17 December 2014

#47 Summary of The mammalian transport system

 1. Blood is carried away from the heart in arteries, passes through tissues in capillaries, and is returned to the heart in veins. Blood pressure drops gradually as it passes along this system.

2. Arteries have thick, elastic walls, to allow them to withstand high blood pressures and to smooth out the pulsed blood flow. Capillaries are only just wide enough to allow the passage of red blood cells, and have very thin walls to allow effi cient and rapid transfer of materials between blood and cells. Veins have thinner walls than arteries and possess valves to help blood at low pressure flow back to the heart.

 3. Blood plasma leaks from capillaries to form tissue fluid. This is collected into lymphatics as lymph, and returned to the blood in the subclavian veins. Tissue fluid and lymph are almost identical in composition; both of them contain fewer plasma protein molecules than blood plasma, as these are too large to pass through the pores in the capillary walls.

 4. Red blood cells are relatively small cells. They have a biconcave shape and no nucleus. Their cytoplasm is full of haemoglobin.

 5. White blood cells include phagocytes and lymphocytes. They all have nuclei, and are either
spherical or irregular in shape.

6. Red blood cells carry oxygen in combination with haemoglobin. Haemoglobin picks up oxygen at high partial pressures of oxygen in the lungs, and releases it at low partial pressures of oxygen in respiring tissues. A graph showing the percentage saturation of haemoglobin at diff erent partial pressures (concentrations) of oxygen is known as a dissociation curve. At high carbon dioxide concentrations, the dissociation curve shifts downwards and to the right, showing that haemoglobin releases oxygen more easily when carbon dioxide concentration is high. This is known as the Bohr effect.

 7 Carbon dioxide is mostly carried as hydrogencarbonate ions in blood plasma, but also in combination with haemoglobin in red blood cells and dissolved as carbon dioxide molecules in blood

 8 At high altitudes, the partial pressure of oxygen is so low that altitude sickness can be caused, which can be fatal. The body can adapt to gradual changes, however, by producing more red blood cells and haemoglobin.


Human circulation system

1. Multiple-choice test
1. Which description of blood vessels is correct?

A   Arteries have thick walls of smooth muscle with valves at intervals.
B   Arteries near the heart have large numbers of elastic fibres in their thick walls.
C   Capillary walls consist of a layer of endothelium surrounded by collagen fibres.
D   Small veins have thin walls made entirely of smooth muscle.

2. Which comparison of blood pressures is correct?

A   The pressure in arterioles is lower than in venules.
B   The pressure in capillaries is lower than in small veins.
C   The pressure in small arteries is higher than in large veins.
D   The pressure in the vena cava is higher than in capillaries.

3 Which statement about veins is not correct?

A   Blood is forced through a semilunar valve by the contraction of smooth muscle fibres in the wall of the vein.
B  Semilunar valves allow blood to move towards the heart but  not away from it.
C  Semilunar valves are formed from the endothelium and are moved by changes in blood pressure.
D  The pressure needed for blood flow in a vein is produced by contraction of nearby skeletal muscles.

4 Which of the following describe a phagocyte?
1   lobed nucleus
2   spherical nucleus
3   small granules in the cytoplasm
4  very little cytoplasm
5   smaller than a red blood cell

A   1, 3 and 5 only
B   2, 4 and 5 only
C   1 and 3 only
D   2 and 4 only

5. Which of the following describes a molecule of haemoglobin?

A    A molecule made up of four haem groups, each of which binds reversibly to an atom of oxygen.
B    A molecule made up of a single haem group which binds irreversibly with a molecule of oxygen.
C    A protein with quaternary structure, consisting of a single globin polypeptide attached to a haem group.
D    A protein with quaternary structure, consisting of two α- and two β-globin polypeptides, each attached to a haem group.

6 Which of the following word equations, showing reactions in a red blood cell, includes a mistake?

A    haemoglobin + oxygen oxyhaemoglobin
B    oxyhaemoglobin + hydrogen ions haemoglobinic acid
C    carbon dioxide + water carbonic acid
D    haemoglobin + carbon dioxide carboxyhaemog

7 The red blood cell count of humans increases when they remain at high altitudes.
What is the effect of this?

A    It increases the Bohr effect.
B    It compensates for the lack of oxygen at high altitudes.
C    It reduces the amount of haemoglobin per red blood cell.
D    It increases the percentage saturation of haemoglobin with oxygen.

8. The graph shows dissociation curves for haemoglobin at two different concentrations of carbon dioxide.

What may be concluded from the graph?

