7.1 Gaseous Exchange Control in Mammals

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CHAPTER 7 : GASEOUS EXCHANGE & ITS CONTROL

Lesson Objective:

7.1b Describe the structures of haemoglobin & itscharacteristics as respiratory pigments

7.1 GASEOUS EXCHANGE & CONTROL IN MAMMALS7.1 GASEOUS EXCHANGE & CONTROL IN MAMMALS

Characteristics of Hb as respiratory pigmentCharacteristics of Hb as respira tory pigment

Shows high affinity for O 2 (easy to bind O 2)especially in an environment with high oxygen partial pressure ( Po 2) such as in the lungs.

Hb binds O 2 to form oxyhaemoglobin (HbO 2).HbO 2 also release O 2 easily when Po 2 such as in the tissue area.

Shows allosteric cooperativity .Binding of O 2 to 1 subunit induces the remaining subunits to change their shapes slightly so thattheir affinity for O 2 increases too.


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Lesson Objective:

7.1c Explain the transport of oxygen by haemoglobin. Show the characteristicsof oxygen dissociation curve of haemoglobin & myoglobin


Oxygen Dissociation Curve Shows the amounts of O 2 bound to Hb at different Po 2 .

At Po 2 of ~100 mmHg (typical in the lungs), Hb is about 98% saturated with O 2.

At Po 2 of 40 mmHg (common in the tissue area), Hb is only ~70% saturated.

Hb will release its reserve of O 2 to tissues that are very active metabolically (Po 2 ~10 mmHg) and only~20% saturated.

The curve is S-shaped. Cooperative oxygen binding and release is evident in the O 2 dissociation curve for Hb.

When Po 2 , the tendency of HbO 2 to release O 2 is greater.

When Po 2 , the amounts of O 2 bound to Hb (O 2 saturation of Hb) .

The steep slope in the curve shows the characteristic of Hb which releases O 2 fast when O 2 isneeded for cell respiration.


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Lesson Objective:




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Another protein that can bind O 2 ismyoglobin .

It functions as an O 2 store in the muscles.

It has a greater affinity towards O 2 (whencompared with Hb).

It remains fully saturated with O 2 even at lowPo 2 (when Hb starts to release O 2).

The O 2 dissociation curve for myoglobin issituated well to the left of the O 2 dissociation curve for Hb.

It only releases O 2 when Po 2 falls very low (asin severe muscular exertion).

Lesson Objective:



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Lesson Objective:

7.1d Compare oxygen dissociation curves of haemoglobinas in foetus & adult


Hb in fetus is slightly different from theHb of adults.

Fetal Hb has a higher affinity towardsO 2 (when compared with Hb of themother).

This ensures that the fetus will get thesupply of O 2from the mothers bloodwhen Po 2 is low in the uterus.

The O 2 dissociation curve for fetal Hb issituated to the left of the O 2 dissociation curve for the mothers Hb.

20 40 60 80 100

50

100

Partial pressure of oxygen (mm Hg)

Percentageoxyhem

oglobin

Saturation(%

)

Adult (maternal)hemoglobin

Fetal hemoglobin


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Lesson Objective:

7.1c Explain the transport of carbon dioxide byhaemoglobin.

CO 2 is transported in blood between respiring tissues and the lungs in 3 ways

1. As dissolved CO 2 in blood plasma

CO 2 diffuses into the blood plasma and moves towards the lungs as dissolved CO 2

Only about 7% as CO 2 is not so soluble in water

Carbon dioxide transport

2. As carbamino-haemoglobin

CO 2 diffuses into the blood plasma and enters the erythrocyte

Inside, the CO 2 binds to amino group of the Hb to form carbamino-haemoglobin

The carbamino-haemoglobin will be transported in the erythrocyte until it reaches the lungs area

About 23% of the CO 2 is transported this way


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Lesson Objective:


Carbon dioxide transport3. As bicarbonate ions About 70% of the CO 2 diffuses into the blood plasma, enters the erythrocyte and reacts with water

to form carbonic acid (H2CO 3)

This is catalyzed by carbonic anhydrase

The H 2CO 3 dissociates into H + and bicarbonate ion (HCO 3-)

The H + attaches to Hb While the HCO 3- diffuses into the plasma and carried towards the lungs

When the HCO 3- diffuses out from erythrocyte, the charge in the blood cell will change

Chloride ion (Cl -) will move from the plasma into the erythrocyte to keep the charge in the cellconstant

This is the chloride shift

*All of the processes that occur in the tissue area are reversed in the lungs


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Lesson Objective:


Carbon dioxide transport

From lungs


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Lesson Objective:

7.1e Explain the Bohr effect due to partial pressure of carbon dioxide


The CO 2 partial pressure (Pco 2) also has an effect on the O 2 dissociation curve of Hb.

Metabolically active tissues produce a lot of CO 2 which reacts with water to form carbonic acid whicheventually can form H + .

This lowers the pH (more acidic) of the blood at the active tissues.

This induces the Hb to release more O 2 to be supplied to the active tissues while the Hb picks up the H + toform haemoglobinic acid (to prevent harmful changes in blood pH).

At the same Po 2 , O 2 saturation of Hb is lowered when the pH is lowered.

This shifts the O 2 dissociation curve to the right as a drop in pH lowers the affinity of Hb towards O 2 .

This is the Bohr shift ( Bohr effect ).


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Lesson Objective:

7.1e Explain the Bohr effect due to partial pressure of carbon dioxide

7.1 Gaseous Exchange Control in Mammals

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