TRANSPORT IN ANIMALS [part 3]

A) General characteristics of a circulatory system

B) The development of blood systems in animals

C) Composition of blood

D) The circulatory system

E) Formation of tissue fluid

F) The heart

G) Functions of mammalian blood

H) Oxygen dissociation curves –

The Bohr shift

Oxygen transportHaemoglobin: 4 haem prosthetic

groups linked to 4 globin polypeptide chains

1 Fe2+ in 1 haem

1 Fe2+ can combine loosely with 1 O2 molecule

four molecules

of oxygen.

One Hb molecule carries:

Oxygenation: Hb + 4O2 <--> Hb(O2)4 (oxyhaemoglobin)

readily reversible

Fe2+ in haemoglobin must remain Fe2+

certain chemicals can oxidise Fe2+ to Fe3+ producing derivatives of haemoglobin which cannot carry oxygen

e.g. carboxyhaemoglobin contains Fe3+

CO reaction: Hb + CO --------> HbCO (carboxyhaemoglobin)

very high affinity (230X greater than for O2)

Under which condition does Hb :

LOW partial pressure of O2 : capillaries supplying metabolically active tissues

HIGH partial pressure of O2: lungs

Bonds holding O2 to Hb become unstable & O2 is released

Bind to O2?

Release O2?

Oxygen dissociation curves

Oxygen tensiondetermines the amount of oxygen that

can combine with haemoglobin is expressed as a partial pressure is the fraction of oxygen found in the air

The oxygen dissociation curve is a plot of the:

% oxygen saturation of blood against

the partial pressure of oxygen

What is O2 saturation?

% s

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of

hae

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Oxygen tension /mmHg

Oxygen saturation is the:fraction or percentage of all the haemoglobin

binding sites that are currently occupied by O2

A 100% saturation of Hb: is rarely achieved

At a partial pressure of 0%: no O2 is attached to Hb

tension at which 95% of the pigment is saturated with O2

tension at which 50% of the pigment is saturated with O2

Loading tension: Unloading tension:

The oxygen dissociation curve shows that when haemoglobin is exposed to a gradual increase in

O2 tension, it absorbs:

1. O2 rapidly at first

2. but more slowly as the O2 tension continues

to rise

a big fall in Hb saturation, so

that O2 is released to the tissues where

need is greatest

Over the steep part of the curve, a small decrease in O2 tension:

Results in:

The curve is S (sigmoid)-shaped due to: positive

cooperativity

shape of Hb molecule is altered as each O2 molecule is taken up

each one facilitates much faster uptake of oxygen than the previous one

affinity for O2 INCREASES as each O2 molecule is taken up

2. When an O2 molecule combines with the iron atom

of a single haem unit, it distorts its shape slightly.

The whole molecule changes shape accordingly.

1. Deoxy-haemoglobin has a relatively low affinity for O2.

3. The 2nd molecule binds more easily, and the 3rd & 4th even more easily.

When Hb gives up its O2:

The 1st O2 molecule is released: very rapidly

The 2nd, 3rd, 4th molecules are released: less rapidly at a much reduced partial pressure

of O2

Changes in haemoglobin affinity for oxygen

The term "affinity"

is used to describe the attraction of oxygen to haemoglobin binding sites

affinity changes with: variation in pH temperature CO2 

Traditionally the curve starts with: pH at 7.4 temperature at 37C partial pressure of

CO2 at 40 mmHg

changes from these values are called "shifts"

Left shift

Right shift

pH

temperature

pH

temperature

norm

al

increases oxygen's affinity for haemoglobin

A LEFT shift:

A RIGHT shift:

decreases oxygen's affinity for haemoglobin

blood holds onto O2

less O2 is released to tissues

A LEFT shift:

A RIGHT shift:

blood releases O2 more readily

more O2 is released to tissues

minimal effects on O2 loading in the lung:

because the O2 dissociation curve is still fairly flat at a PO2 of 100 mmHg.

Shifts in O2 dissociation curves have:

maximal effect on O2 unloading at the tissues:

because the curve is steep at a vein (e.g. 40 mmHg).

The Bohr Effect or Shiftis a shift to the RIGHT in the O2 dissociation

curve in regions with an increased partial pressure of CO2 (fig. 21)

Low CO2 tension [2 kPa]

Medium CO2 tension [5 kPa]

High CO2 tension [8 kPa]

The effect of increased CO2 is to cause O2 to be released from the Hb molecule

H+ released: combine with haemoglobin and make it less able to carry O2

CO2 has this effect because when it dissolves it forms a weak acid:

How does CO2 make Hb release O2?