A    P is at a higher concentration of carbon dioxide than Q.
B    P is at a lower pH than Q.
C    Q shows haemoglobin that is more saturated with oxygen than P.

D    Q shows haemoglobin with a lower affinity for oxygen than P.

9. The diagram shows dissociation curves for adult haemoglobin, fetal haemoglobin and myoglobin. Myoglobin only releases oxygen when concentrations are very low. Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin does.

10 The statements describe blood, tissue fluid and lymph in a capillary bed.

  •  W lacks large plasma proteins and red blood cells and has a higher water potential than Z.
  •  X is at a lower pressure than Y and contains red blood cells and large plasma proteins.
  • Y is at a higher pressure than W and contains red blood cells and large plasma proteins.
  • Z is at a lower pressure than Y and lacks red blood cells.

Which row identifies W, X, Y and Z?

Answers to Multiple choice test

1. B
2. C
3. A
4. C
5. D
6. D
7. B
8. D
9. D
10. D

2. End-of-chapter questions

1 The diagram shows the changes in blood pressure as blood flows through  the blood vessels in the human  systemic circulatory  system.

The micrograph shows  an  artery  and  a vein.

3. Constructa table  comparing    the  structure  of arteries,   veins  and  capillaries.   Include   both  similarities   and  differences, and  give reasons  for  the  differences   which   you  describe.

4. Constructa table  comparing    blood   plasma,   tissue  fluid  and  lymph.

5. Explain how the  structure   of haemoglobin   enables  it to carry  out  its functions.  (You may  wish  to remind you  about  the  various levels of structure  of a protein   molecule   such  as Hb)

6. The  following statements    were  all made   by candidates    in examination     answers.   Explain   what   is wrong   with  each statement.

a  Oxyhaemoglobin    gradually   releases  its oxygen   as it passes  from  the  lungs  to a muscle. The b The strong  walls  of arteries   enable   them   to pump   blood   around   the  body.
c  Each red blood  cell can  combine    with  eight  oxygen  atoms.
d Red blood  cells have  a large  surface   area  so that  many   oxygen  molecules   can  be attached.

7    Carbon    dioxide   is transported     in  the  blood   in various   forms.
   a    Describe   how  carbon   dioxide   molecules   reach  red  blood   cells from  respiring   cells.                                                          [:
The  figure  shows  part  of a capillary   network   and  some  cells of the  surrounding     tissue.

  b    State  three   ways  in which   the  blood   at Y differs  from  the  blood at X other than   in  the  concentration  of carbon   dioxide.      

An enzyme   in  red  blood   cells catalyses   the  reaction   between   carbon   dioxide   and  water  as blood  flows  through respiring   tissues.

c    i Name   the  enzyme   that  catalyses   this  reaction.           [1]                                                                                                          
    ii Explain   the  significance of this  reaction   in  the  transport of carbon   dioxide.    [3]                                                          
d    The  figure  below  shows  the  effect  of increasing    the  carbon   dioxide   concentration    on  the  oxygen  dissociation curve  for  haemoglobin.

i State the percentage   saturation    of haemoglobin     with  oxygen  at a partial   pressure   of 5 kPa  of oxygen  when   the partial pressure  of carbon dioxide   is:
    1.0 kPa
    1.5 kPa                                                                                            [1]

ii  The percentage  saturation of haemoglobin with oxygen  decreases   as the  partial   pressure   of carbon   dioxide increases.Explain  how  this  happens.                                 [2]

iii Name the effect of increasing   carbon   dioxide   concentration     on  the  oxygen  dissociation    curve.                                                                                                                [1] 
iv   Explain the importance    of the  effect  of carbon   dioxide   on  haemoglobin     as shown   in the  figure.                                                                                                                   [3]
 [Total:   16]

[Cambridge International AS and A Level Biology 9700  Paper 21,  Question 2, June 2011]

8. Mammalh save a closed,  double   circulation.
a State what is meant   by the  term  double   circulation.          [1]

The figure below shows  part  of the  circulation  in a mammalian tissue.  The  central   part  is enlarged   to show  a capillaray,cell supplied   by the  capillary,   and  vessel  Z.

 b    Explainwhy the  wall  of the  artery   is thicker   than   the  wall  of the  vein.      [2]
c    Suggest one  role for  the  pre-capillary   sphincter    muscle   shown   in the  figure.   [1]
d    With reference  to  the  figure,  describe   the  role  of capillaries   in forming   tissue  fluid.   [3.]
e  i Describe three  ways  in which   plasma   differs  from  tissue  fluid.                    [3]
    ii Name the fluid  in vessel  Z.                                                                              [1]


[Cambridge InternationalAS   andA  Level Biology 9700  Paper 2, Question 4, November 2008]

3. End-of-chapter answers

    1 C
    2 D

5  Points that could be made include:

•  The haemoglobin molecule is a protein with quaternary structure. Hydrogen bonds, ionic bonds and van der Waals forces hold the protein
in its three-dimensional shape, which is important for its function.

