overview of respiration and respiratory mechanics (R1) Flashcards
internal respiration
- refers to intracellular mechanisms which consumes oxygen and produces carbon dioxide
- (body systems are made up of cells which need constant supply of oxygen to produce energy and function and the carbon dioxide produced by the cellular reactions must continuously be removed from our body)
- food + oxygen = energy + carbon dioxide
external respiration
refers to the sequence of events that lead to the exchange of oxygen and carbon dioxide between the external environment and the cells of the body
4 steps involved in external respiration
- ventilation (mechanical process of moving gas in and out of the lungs)
- gas exchange between alveoli and blood (the exchange of oxygen and carbon dioxide between the air in the alveoli and the blood in the pulmonary capillaries)
- gas transport in the blood (the binding and transport of oxygen and carbon dioxide in the circulating blood)
- gas exchange at the tissue level ( the exchange of oxygen and carbon dioxide between the blood in the systematic capillaries and the body cells)
3 body systems involved in external respiration
- respiratory system
- cardiovascular system
- haematology system
ventilation definition
the mechanical process of moving air between the atmosphere and alveolar sacs
air flow in relation to pressure gradients
air flows down pressure gradient from a region of high pressure to a region of low pressure
what happens to the thorax and lungs during inspiration
they expand as a result of contraction of inspiratory muscles
relationship between intra alveolar pressure and atmospheric pressure during inspiration
the intra-alveolar pressure must become less than the atmospheric pressure for air to flow into the lungs during inspiration (before inspiration the intra- alveolar pressure is equivalent to the atmospheric pressure)
boyle’s law
at any constant temperature the pressure exerted by a gas varies inversely with the volume of the gas- as the volume of a gas increases the pressure exerted by the gas decreases
what 2 forces hold the lungs to the thorax/ thoracic wall
- intrapleural fluid cohesiveness (the water molecules in the intrapleural fluid are attracted to each other and resist being pulled apart. Hence the pleural membranes tend to stick together)
- the negative intrapleural pressure (the sub-atmospheric intrapleural pressure creates a transmural pressure gradient across the lung wall and across the chest wall. So the lungs are forced to expand outwards whilst the chest is forced to squeeze inwards)
transmural pressure gradient across lung wall
= intra alveolar pressure (760mmHg) minus intra pleural pressure (756mmHg)
transmural pressure gradient across thoracic wall
= atmospheric pressure (760mmHg) minus intrapleural pressure (756mmHg)
3 pressures important in ventilation
- atmospheric pressure
- intra-alveolar pressure
- intrapleural pressure
atmospheric pressure
the pressure exerted by the weight of the gas in the atmosphere on objects on the earths surface- 760mmHg at sea level)
intra-alveolar pressure
the pressure within the alveoli - 760mmHg when equilibrated with atmospheric pressure
intra-pleural pressure
the pressure within the pleural sac- the pressure exerted outside the lungs within the thoracic cavity (usually less than the atmospheric pressure at 756mmHg)
inspiration
active process depending on muscle contraction (inspiratory muscles)
how is the volume of thorax increased during inspiration
- the volume of the thorax is increased vertically by the contraction of the diaphragm flattening out its dome shape (major inspiratory muscle supplied by phrenic nerve from C3,4,5)
- the external intercostal muscle contraction lifts the ribs and moves out the sternum (bucket handle mechanism) this increases anterior-posterior dimension of thoracic cavity (and side to side dimensions)
what effect does the increase in size of lungs during inspiration have on the intra alveolar pressure
causes intra alveolar pressure to fall, this is because air molecules become contained in a larger volume (boyles law)
what happens following a decrease in intra alveolar pressure
air enters the lungs down its pressure gradient until the intra alveolar pressure becomes equal to the atmospheric pressure
normal expiration
passive process brought about by the relaxation of inspiratory muscles, the chest wall and stretched lungs recoil to their pre-inspiratory size because of their elastic properties, the recoil of the lungs makes the intra alveolar pressure rise (because air is contained in a smaller space - boyles law), the air then leaves the lungs down its pressure gradient until the intra alveolar pressure becomes equal to the atmospheric pressure
changes in intra alveolar and intra pleural pressures during the repiratory cycle
- during inspiration, intra alveolar pressure decreases below atmospheric then increases to atmospheric and intrapleural pressure decreases below 756mmHg
- during expiration, intra alveolar pressure increases above atmospheric pressure then decreases to atmospheric pressure, and intra pleural pressure increases to 765mmHg
pneumothorax
air in the pleural space, abolishes transmural pressure gradient
-can either be traumatic pneumothorax (due to a puncture wound in the chest wall) or spontaneous pneumothorax (hole in lung) and both result in collapsed lung
what causes the lungs to recoil during expiration(gives the lungs their elastic behavior)
- elastic connective tissue in the lungs causes the whole structure to bounce back into shape
- (even more importantly) the alveolar surface tension
alveolar surface tension
attraction between water molecules at liquid air interface, which, in the alveoli this produces a force which resists the stretching of the lungs
what would happen to the alveoli if they were lined with water alone
the surface tension would be too strong so the alveoli would collapse
law of LaPlace
according to the law of Laplace, the smaller the alveoli(with radius, r) the higher tendency to collapse
- P = 2T divided by r( where P= inward collapsing pressure, T= surface tension, r= radius of bubble, if you regard the alveoli as spherical bubbles)
pulmonary surfactant
complex mixture of lipids and proteins secreted by typeII alveoli, it lowers alveolar surface tension by interspersing between the water molecules lining the alveoli, lowers the surface tension of smaller alveoli more than that of the large alveoli, therefore prevents the alveoli from collapsing and emptying their air contents into the larger alveoli
respiratory distress syndrome of the new born
developing fetal lungs are unable to synthesize surfactant until late in pregnancy therefore premature babies may not have enough pulmonary surfactant causing respiratory distress syndrome of the new born, baby has to make strenuous inspiratory efforts in an attempt to overcome the high surface tension and inflate the lungs
alveolar interdependence
another factor which keeps the alveoli open, if an alveolus starts to collapse the surrounding alveoli are stretched and then recoil exerting expanding forces in the collapsing alveolus to open it
forces that keep the alveoli open
- transmural pressure gradient
- pulmonary surfactant (which opposes alveolar surface tension)
- alveolar interdependence
forces promoting alveolar collapse
- elasticity of stretched pulmonary connective tissue fibers
- alveolar surface tension
changes in intra alverolar and intra pleural pressures during the respiratory cycle/cause of these changes/relationship between intra alveolar/intra pleural pressures and the transmural pressure gradient
- during inspiration the diaphragm and lungs contract increases the size of the lungs which causes intra alveolar pressure to fall, this is because air molecules become contained in a larger volume, a pressure gradient now exists between the atmosphere and alveoli and air flows into the lungs causing the intra alveolar pressure to rise as the air in the lungs becomes contained within a smaller space, intraalveolar pressure now exceeds atmospheric pressure in expiration and air flows out of the alveoli, decreasing the intralveolar pressure
- during inspiration the intrapleural pressure (the pressure within the pleural sac- the pressure exerted outside the lungs within the thoracic cavity) decreases as a result of rib cage expanding etc etc, however increases again during expiration due to elastic recoil (as the lungs decrease in size/recoil, the intercostals and diaphragm relax and lower the ribcage increasing the pressure exerted outwith the lungs within the thoracic cavity)
- the transmural pressure gradient across the lung wall (=intra alveolar pressure - intrapleural pressure) therefore fluctuates between inspiration and expiration due to these changes in intra alveolar/intra pleural pressure