An increase in blood temperature causes a shift to the right

Myoglobinis a red pigment – has

one polypeptide chain

Location: in skeletal muscles and is the reason

why meat appears red

Myoglobin

Affinity for O2 :

great Acts as:

a store of O2 in resting muscles

Releases O2: only when supplies of oxyhaemoglobin

have been exhausted

Myoglobin O2 dissociation curveis displaced well to the left of haemoglobin

Haemoglobin[sigmoidal]

Myoglobin

[hyperbolic]

0

20

40

60

80

100

0 4 8 12 16

Arterial PO2 (kPa)

Sa

tura

tio

n (

%)

Oxygen dissociation curves for various animals

curves shifted to the right

high metabolism so need O2 to be released readily

Small animals (shrews/mice) & birds have:

South American Llamas live at high altitudes. Where do you think their

oxygen dissociation curve would lie?

S-shaped hemoglobin curveReleases much Becomes saturated

O2 at tissues with O2 at lungs

High affinity onlyCan’t release much O2 to tissues

Low affinity onlyDoesn’t hold on to But can’t pick up much O2 at tissues much O2 at lungs

Advantages of “S-shaped” curve for Hb-O2 association

Question: MAY, 2010

Use your knowledge of biology to describe the selective advantage of each

of the following adaptation.

The haemoglobin of certain mud-dwelling organisms has an oxygen-dissociation curve shifted well to the left of that of human haemoglobin. (5)

Comparison of the O2 dissociation curves of foetal & maternal haemoglobin

To enable it to obtain O2 from the mother’s haemoglobin in the

placenta.

Foetal haemoglobin has a greater affinity for O2

than adult haemoglobin.

O2 must easily dissociate from the maternal haemoglobin to the foetal haemoglobin

therefore, easily transferred from maternal to foetal blood

Question: SEP, 2011The graph in Figure 2 shows the variation in oxygen saturation of foetal and maternal haemoglobin in relation to partial pressure of oxygen.

1. Based on information given in the graph in Figure 2, describe the activity of maternal haemoglobin under different partial pressures of oxygen. (4)

Haemoglobin's affinity for oxygen increases as successive molecules of oxygen bind. More molecules bind as the oxygen partial pressure increases until the maximum amount that can be bound is reached. As this limit is approached, very little additional binding occurs and the curve levels out as the haemoglobin becomes saturated with oxygen. Hence the curve has a sigmoidal or S-shape. At pressures above about 60 mmHg, the standard dissociation curve is relatively flat, which means that the oxygen content of the blood does not change significantly even with large increases in the oxygen partial pressure.

2. How does the activity of foetal haemoglobin differ from that of maternal haemoglobin? (2)

Foetal haemoglobin has a higher affinity for oxygen than the maternal one. It has a higher saturation with oxygen at all partial pressures.

3. Draw another curve on the graph in Figure 2 showing the probable shape of the analogous curve for maternal myoglobin. (2)

4. Explain the difference between the curve for maternal haemoglobin and that for maternal myoglobin. (2)

Myoglobin has a higher affinity for oxygen than haemoglobin. Myoglobin releases its oxygen at very low partial pressures of oxygen.

5. How would the curve for maternal haemoglobin be expected to differ under conditions of high partial pressure of carbon dioxide? (2)

Shifted to the right.

Affinity of haemoglobin to oxygen is reduced so that this gas is released to the cells which need it.

Carbon monoxide & haemoglobin

Result: O2 does not combine with

Hb & so it is not transported

Hb + CO

Hb combines with any CO available in preference to O2

[a relatively stable compound carboxyhaemoglobin]

HbCO

Transport of carbon dioxideCO2 must not be allowed to accumulate in the

body

it forms an acid in solution that could lead to fatal changes in blood pH

BECAUSE

Carbonic acid

CO2 is carried in blood in three ways:

In solution

Combined with haemoglobin

[carbamino-haemoglobin]

As hydrogen carbonate 85%

10-25%

5%

Carbonic anhydrase in RBC catalyses reaction to form H2CO3

Some H2CO3 dissociates & the H+ displace the O2 from the haemoglobin

this is the basis of the Bohr effect

By accepting H+, haemoglobin acts as a BUFFER molecule and so enables large quantities of carbonic

acid to be carried to the lungs without any major changes in blood pH

diffuse out of the RBC into the plasma combine with Na to form NaHCO3

this is called the chloride shift

Majority of the HCO3- ions:

the RBC becomes

positively charged as

it loses negative ions

Cl- ions diffuse into

the RBC

CO2 forms at the lungs & is exhaled

AT ACTIVE TISSUES AT LUNGS

THE END