respiratory control Flashcards
gas diffusion
Movement of gas throughout the respiratory system occurs via diffusion.
1) Large surface area for gas exchange.
2) Large partial pressure gradients.
3) Gases with advantageous diffusion properties.
4) Specialised mechanisms for transporting O2 and CO2 between lungs and tissues.
what is partial pressure
Sum of the partial pressures (mmHg) or tensions (torr) of a gas must be equal to total pressure.
blood circulation
Gases move down their pressure gradients.
Pulmonary circuit: O2 enters blood, CO2 leaves
Systemic circuit: O2 leaves blood, CO2 enters.
gas concentration gradient
Pulmonary Capillary
Alveolar Air Venous Blood
PO2 100 –>40
PCO2 40 <–46
Systemic capillary
Tissues Arterial Blood
PO2 <40 < – 100
PCO2 > 46—> 40
Pressure gradient for O2 is much bigger than for CO2.
CO2 is more diffusible.
alveolar-capillary network
Gas exchange occurs through dense mesh-like network of capillaries and alveoli.
Distance between alveoli and red blood cell 1-2µm:
Type 1 alveolar epithelial cell, capillary endothelial cell and basement membrane.
Red blood cells pass through capillaries in less than 1 second – sufficient time for CO2 and O2 gas exchange
dissolved O2
Dissolved O2 is measured clinically in an arterial blood sample PaO2.
Only a small percentage of O2 in blood is in the dissolved form.
Amount of dissolved O2 in blood is proportional to its partial pressure.
For each mmHg of PO2 there is 0.003 ml O2/100 ml blood.
example: Arterial blood (PaO2) = 100 mmHg:
contains 0.3 ml O2/100ml blood (3ml O2/litre of blood).
haemoglobin
Second O2 transport system in place: Haemoglobin.
Haemoglobin (Hb) is major transport molecule for O2 found in red blood cells.
Hb: four heme (iron porphyrin compounds) groups joined to globin protein
(two α chains and two β
chains polypeptide chains).
Each heme group contains
iron in the reduced ferrous
form (Fe++), which is the
site of O2 binding.
binding and dissociation of O2
280 million Hb molecules/red blood cell.
Binding and dissociation of O2 with Hb occurs in milliseconds to facilitate transport – necessary because red blood cells in capillaries for 1 second only.
oxyhemoglobin dissociation curve
O2 binding to Hb is reversible.
Clinical significance of:
flat portion: drop in PO2 from 100 to 60 mmHg has minimal effect on Hb saturation.
Steep portion: large amount
of O2 is released from Hb with only a small change in PO2, facilitating release into tissues
CO2 production
Normal healthy conditions:
- 200 ml CO2 / min produced
- 80 molecules CO2 expired by lung for every 100 molecules of O2 entering.
CO2 transport
CO2 carried in blood in three forms:
7% dissolved.
23% bound to
haemoglobin (Hb).
iii) 70% converted to
bicarbonate.
bicarbonate
Direction and speed of the reaction is determined by the concentration gradients.
Rightwards in systemic capillaries – CO2 produced by tissues expelled into blood.
Leftwards in pulmonary capillaries – CO2 to be expelled into alveoli.
The CO2 to HCO3- pathway plays a critical role in regulation of H+ ions and in maintaining acid- base balance in body.
co2 and acidity
The concentrations of CO2, bicarbonate and hydrogen ions are linked.
This means changes in bicarbonate concentrations can be used to stabilise the pH.
This is a buffer reaction.
The high bicarbonate concentration makes buffering reaction strong.
the v/q ratio
The V/Q ratio is the ratio of ventilation to blood flow.
Ratio can be defined for single alveolus, a group of alveoli or entire lung.
Single alveolus: ratio defined as alveolar ventilation divided by capillary flow.
Lung: ratio defined as total alveolar ventilation divided by cardiac output.
