Respiratory physiology 4 Flashcards
Describe the ventilation-perfusion relationship
Ideally match each other but not the same across the whole lung so can get a mis-match
Ventilation is describing the air getting to the alveoli
Perfusion describes the local blood flow
L/min for both
Describe ventilation and perfusion at base of lungs
Blood flow is higher than ventilation
This is because arterial pressure exceeds alveolar pressure which compresses the alveoli
Describe ventilation and perfusion at the apex of the lungs
Blood flow is lower than ventilation
Because arterial pressure is less than the alveolar pressure which compresses he arterioles
Alveoli compress the vessels in lungs
What is the perfectly matched ventilation: perfusion ratio?
1.0
Mismatch1 at base - ventilation<Perfusion< 1.0
Mismatch 2 at apex - ventilation>Perfusion > 1.0
What percentage of the healthy lung performs well in matching blood and air?
Over 75%
Majority of mismatch is at the apex
autoregulated to keep ratio close to 1.0
Describe what happens when blood flow is more than ventilation
If ventilation decreases in a group of alveoli, PCO2 increases and PO2 decreases.
Blood flowing past these will not be oxygenated
Dilution of oxygenated blood from better ventilated areas
Blood is then shunted from right side to left side of heart without undergoing gas exchange
Describe autoregulation when blood flow is more than ventilation
Decreased tissue PO2 around under ventilated constricts their arterioles, diverting blood to better ventilated alveoli.
Increased PCO2 also causes mild bronchodilation
Describe autoregulation when ventilation is more than blood flow
Alveolar PO2 rises which causes pulmonary vasodilation
PCO2 falls so there is mild bronchial constriction
What is Alveolar dead space?
When ventilation exceeds blood flow so more air that what can participate in gas exchange
Occurs small amount in apex of lungs but can be pathologically in pulmonary embolism
Describe the term shunt
Describes the passage of blood through areas of the lung that are poorly ventilated
Is the opposite of alveolar dead space
What is anatomical dead space?
Refers to the air in the conducting zone of respiratory tract unable to participate in gas exchange as walls of airways in region are too thick
Explain Shunting and dead space on RSA
RSA is respiratory sinus arrhythmia
During inspiration there is increased alveolar dead space
During expiration there is increased shunt
RSA ensures ventilation: perfusion ratio remains close to 1 - increased vagal activity during expiration phase
What is physiologic dead space
Alveolar dead space and anatomical dead space
What are the 2 forms O2 travels in the blood?
In solution in plasma - 3ml per litre of plasma
Bound to haemoglobin protein in red blood cells - 197ml of O2 out of 200ml
How is CO2 transported in the blood?
The bulk of CO2, 77% is transported in solution in plasma due to solubility of CO2
23% is stored within haemoglobin
What is the O2 demand of resting tissues?
250ml/min
What percentage of arterial O2 is extracted by peripheral tissues at rest?
Only 25% as Haemoglobin and arterial O2 content provide 1000ml/min
Describe the structure and function of haemoglobin
Each Hb contains 4 haem groups which each contain one Fe2+ which binds one molecule of O2
Each RBC is packed with O2, just Hb can carry only 4
What is the major determinant of of the degree at which Hb binds with O2?
Partial pressure of O2 in the blood which is equal to PP in alveoli
Why is partial pressure of O2 in plasma important?
Hb effectively sequesters O2 from the plasma which maintains PP gradient that continues to suck O2 out of the alveoli until Hb becomes saturated
How long does it take for Hb to become saturated?
0.25s after contact with the alveoli and the total contact time is 0.75s
Explain the oxygen - haemoglobin dissociation curve
Hb is 98% saturated at normal systemic arterial PO2 of 100 mm Hg
Even at PO2 of 60mm Hg there is still 90% saturation
This means there is a normal uptake of O2 by blood when PO2 is relatively reduced
At normal venous PO2 (at peripheral tissues) - 40 mm Hg there is still 75% saturation
What is anaemia?
Any condition where the O2 carrying capacity of the blood is compromised
Ex. iron deficiency, haemorrhage or B12 deficiency
What would happen to PO2 in anaemia?
Nothing
PO2 is normal despite total blood O2 content being low
Is it possible for RBCs to be fully saturated with O2 in anaemia?
RBCs would still be fully saturated with O2 as PO2 is normal
The problem could be with Hb deficiency or not enough RBCs
What is the effect of pH on the saturation of Hb?
Fallen pH can cause acidosis which causes extra O2 to be given off so tissues receive more - curve fallen
Increased pH causes alkalosis which means Hb holds onto O2 more - curve has increased
Describe the effect of temperature on the saturation of Hb
An increase in temp. causes the curve to fall - Hb affinity is less for O2 so more given to tissues
A decrease in temp. causes the curve to increase - can be dangerous as Hb hangs onto O2 more
What effect does conc. of DPG have to Hb saturation?
An increase in DPG conc. causes decrease in curve - saturation decreases
No DPG causes increase in Hb saturation
More DPG in hypoxia, lung disease, heart disease
What is the effect of PCO2 on the Hb saturation?
Increase in PCO2 to 80mm Hg causes decrease in saturation so extra O2 given to tissues
Decrease in PCO2 to 20mm Hg causes increase in affinity for O2 so Hb hangs onto O2
What is Carbon monoxide?
CO forms from the incomplete combustion of carbon fuel
Characterised by hypoxia, anaemia, nausea, headache and cherry res skin and mucous membranes
Potential brain damage and death so treatment is 100% oxygen to increase PaO2
Explain CO binding to haemoglobin
Is highly toxic
CO binds to Hb to form carboxyhaemoglobin with an affinity to O2 x250 more
So binds O2 readily and dissociates very slowly
What happens to CO2 when it diffuses from the tissues into the blood?
7% remains dissolved in plasma and erythrocytes
23% combines in erythrocytes with deoxyhaemoglobin to form carbamino compounds
70% combines in the erythrocytes with water to form carbonic acid
What happens to the carbonic acid formed from CO2 in blood?
Dissociates to yield bicarbonate and H+ ions
Most of bicarbonate moves out of erythrocytes into plasma in exchange for Cl- and excess H+ ions bind to deoxyhaemoglobin
This is the Cl- shift
What is PaO2?
Arterial partial pressure of O2
Refers purely to O2 in solution in plasma and is determined by O2 solubility and partial pressure of O2 in gaseous phase
What are the different types of haemoglobin?
92% of Hb is in RBCs in form of HbA
Remaining 8% is made of HbA2, HbF and glycosylated
Hb
Myoglobin is another O2 carrier found in cardiac and skeletal muscle
Why does HbF (feotal Hb) and myoglobin have a higher affinity to O2 than HbA?
Is necessary for extracting O2 from maternal/ arterial blood
What is hypoxaemic hypoxia?
Most common
Reduction in O2 diffusion at lungs due to increased PO2 atmos. or tissue pathology
What is Anaemic hypoxia?
Reduction in O2 carrying capacity of blood due to anaemia (RBC loss or iron deficiency)
What is stagnant hypoxia?
Heart disease results in inefficient umping of blood to lungs/ around the body
What is histotoxic hypoxia?
Poisoning prevents cells utilising O2 delivered to them ex. CO
What is metabolic hypoxia?
Oxygen delivery to the tissues does not meet increased O2 demand by cells
