Respiratory System: 02 and C02 Transport

Question 1

Describe the oxygen- Hb dissociation curve

Answer 1

The curve represents the affinity of Hb for 02. The sigmoidal shape is produced by:
Cooperative binding (02-Hb affinity increases as more oxygen molecules bind due to changes in the protein shape.)
Saturation of 02 sites (as binding sites fill it gets harder to bind further 02).

Question 2

What is the effect of the high oxygen binding affinity of Hb?

Answer 2

Low PO2 is needed for high Hb binding site saturation.
Also, Hb will release oxygen if the surrounding P02 decreases, such as in respiring tissues.
This means Hb readily takes large quantities of oxygen at respiratory surfaces and gives it up to respiring tissues.

Question 3

What factors decrease 02-Hb binding affinity?

Answer 3

Increased CO2
increased temp
increased concs of 2,3-DPG (glycolysis intermediate, produced in RBCs)
decreased pH (acidosis)
All these decrease O2-Hb binding affinity, decreasing 02 saturation at a given PO2 and releasing O2 from Hb. This shifts the dissociation curve to the right in the Bohr effect

Question 4

What factors increase 02-Hb binding affinity?

Answer 4

Decreased CO2, decreased temp, increased pH (alkalosis) and decreased concs of 2,3-diphosphoglycerate increase O2-Hb binding affinity, increasing 02 saturation at a given PO2 and accumulate O2 on Hb. The Bohr curve shifts to the left

These effects prioritise O2 release to tissues w greatest demand- where PO2 & pH is lowest and CO2 is highest.

Question 5

Describe anaemia

Answer 5

Anaemia is a decrease in the number of RBCs per unit of blood volume. Decrease in RBC density reduces Hb conc, total 02 binding sites, and 02 carrying capacity.

Hb affinity is unchanged (at any PaO2, the % Hb binding sites occupied by oxygen stay the same, solo hay less of them).

So Hb-02 saturation and 02 partial pa in the plasma=normal, but overall total O2 content, oxyHb and deoxyHb conc of the blood decrease, causing pale skin and conjunctiva.

Question 6

Draw a graph to show effect of anaemia on oxygen transport

Answer 6

O2-Hb binding affinity is unaffected (the shape of the curve is the same).

But, less number of Hb molecules available per unit of blood means fewer binding sites. This means that the curve saturates at a lower oxygen content and total oxygen carrying capacity is reduced.

Question 7

Describe carbon monoxide poisoning and compare it to anaemia

Answer 7

Like anaemia, CO poisoning decreases 02 carrying capacity due to less available Hb binding sites.

Unlike anaemia, in CO poisoning overall Hb conc in the blood remains constant. CO displaces O2 at Hb binding sites (greater affinity). Less O2 can bind so less is transported.

ABG readings (SaO2) fall as they compare oxyHb conc to total Hb. Pulse ox readings (SpO2) may stay normal as it can’t differentiate between O2-Hb and CO-Hb. CarboxyHb is red, so hypoxia occurs sin cyanosis

Question 8

Draw a graph to show the effect of CO on 02 transport

Answer 8

Oxygen-haemoglobin binding affinity is slightly increased due to inhibition of 2,3-DPG production by CO.

Also, total oxygen carrying capacity is reduced. The curve saturates at a lower oxygen content because CO binds at the same site as oxygen w higher affinity, displacing it. So the number of available binding sites for oxygen per unit of blood decreases.

Question 9

Describe and explain cyanosis

Answer 9

Cyanosis: blue-purplish skin + tissue discolouration when hay excessive deoxyHb conc within the blood.

Central cyanosis: discoloration of the core, mucous membranes and extremities. Causes: inadequate oxygenation of blood within the lungs (from hypoventilation, gas exchange defects or V/Q mismatch).

Peripheral cyanosis (discoloration confined to the extremities) reflects inadequate oxygen supply to only these tissue (e.g. due to small vessel circulation problems)

Question 10

Describe the role of erythropoietin in chronic hypoxia

Answer 10

The kidney secretes EPO in response to hypoxia to induce rbc production.

