Oxygen transport and consumption

Question 1

What is Huffner’s constant

Answer 1

The maximum oxygen carrying capacity of Hb is 1.34 ml O2/ gram Hb

Question 2

What is the formula for arterial oxygen content per 100 ml of blood?

Answer 2

CaO2 = 1.34 [Hb].(SaO2/100%) + (0.023.PaO2)

Question 3

How much O2 does the body use at rest per minute?

How is this calculated

Answer 3

VA x (FiO2 - FeO2) = Oxygen consumption in mL/min

250 ml/minute

Question 4

How many Hb molecules exist in each RBC

Answer 4

500 million

Question 5

What does a Hb molecule consist of?

Answer 5

1. Four linked polypeptide chains (2 alpha and 2 beta = HbA)

2. Each polypeptide chain has a haem group (porphyrin ring containing a Fe2+ ion)

Question 6

Where does O2 bind on the Hb and how many O2 molecules can bind to one Hb

Answer 6

1. To the Fe2+

2. Four x O2 per Hb

Question 7

What causes the sigmoid shape of the OHDC

Answer 7

Co-operative binding
1st O2 - difficult binding
2nd and 3rd O2 - easy binding
4th O2 - fewer sites but higher affinity - moderately difficult binding

Question 8

Define P50 on the OHDC

Answer 8

This is the PO2 value on the curve at which 50% of Hb is saturated and under normal conditions is 3.5 kPa. When the OHDC is shifted to the right or to the left, this is described by the change in P50.

Question 9

At rest how much O2 is extracted from the tissues

Answer 9

25% (SvO2 = about 75% and PvO2 about 5.3)

Question 10

What is normal PaO2 and SaO2

Answer 10

13 kPa and 99% SaO2

Question 11

How can the difference between Atmospheric partial pressure of O2 of 21 kPA and arterial partial pressure of 13kPa be explained?

Answer 11

By the oxygen cascade graph

1. Upper airways - humidification –> addition of water vapor
2. Dilution by CO2 in the alveoli (AGE)
3. Physiological shunt

Question 12

How is the oxygen saturation non-invasively measured

Answer 12

Pulse oximeter

The different absorption characteristics of deoxy-Hb ad oxy-Hb for red and infrared light are used to measure SaO2 non-invasively. Only the pulsatile signal is extracted - therefore, the value for arterial blood is obtained.

Closely reflects CaO2 when Hb is normal

Question 13

What is the truest measure of oxygen present in blood?

Answer 13

O2 content (CaO2)
Correlates to SaO2 if Hb is normal

When fully saturated 1 g of Hb holds 1.34 ml of O2

CaO2 = O2 bound to Hb + O2 dissolved

= 1.34 x [Hb] x SaO2 + (0.23 xPaO2)
= 200ml O2/L blood if normal values are used

Question 14

What is the formula for O2 delivery?

Answer 14

DO2 = CO x CaO2
= 200 x 5
= 1L/min
= 1000 mL O2/min
(rate of O2 consumption is 25% of this (250 mL O2/min) leaving venous blood saturated with 75% with a content of 150ml O2/L blood

Question 15

What increases the rate of O2 consumption (VO2) and how is the extra O2 provided?

Answer 15

Shivering
Pregnancy
Childhood
Thyrotoicosis
Sepsis
Surgery

Extra O2 from:

1. Increased CO
2. Increased VA
3. Increased OER

Question 16

What is VO2 max?

Answer 16

The maximum extent to which an individual can increase his/her O2 consumption during exercise –> increasingly being used as a measure of cardiovascular fitness

Question 17

What is the Bohr effect

Answer 17

Active tissues increase CO2, H+, Temperature and 2,3DPG which shift the OHDC to the right –> O2 is more easily offloaded to the active tissues

Question 18

When is cyanosis detectable

Answer 18

When deoxy-Hb > 5g/dL (with Hb of 15 this is 33% of the total with the remaining 67% being saturated)

Question 19

Cyanosis is detectable when 5g/dL of deoxyHb is present. With Hb of 15 this accounts for 33% of the Hb with the remaining 67% of Hb saturated. Why then in practice is cyanosis visible in patients with SaO2 is around 85%

Answer 19

Cyanosis is observed in CAPILLARY BLOOD
The pulse oximeter reading is based on the arterial value.

The arterial SaO2 is higher in arterial blood because

1. More oxygen present as has not been consumed by tissues yet
2. Capillary blood: OHDC P50 is slightly shifted to the right meaning that less O2 will be attached to the Hb at the same PO2.

Question 20

How does anaemia affect CaO2

Answer 20

Anaemia reduces CaO2 in proportion to the fall in Hb (as most O2 is carried bound to Hb)

Question 21

How would the body compensate to the reduction in CaO2 caused by anaemia

Answer 21

Increasing CO by increasing HR

Question 22

To what extent can the CO be increased during exercise

Answer 22

x 5 (HR can increase 3 times and SV can increase 1.5 times)

Question 23

To what extent can O2 extraction ratio be increased during exercise?

Answer 23

OER can double (From 0.25 to 0.5)

Question 24

To what extent can O2 consumption be increased during exercise?

Answer 24

CO can increase 5 times
OER can increase x 2

Therefore - overall increase of 10 x