/pressure
o How easy it is for lungs to move out to take air in
o Is the stretchiness of the lungs
o If stiff = harder
o 2 opposing forces – alveoli wants to collapse but chest wall wants to
expand
1. Inspiratory muscles must overcome the resistance of the chest wall for
lung tissue and airways to inflate the lungs
2. Ease of stretching the lung depends on consistency of the lung
parenchymal tissue (alveoli)
3. Surface tension of the air-fluid interface lining the alveoli
4.
o On inhalation – collagen fibrils in parenchyma stretch and the alveoli are
pulled to expand (recoil on exhalation)
o In fibrosis – you get decreased compliance
More collagen fibrils = less compliance
More fibroblasts in interstitium = thickened area (stiff)
o Emphysema = increased compliance
o Fibrosis = decreased compliance
o Decrease compliance = decreased lung volume
Surface tension of
alveoli
-contributes to lung
compliance
o Sit on peripheral
o Lungs are held up by surfactant – usually not a problem unless you take
in water and wash it out (near drowning)
o Surfactant:
Mixture of phospholipids, proteins and ca2+
Reduces surface tension by 7-40%
Not produced until 6-7th month of gestation
-aies a’t reathe properly he or preature
Secreted by type II alveolar cells
Approximately 100ml in adult lung
o It is easier to inflate saline lungs than air filled lungs
o Elastic forces count for 1/3 of lung compliance and surface tension
accounts for remaining 2/3
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Respiratory 2&3 pulmonary ventilation and lung mechanics and, principles of gas exchange. The 2 membranes are separated by a thin layer of fluid which allows easy movement. Intrapleural pressure = always negative to keep lungs held up in the chest wall. If >0 lu(cid:374)gs (cid:449)ill fall a(cid:449)ay fro(cid:373) the (cid:272)hest (cid:449)all a(cid:374)d lu(cid:374)gs (cid:449)ill (cid:272)ollapse (cid:894)(cid:272)a(cid:374)"t take i(cid:374) air(cid:895: air flow depends on the volume of chest and whether diaphragm/muscles are contracting. Contraction = chest cage out and air moves in. Once muscles stop contracting -> chest cage goes back to normal: how the lung behaves during ventilation, boyles law, air always moves from high to low and is dependent on pressure differences, factors affecting pulmonary ventilation: How lugs inflate and how air gets into lungs. If decreases = makes air harder to get through: radius decreases as you move down the pulmonary tree, poiseuille"s la(cid:449):