O2 transport from lungs to tissues depends on Hb conc and PaO2, so increased Hb can compensate for chronic hypoxia. Hb saturation will decrease due to reduced PaO2, but this is compensated by more total Hb.

Increased EPO secretion also occurs in high altitude (due to the chronic hypoxia). The resulting rbc increase per unit of plasma is polycythaemia.

Question 11

How is C02 transported in the blood?

Answer 11

Carbon dioxide is transported in the blood in three forms:

As carbonic acid: CO2 reacts w water to form carbonic acid (catalysed by CA). This then partially ionises to H+ and HCO3- (see below).

Bound to Hb: CO2 binds to Hb at diff sites than O2 and w less affinity. So a lower % of CO2 is transported like this.

Dissolved in plasma: because tiene higher solubility than 02

Question 12

Why does increased carbon dioxide decrease 02-Hb binding affinity?

Answer 12

More C02 = lower pH = lower 02-Hb affinity = less 02 carried.
This is bc when CO2 and H+ bind to Hb they induce a conformation change in Hb.
This changes the structure of the O2 binding site, altering O2-Hb binding affinity.
These effects and the shift they induce in the 02-Hb dissociation curve= Bohr effect:

Question 13

Using this diagram, discuss CO2 transport in the tissues

Answer 13

1. C02 produced by respiring cells dissolves in plasma and enters rbcs
2. C02 reacts w water to form H2C03 inside rbcs (catalysed by CA). This removal of C02 from step 2 allows more CO2 to diffuse into the rbc
3. H2C03 ionises to HC03- + H+. Rbcs are impermeable to H+ so it can’t leave
4. DeoxyHb acts as a buffer and binds H+. This prevents H+ accumulation inside rbcs and allows step 2 to continue
5. Increased HC03- creates a diffusion gradient, so it leaves the cell. Its exchanged for Cl- to maintain electric neutrality

Question 14

Using the diagram below, explain C02 transport in the lungs

Answer 14

1. Low PaC02 creates a diffusion gradient. C02 diffuses out the RBC into the airspace
2. Increased Pa02 increases 02-Hb binding. 02Hb binds less H+ than de02Hb, increasing free H+
3. This leads to increased H2C03 and C02 which increases C02 plasma sat.
4. HC03- decreases as it binds to the free H+. This forms a diffusion gradient, HC03- enter the RBC in exchange for Cl-

Question 15

What is the Haldane effect?

Answer 15

H+ binding to Hb acts as a buffer and removes H+ from the surroundings. This shifts H2CO3 —> H+ + HCO-3 to the right.

This means dissolved CO2 is pulled out the plasma and stored as bicarbonate. This lost CO2 is replaced by fresh CO2 produced by tissues, so overall CO2 level carried in the blood increases.

But, when O2 binds Hb, a conformational change reduces Hb affinity for CO2 and H+, decreasing CO2 carrying capacity. This interaction between CO2 and O2 transport= the Haldane effect, and is why venous (less oxygenated) blood can carry more CO2 than arterial blood.

Question 16

What is a dangerous consequence of the Haldane effect in patients with COPD?

Answer 16

Dangerous to start supplemental O2 too quickly in severe COPD patients:
Chronic hypoventilation means C02 rises in the body. COPD patients have greater C02 blood capacity due to low 02 levels and the Haldane effect (they’re hypercapnic and hypoxaemic).

When 02 suddenly increases (supplemental O2), CO2 is displaced from the blood (less affinity). This leads to sudden v high C02 in the body and acidaemia.
You could increase ventilation to remove excess CO2, but cannot in COPD patients due to hypoventilation and lung deterioration.

Question 17

How does the bicarbonate buffering system maintain blood pH?

Answer 17

Question 18

How do the lungs & kidneys maintain blood pH homeostasis?

Answer 18