Lecture 19 resp physio O2 and CO2 transport Flashcards
O2 transport
O2 is carried in the blood in two forms:
what are they?
- Dissolved O2
2. Combined with haemoglobin (Hb)
- Dissolved O2
very ineffective for O2 transport
Need to use Haemoglobin (Hb) to carry O2
why is dissolved O2 very ineffective for O2 transport?
- For each mmHg, PO2 can only dissolve 0.03 ml of O2 per litre of blood
- Arterial blood with PO2 ~100mmHg contains only 3 mL of dissolved O2 per litre of blood
combined with Hb
O2 forms an easily reversible combination with Hb to give oxyhemoglobin
O2 + Hb ⟷ HbO2
binding depends on PO2 - dissociation curve - saturation
O2 saturation (“How full is the Hb?”)
whats O2 saturation of Hb?
percentage of the available binding sites that have O2 attached.
Varies with PO2
O2 saturation (“How full is the Hb?”)
Arterial blood: PaO2 = 100 mm Hg
whats the SaO2?
~ 97%.
O2 saturation (“How full is the Hb?”)
Venous blood: PvO2 = 40 mm Hg
whats SvO2?
~ 75%.
what curve is Oxygen hemoglobin?
sigmoidal
Oxygen-haemoglobin curve
what does the Upper flat part of the curve explain?
moderate changes in PO2 around the normal value (~100 mm Hg) have only small effects on the % saturation
the amount of O2 carried by arterial blood.
i.e some reserve capacity.
Oxygen-haemoglobin curve
what does the Steep part of curve at lower PO2 explain?
helps with loading of Hb in lungs AND unloading of O2 to the tissues.
Small changes in PO2 result in large changes in amount of O2 bound to Hb.
O2-Hb dissociation curve varies with
conditions to improve O2 uptake / delivery
O2-Hb dissociation curve
what effects results in a right shift in curve?
Increase PCO2
Decrease pH bohr effect
Increase Temperature (exercise)
O2-Hb dissociation curve
what effects results in a left shift in curve?
Increase PCO2
Increase pH bohr effect
Decrease temperature (exercise)
O2 Content (How much O2 is the blood carrying?)
whats the equation?
O2 content = O2 capacity x saturation ( + dissolved)
O2 content = O2 capacity x saturation ( + dissolved)
O2 content of the blood
- ml O2 / litre blood
Hb is the [haemoglobin]
- g/litre
Sat percentage saturation of Hb
- PaO2 is in mm Hg
O2 Content (How much O2 is the blood carrying?)
O2 content = O2 capacity x saturation ( + dissolved)
= 1.34 x Hb x (Sat / 100) + 0.03 x PaO2
O2 carrying capacity
- 1.34 x Hb x (Sat / 100)
O2 dissolved
- 0.03 x PaO2
O2 content of the arterial blood
Assume 150g of Hb/litre; PaO2 100 mm Hg; SaO2 98%
1.34 x Hb x (Sat / 100) + 0.03 x PaO2
O2 content of arterial blood (ml O2/litre blood) = 200 Arterial O2 content
O2 content of the venous blood
Assume 150g of Hb/litre; PvO2 40 mmHg; SvO2 74%
1.34 x Hb x (Sat / 100) + 0.03 x PvO2
O2 content of venous blood (ml O2 /litre blood) = 150 Venous O2 content
whats the Arterial (a) - Venous (v) O2 difference?
amount of O2 extracted by tissues
Arterial content = 200 ml O2 /litre blood
Venous content = 150 ml O2 /litre blood
whats the a-v difference?
50 ml O2 /litre blood
50 ml O2 was extracted from each litre of blood by the tissues and used in metabolism
Arterial (a) - Venous (v) O2 difference
How many litres/min flow to tissues?
CO = 5 litres/min
a - v difference = 50 ml O2 /litre blood
(5x50)
Total O2 extracted by tissues = 250 ml/min (VO2)
CO2 transported in 3 forms
what are they?
- Dissolved in plasma - 20 times more soluble than O2 (~7%)
- As bicarbonate (70%)
- Combined with proteins as carbamino compounds (23%)
Formation of bicarbonate (70%)
CO2 + H20 ⟷ H2CO3 ⟷ H+ + HCO3-
⟷
- Carbonic anhydrase
what do you NEED to maintain normal levels of for metabolic and biochemical stability (e.g pH)?
PaO2 and PaCO2
O2 uptake and CO2 production quite variable so how is PaO2 and PaCO2 normally kept within close limits?
tight control of ventilation.
Effectors ⟶ Sensors ⟶ Central controller ⟶ Effectors
Triangle
what are the effectors?
what are the sensors?
what is the central controller?
- Respiratory muscles
- Chemoreceptors, Lung and other receptors
- Pons, medulla, other part of brain
where are central chemoreceptors located?
Located medulla (brainstem)
Ventrolateral surface of medulla
what are Central chemoreceptors?
respond to pH change
Sensitive to the PCO2 but not PO2 of blood
Central chemoreceptors
CO2 diffuses out of the cerebral capillaries what does this do?
→ changes pH of the cerebrospinal fluid (CSF)
→ central chemoreceptors respond to pH change
where are peripheral chemoreceptors located?
Carotid and aortic bodies
what are peripheral chemoreceptors?
responds to decrease arterial PO2
limited response to PCO2 changes
Rapidly responding
