Capillary Function and Blood Gases

 

 

·      Blood consists of red blood cells (erythrocytes), white blood cells (lymphocytes) and platelets, suspended in the liquid blood plasma.

·      Red cells contain haemoglobin and are involved in the transport of oxygen and carbon dioxide by the blood.

·      White cells are a major part of the immune system.

·      Platelets are cell fragments involved in clotting of the blood when blood vessels are damaged.

·      Blood plasma is mainly water.

·      It contains many substances in solution, including;

·      ions - sodium, potassium, chloride, bicarbonate and others,

·      t

·      hormones - insulin, glucagon and many others,

·      urea,

·      proteins.

·      The proteins are of various types, including;

·      enzymes,

·      antibodies (to fight infections),

·      albumins,

·      globulins (to carry lipids),

·      fibrinogen.

·      When blood clots, the fibrinogen changes into insoluble threads of fibrin.

·      These threads form a mesh over a break in the wall of a blood vessel.

·      Platelets get caught in the mesh and a clot forms.

·      Blood serum is what remains when the fibrin/fibrinogen is removed from blood plasma.

·      Tissue fluid is the liquid which is found between/surrounding all the cells of the body.

·      It is formed from components of the blood plasma which leave the blood as it passes through the capillaries.

·      Most of the large components of the blood can not cross the capillary wall, so tissue fluid does not contain blood cells (except for certain white cells), platelets, or large plasma proteins.

·      Water, mineral ions, glucose and amino acids can cross the wall and are found in similar concentrations to those in blood plasma.

·      Tissue fluid has a lower concentration of oxygen and a higher concentration of carbon dioxide than plasma.

·      Lymph is formed when surplus tissue fluid enters lymph capillaries.

·      These capillaries are dead-end vessels which merge to form larger lymph vessels.

·      The lymph is eventually returned to the blood, to form part of the plasma again.

·      Lymph is very similar in composition to tissue fluid but may contain fats (absorbed in digestion), more protein and white blood cells (in the lymph nodes).

 

The exchange of substances between the blood and tissue fluid across the wall of the capillary.

·      At the arterial end of a capillary, the blood pressure is relatively high.

·      This hydrostatic pressure forces water through the wall of the capillary into, and becoming part of, the tissue fluid.

·      Glucose and mineral ions leave by diffusion.

·      Oxygen diffuses out into the tissue fluid along a concentration gradient - since the cells are constantly using up oxygen in respiration.

·      Carbon dioxide and urea diffuse into the plasma along concentration gradients.

·      Blood cells and plasma proteins are too large to get through the capillary wall.

·      The loss of water lowers the volume of the blood and this, together with friction, reduces the blood pressure as the blood flows towards the venous end of the capillary.

·      The loss of water also increases the concentration of the plasma proteins, making the water potential of the blood more negative.

·      This lower blood pressure and water potential allows water to flow back into the blood from the tissue fluid, along a water potential gradient by osmosis. You may have been taught this as Starling’s Hypothesis.

·      The water potential gradient is not large enough to reabsorb all of the water that leaves the capillary.

·      This means that the volume of the tissue fluid would tend to increase (and blood volume decrease).

·      The surplus tissue fluid is drained away by the lymphatic system and returned to the blood.

 

You should understand how oxygen and carbon dioxide are loaded, carried and unloaded by the blood. You should understand the buffering effect of haemoglobin.

 

·      Oxygen. The vast majority of the oxygen in the blood is carried in red blood cells (erythrocytes).

·      It is chemically bound to haemoglobin inside the cells.

·      Each haemoglobin molecule can hold up to four oxygen molecules.

·      Haemoglobin is a protein with specific receptor/binding sites on its surface for oxygen.

·      The oxygen fits into the receptor sites because its shape fits that of the receptor site.

Note. Haemoglobin is not an enzyme but many of its characteristics are similar to an enzyme’s.

·      Oxyhaemoglobin is formed when oxygen binds to haemoglobin.

 

            Haemoglobin + Oxygen Û Oxyhaemoglobin

 

·      The binding of oxygen is reversible,

·      depending upon the concentration of oxygen and the strength of the affinity/attraction of haemoglobin for oxygen.

·      Blood entering the lung capillaries has little oxygen bound to its haemoglobin.

·      The capillaries surround the alveoli of the lungs, which contain a high concentration of oxygen.

