Gas Exhange, Transport and Breathing Control
Changes in Lung VOL/P in Inspiration and Expiration
Before Inspiration:
- Pavleoli = Patmosphere, no flow
Inspiration:
- pleural pressure decreases due to inspiratory muscle contraction
- alveolar pressure decreases due to decompression
- establishes pressure gradient from atmosphere to alveoli
End of Inspiration:
- inspiration ends when contraction of inspiratory muscles decreases
- allows lung recoil pressure to catch up and equal pleural pressure
- Palveoli = Patmosphere + flow stops
Onset of Expiration:
- starts when inspiratory muscles stop contracting
- lung recoil presure greater than pleural pressure = + Palveolar and expiratory flow
Pressure, Flow and Vol Change During Respiratory Cycle
1.
- before inspiration, Ppl equal and opp to lung recoil pressure
- Palv is 0, no flow
2.
- when muscles contract, Ppl and Palv decrease
3.
- Inspiratory muscles start to relax at end of inspiration
- lung recoil pressure catches up to Ppl and Palv returns
- more negative Ppl, higher lung vol
4.
- In expiration, Ppl and Palv increases, air forced out of lungs
Airway Resistance and Airflow
- Determined of resistance to airflow is radius of conducting airway
- ANS controls contraction of smooth muscle in walls of bronchioles
Dilation by adrenergic system:
Directly: NE
Indrectly: E
Alveolar Stability
- magnitude of collapsing pressure directly proportional to surface tension and radius
1. pulmonary surfactant
2. alveolar independence
Pulmonary Surfactant
- mix of lipids and proteins secreted by type 2 alv cells
- lowers alveolar surface tension
1. reduces work of lungs
2. reduces recoil pressure of smaller alveoli
Alveolar Interdependence
- when alveolus in a group of interconnected alveoli starts to collapse, surrounding alveoli are stretched by collapsing alveolus
- neighboring alveoli recoil, they pull outward on collapsing alveolus
Ficks Law of Diffusion in Gas Exchange
O2 and CO2 are dependent on:
1. permeability of the barrier
2. surface area of the lung
3. pressure gradient
4. membrane thickness
Effect of Diffusion Coefficient on Gas Exchange
- rate of gas transfer directly proportional to the diffusion coefficient
- Coe for CO2 is 20x O2, more soluble in tissue
Effect of Partial Pressure Gradient on Gas Exchange
- exchange across systemic capillaries occurs down partial pressure gradients
- By equilibration in the alveoli, O2 in systemic capillaries is high Ppartial compared to tissue cells
Diffusion at Cellular Level
1. cells use O2 from metabolism, creating a low PO2
2. Cells make CO2 from metabolism creating a high PCO2
3. Creates concentration gradient for CO2 and O2 to be exchanged at lvl of tissue
Diffusion At Alveoli
1. venous blood entering the capillary has low PO2 and high PCO2
2. Alveoli has high PO2 and low PCO2
- O2 moves into capillary in exchange for CO2
Effect of SA and Membrane Thickness on Gas Exchange
- poor gas exchange occurs when the thickness is pathologically increased between air and blood
- thickness increases = gas transfer decreases
- emphysema, pulm oedema, pulm fibrosis, pneumonia
O2 and Hypoxic Vasoconstriction
In the lungs:
- Oxygen delivered by ventilation (airflow) = like the faucet
- Oxygen taken up by blood (perfusion) = like the drain
- Alveolar PO₂ (oxygen level in air sacs) = like the water level
If oxygen in a lung region drops (low alveolar PO₂ → hypoxia):
- The blood vessels in that area constrict (vasoconstriction)
- This reduces blood flow to that poorly ventilated region
- Blood is redirected to better-ventilated areas
Hemoglobin
- 4 globular protein chains wrapped around iron heme group
- each heme group binds to 1 O2 = 4 o2/heme
- hemoglobin is the o2 carrying protein in RBC
PO2 and Hemoglobin Saturation
- more than 98% of O2 in blood is bound to hemoglobin in RBC
- less than 2% is dissolved in plasma
Oxygen Bound to Hemoglobin
- hemoglobin combines with o2 as o2 diffuses from alveoli into pulm capillaries
- approx 2% o2 dissolved in plasma
At Alveolar-capillary interface:
- more free o2 dissolves in capillaries
- increased free o2 stresses system, more o2 binds
At capillary Tissue interface:
- o2 taken up by tissues, less free o2 in blood
- more o2 offloaded to tissues
Hemoglobin Binding Affinity
Binding Affinity
- the strength of interaction between 2 molecules reversible interact
Red line
- binding between Hb and O2
- high binding high Po2 (lungs)
- low binding low Po2 (tissues)
Blue line
- Hb binding affinity can change
- Right shift = decrease affinity
- Left Shift = increase affinity
Factors that Decrease Hb-O2 Affinity
1) increased temp
2) increased 2,3-bisphosphoglycerate prod
3) increased PCO2 from tissue to systemic capillaries
4) inceased H+ prod (decrease pH)
BPG and Temp Binding Affinity
BPG
- created on chronic hypoxia in RBC
- More 2,3-BPG causes O2 offloading to tissues
- helps combat chronic hypoxia
Temp
- temp increases, Hb less likely to bind to O2
- more likely to offload tissues
- more O2 supplied to working muscle
PCO2 and Blood pH Binding Affinity
PCO2:
- increased in blood
- preferentially binds to Hb
- shifts curve to right
pH:
- increased acidity of blood
- anything the creates H+
- O2 offloads to tissues easier
- lactic acid
CO2 Transport
1) physically dissolved in plasma (7%)
2) chemically bound to Hb (23%)
3) transported as bicarbonate ions (70%)
- CO2 combines with water to form carbonic acid
- Carbonic acid dissociates into H+ and bicarbonate
Regulation of Ventilation
- breathing occurs unconsciously
- skeletal muscles that control breathing dont contract spontaneously
- breating controlled both autmaticcaly and voluntarily to a point
- CO2, O2, H+ are sensed by chemoreceptors in the brain and periphery
Dorsal Respiratory Group
- Inspiratory centre
- phrenic and intercostal nerve stim of diaphragm and intercostals
- gens rhythmic cycle
- input from vagal and glossopharyngeal nerves
Ventral Respiratory Group
- involved in forced inspiration and expiration
Pontine Respiratory Group
- Constantly inhibits medullary centres to smooth out inspiration
Hypoxic Vasoconstriction
- Diverts blood flow away from poorly ventilated areas to well-ventilated areas