•  The primary structure of each polypeptide chain determines how the chain will fold and where the bonds will form, thus determining its three- dimensional shape.

•  The haemoglobin molecule has R groups with small charges on its outer surface (hydrophilic R groups), which help to make it soluble in water. This allows it to dissolve in the cytoplasm of a red blood cell.

•  Each haemoglobin molecule is made up of four polypeptide chains, each with a haem group at its centre. Each haem group can bind reversibly with one oxygen molecule.

•  When one oxygen molecule binds with one of the haem groups, it slightly changes the shape of the haemoglobin molecule so that it becomes easier
for more oxygen molecules to bind with the other haem groups.

6  a  The word ‘gradually’ is not correct. The partial pressure of oxygen is high in the lungs and low in muscle. It does not change gradually as the blood flows from the lungs to the muscle, because it is only when it gets to the muscle that the blood is in contact with anything that is using oxygen. While it is inside an artery, it remains fully oxygenated. The blood is only exposed to a low partial
pressure of oxygen once it enters a capillary inside a respiring tissue, such as a muscle. Capillary walls, unlike those of arteries, are thin and easily permeable to oxygen.

b Arteries do not pump blood. Their strong walls, which are also elastic, enable the artery to expand and recoil as pulses of high-pressure blood pass through. The recoil of the artery wall does help to give the blood a further ‘push’ in between these pulses, but this is not ‘pumping’ and is due only to elasticity, not to muscle contraction.

c  This should say: Each haemoglobin molecule can combine with eight oxygen atoms. A red cell is huge compared with a haemoglobin molecule. One red cell contains well over 200 million haemoglobin molecules.

d Red blood cells do have a large surface area, but oxygen does not attach to their surface. The large surface area allows more oxygen to diffuse in and out at any one time, therefore increasing the rate at which the cell can take up and release oxygen. Once inside the cell, the oxygen does not attach to its surface, but to the haemoglobin molecules within its cytoplasm.

Exam-style questions

7  a  reference to diffusion;
down concentration gradient;
through the wall of a capillary;                                                            [max. 2]

b lower pressure;
             lower concentration of oxygen; lower concentration of glucose
             lower water potential;
             lower concentration of proteins/amino acids/fatty acids/other named nutrient;
             higher concentration of carbon dioxide/urea;                                            [max. 3]

            c  i     carbonic anhydrase;                                                                            [1]
            ii    hydrogencarbonate ions diffuse out of red blood cells;
                 (hydrogencarbonate ions) are transported in solution in blood plasma;
                  conversion of CO2  to hydrogencarbonate reduces concentration of CO2  in the blood;
                  which maintains diffusion gradient for CO2  to diffuse into the blood from respiring  tissues;                                                                                              [max. 3]

       d i     73%, 62%;                                                 [1]
          ii    presence of carbon dioxide causes affinity of haemoglobin for oxygen to decrease;
   hydrogen ions (from the dissociation of H2CO3) bind with haemoglobin;
                  cause change in shape of Hb molecule; [max. 2] 
           iii  Bohr effect;                                        [1]
           iv   causes more release of oxygen (than if this effect did not occur);
                 in respiring tissues;
                where demand for oxygen is high/where production of carbon dioxide is high;          [3]
[Total: 16]

8  a  blood goes through heart twice on one complete circuit of the body;                                              [1]

b has more smooth muscle/elastic tissue;
to withstand higher (blood) pressure;
to withstand fluctuating (blood) pressure;                                                                             [max. 2]
     c  to prevent blood flowing into the capillary bed/to divert blood to other capillary beds;   [1]                                                                                                                                                         
     d permeable walls/reference to pores in walls;
         allow water/dissolved ions/dissolved substances (from plasma) to pass out;
        do not  allow large protein molecules/cells to pass out;
        reference to greater hydrostatic pressure inside capillary than in tissue fluid;                 [max. 3]
    e  i     (plasma contains) more proteins
       has lower water potential;
       has lower, carbon dioxide/HCO3 concentration;
       has greater glucose concentration;
       has greater oxygen concentration;       [max. 3]
ii              lymph                      [1]

[Total: 11]

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