·      This creates a concentration gradient for the diffusion of oxygen.

·      Oxygen dissolves in the liquid lining the inside of the alveoli and then diffuses through the wall of the alveoli and capillary, into the blood plasma.

·      It then diffuses into the red cells and is bound to haemoglobin.

·      This takes it out of solution, effectively keeping the concentration of dissolved oxygen low in the blood and maintaining the concentration gradient.

·      As the haemoglobin becomes saturated with oxygen, it is carried away from the lungs as the blood flows and replaced with more oxygen poor haemoglobin.

·      This also maintains the concentration gradient for oxygen diffusion.

·      The haemoglobin in the lung capillaries has a high affinity for oxygen, due to the excretion of carbon dioxide by the lungs and the resulting changes in blood chemistry (see carbon dioxide transport later in this section).

·      Respiration takes place all the time in the tissues of the body.

·      This means that the cells are continuously using oxygen.

·      As a result, the concentration of oxygen in the tissues is lower than in the blood.

·      A concentration gradient exists for the diffusion of oxygen from the red cells, into the blood plasma, the tissue fluid and the cells of the tissues.

·      The lowering of the oxygen concentration in the red cells causes the reversal of the binding of oxygen to haemoglobin.

·      Respiration in the tissues also increases the concentration of carbon dioxide.

·      This produces changes in blood chemistry which lower the affinity of haemoglobin for oxygen, causing more oxygen to be released, more easily.

 

·      Carbon dioxide. Carbon dioxide is carried in the blood in three ways.

·      Dissolved carbon dioxide - about 10% is carried as dissolved carbon dioxide molecules in the blood plasma.

·      Carbamino-haemoglobin - about 20% is carried attached to an amino acid in the haemoglobin molecule.

·      Hydrogencarbonate ions - about 70% is carried as these ions in solution in the blood plasma.

·      Respiration in the tissues produces a high concentration of carbon dioxide.

·      This creates a concentration gradient for the diffusion of carbon dioxide from the tissues and into the blood plasma.

·      Most of the carbon dioxide then diffuses into the red cells, because of a low concentration of carbon dioxide in these cells.

·      The concentration is low, because red cells contain an enzyme, carbonic anhydrase, which speeds up the reaction of carbon dioxide with water to produce carbonic acid (effectively ‘using-up’ the carbon dioxide).

·      The carbonic acid then dissociates to produce hydrogencarbonate ions and H+ ions.

 

 

 

 


·      The reaction is reversible, depending upon the concentration of carbon dioxide, and this is important in the lungs (see later).

·      The hydogencarbonate ions diffuse out of the red cells into the plasma, along a concentration gradient.

·      This outflow of negative ions would leave the red cells with an electrical charge imbalance.

·      To keep the charge balance, negative chloride ions (from sodium chloride) diffuse into the red cells.

·      This is the chloride shift.

·      The H+ from the carbonic acid could be a problem, since they tend to lower the pH of (make more acidic) the red cells and the blood.

·      Haemoglobin helps to prevent the pH change by binding to H+.

·      This is haemoglobin acting as a buffer.

·      Binding to H+ lowers the affinity of haemoglobin for oxygen, making it easier for oxygen to be supplied to the tissues.

·      The greater the rate of respiration in the tissues, the greater the concentration of carbon dioxide, the more H+ is produced and the more easily oxygen is released to the tissues that need it.

·      This increase in release/dissociation of oxygen from oxyhaemoglobin caused by increases in carbon dioxide concentration is the Bohr effect.

·       When the blood reaches the lung capillaries, carbon dioxide diffuses out of the blood plasma into the alveoli, along a concentration gradient.

·      This lowers the carbon dioxide concentration in the plasma and causes the reactions involving carbonic anhydrase to go into reverse.

·      As a result, hydrogencarbonate ions are converted back into carbon dioxide and excreted into the alveoli.

·      The other events, involving H+ and chloride ions are also reversed.

In exam questions, the formation of hydrogencarbonate ions might be shown on a diagram of a red cell.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Note that all of the reactions involved in these events are synoptic with sections 1.3, 1.4 and 1.5 of By01.

 

 You should be able to interpret oxygen-haemoglobin dissociation curves.

 

·       The graph shows a dissociation curve for human haemoglobin.

·       Many organisms have haemoglobin but different to human haemoglobin, each with its own dissociation curve. (This is discussed later.)

·       The curve is sigmoid/S-shaped.

·       The partial pressure of oxygen can be thought of as the ‘concentration’ of oxygen.

·       In the lungs and major arteries the % saturation is about 97%.

·       Because the graph levels off at the top, it means that haemoglobin becomes fully saturated with oxygen over a wide range of atmospheric partial pressures of oxygen.

·       This is why you can climb quite high mountains without the need for an oxygen mask!

·       When the body is at rest, the tissues remove oxygen for respiration and leave the haemoglobin about 75% saturated with oxygen.

·       This corresponds to an oxygen partial pressure of just over 5kPa.

·       During exercise, the tissues use far more oxygen, lowering the partial pressure to about 4kPa.

·       From the graph, you can see that this small fall in partial pressure causes a large fall in the saturation of haemoglobin from 75% to about 58%.

·       This is the significance of the steep part of the dissociation curve: a small fall in oxygen in the tissues causes a lot of oxygen to dissociate from haemoglobin.

·       A rise in the rate of respiration causes much more oxygen to be released into the tissues, to maintain the high rate of respiration.

·       A high rate of respiration also produces more carbon dioxide.

·       This is converted into carbonic acid in the red cells.

·       As discussed earlier in this section, this lowers the affinity of haemoglobin for oxygen - the Bohr Effect.

 

 

·      The graph shows the effect of an increase in the acidity of the blood on the dissociation curve for haemoglobin.

·      The curve has moved to the right.

·      The more carbon dioxide produced/ the greater the fall in the pH, the greater the shift to the right.

·      As you can see from the graph, a fall in pH of 0.2 causes 18% extra oxygen to dissociate (it falls from 63% to 45%) from haemoglobin at a partial pressure of oxygen of 4kPa in the tissues.

·      Again, this extra oxygen would help to maintain the high rate of respiration producing the carbon dioxide which lowered the pH.

 

·      You might be given data from different species of organism living in different environments, with different amounts of oxygen available.

·      For example, fish live in water and this contains much less oxygen than air.

·      The fish have haemoglobin which becomes fully saturated at much lower partial pressures of oxygen than the haemoglobin of air-breathing mammals.

·      Worms that live in mud flats (e.g. Lugworms) have even less oxygen available in their environment than fish.

·      They have evolved haemoglobin which is fully saturated at a very low partial pressure of oxygen; much lower than for fish.

·      So, the dissociation curve for fish haemoglobin would be to the left of that for human haemoglobin.

·      The curve for the worm would be to the left of that for the fish.

·      You might be given data on another ‘special case’, the human foetus.

·      The human foetus has slightly different haemoglobin to that of humans once they have been born.

·      Foetal haemoglobin has a dissociation curve to the left of ‘normal’ human haemoglobin.

·      This means that it has a higher affinity for oxygen than the mother’s blood.

·      This allows the foetus to take oxygen from the mother’s blood, across the placenta.

 

 

 

Common Mistakes

 

Not knowing in sufficient detail how carbon dioxide is carried in the blood.

Not appreciating that the reactions involved in carrying oxygen and carbon dioxide are reversible. The conditions in the alveoli of the lungs and the tissues are the opposite of each other: high oxygen concentration/partial pressure in the alveoli and low in the respiring tissues, high carbon dioxide concentration/partial pressure in the tissues and low in the alveoli. These conditions are linked to diffusion gradients created by respiration in the tissues (using up oxygen and producing carbon dioxide) and ventilation of the lungs (removing carbon dioxide and bringing in oxygen).

Not appreciating that the shape of the haemoglobin dissociation curve and changes such as the Bohr effect are linked to providing oxygen to rapidly respiring tissues.

Practice Questions

 

1.

Explain how carbon dioxide produced in respiring tissues is loaded and carried in the blood and finally unloaded in the lungs. (6 marks)

(15 lines were allowed for the answer.)

Northern Examinations and Assessment Board June‘96. Question 8. [Part]

 

2.

The graph shows the oxygen-haemoglobin dissociation curves for a mouse and some other mammals.

 

 

Key:

1 Elephant

2 Horse

3 Cat

4 Mouse
 

 

 

a)

Explain how the haemoglobin of the mouse is able to take up oxygen in the lungs and unload it in the tissues. (5 marks)

(10 lines were allowed for the answer.)

 

b)

i)

Describe the relationship between the position of the oxygen-haemoglobin dissociation curve and body size for the species shown in the graph. (1 mark)

 

 

ii)

Explain the advantage to a mouse of having an oxygen-haemoglobin curve of this shape and position. (3 marks)

(7 lines were allowed for the answer.)

Northern Examinations and Assessment Board June‘97. Question 9.Part.

 

3.

Nephrosis is a kidney condition in which damage to the glomeruli results in large quantities of protein passing into the glomerular filtrate. This protein finally appears in the urine.

 

a)

Suggest why this protein is not reabsorbed into the blood in the proximal convoluted tubule of the nephron. (1 mark)

 

b)

As a result of nephrosis large amounts of tissue fluid accumulate in the body, especially in the ankles and feet.

 

 

i)

Explain why the loss of protein from the blood results in the accumulation of tissue fluid. (2 marks)

 

 

ii)

Suggest why this fluid accumulates especially in the ankles and the feet. (2 marks)

Northern Examinations and Assessment Board June‘95. Question 6. [Part]

 

4.

The graph shows oxygen-dissociation curves for haemoglobin as blood passes through capillaries in the lungs and in the skeletal muscles of an athlete.

 

 

 

Explain how features of the oxygen-dissociation curves for haemoglobin in the lungs and in the skeletal muscles benefit an athlete. (7 marks)

(18 lines were allowed for the answer.)

Northern Examinations and Assessment Board March‘98. Question 8.Part.

 

Model Answers

 

1.

Carbon dioxide diffuses into the blood (in solution);

where some is carried in solution as carbon dioxide;

most moves into red blood cells along a concentration gradient;

there is carbonic anhydrase (in the red cells);

which converts carbon dioxide into carbonic acid/hydrogencarbonate ions;

the hydrogencarbonate ions diffuse out into the blood plasma;

this is the major form in which carbon dioxide is carried;

some is carried attached/bound to haemoglobin molecules;

the movement of hydrogencarbonate out of the red cells is balanced by chloride ions

moving in/the chloride shift;

in the lungs the reactions involving hydrogencarbonate are reversed (because of the

low partial pressure of carbon dioxide in the alveoli);

carbon dioxide diffuses out into the alveoli;

(any 6 points)

 

2.

a)

Oxygen diffuses into the blood (from the alveoli);

the high partial pressure of oxygen in the lungs means that haemoglobin becomes

saturated fully loaded with oxygen;

each molecule of haemoglobin can carry four oxygen molecules;

forming oxyhaemoglobin;

in the tissues, the oxygen partial pressure is lower (due to respiration) and oxygen

is released/unloaded from oxyhaemoglobin;

the higher carbon dioxide levels (from respiration) in the tissues shift the dissociation

curve for haemoglobin to the right/Bohr shift;

this causes more oxygen to be unloaded from haemoglobin for respiration;

the higher carbon dioxide levels also mean more carbonic acid production, producing

H+ ions which also favours the release of oxygen from haemoglobin (as a buffer);  

(any 5 points)

 

b)

i)

The smaller the animal, the further the curve is to the right;

 

 

ii)

Small animals lose more heat;

which needs a higher rate of respiration, to replace the lost heat;

this needs more oxygen;

their haemoglobin releases its oxygen more easily;

(any 3 points)

 

3.

a)

 

The protein molecules are too large to pass through the membranes of the cells of the tubule;

 

b)

i)

The loss of protein makes the water potential of the blood plasma less negative;

this means that less tissue fluid is reabsorbed into the blood in the capillaries;

 

 

ii)

There are not many lymph vessels in the ankles/feet;

and so the tissues do not drain;

 

 

OR

There are few muscles in the ankles/feet;

so not much squeezing of tissues to drain them;

 

4.

Haemoglobin in red cells returning to the lungs has little oxygen, and thus a high affinity for oxygen;

it becomes saturated with oxygen in the lungs;

and this happens over quite a wide range of (lung) oxygen concentrations (as seen

by the flat top to the curve);

in the muscle capillaries there is a high carbon dioxide concentration/low pH (due to a high rate of respiration);

which produces a Bohr effect/shifts the curve to the right;

this reflects a lower affinity of haemoglobin for oxygen;

so, oxyhaemoglobin dissociates/gives up oxygen easily;

which gives more oxygen for respiration in the muscle cells;

(any 7 points)