#51 Summary of Gas exchange

1 Multicellular organisms often have surfaces that are specialised to allow exchange of gases to take place between their bodies and the environment. Alveoli in the lungs form the gas exchange surface in mammals.

 2 In the human lungs, air passes down the trachea and through a branching system of airways to reach the alveoli. The airways are lined by a ciliated epithelium with mucus-secreting goblet cells. The epithelium protects the alveoli by moving a carpet of mucus

towards the throat, where it can be swallowed.

 3 There are C-shaped rings of cartilage in the trachea and irregularly shaped blocks of cartilage in the bronchus to keep the large airways open and so reduce resistance to the fl ow of air. Smooth muscle in the airways contracts and relaxes to adjust the diameter of the airway.

4 The alveoli are lined by a squamous epithelium that gives a short diff usion distance for the exchange of oxygen and carbon dioxide. The alveoli are well supplied with blood by the many capillaries surrounding the gas exchange surface.

 5 The constant flow of blood and the continuous ventilation of the lungs maintain concentration gradients between blood and air for oxygen and carbon dioxide.

 6 Recoil of the elastic fi bres surrounding the alveoli helps to move air out during expiration.

1. Multiple-choice test

1  A large, thick-walled blood vessel lies alongside a bronchus. 

Which row names the vessel and describes its contents?

2 The diagram shows part of the wall of a bronchus in cross-section.

3 The table shows the presence or absence of ciliated epithelium and goblet cells in airways. 

Which row describes a bronchiole?

4 Two airways each have smooth muscle in their walls, but only one has cartilage.

What are the airways?

A  alveolus and bronchus
B  alveolus and trachea
C  bronchiole and bronchus
D  bronchus and trachea

5 The mucus secreted into the airways is a solution of the glycoprotein mucin.

Which statement about mucin contains a mistake?

A  Carbohydrate chains make mucus sticky enough to trap dust particles.
B  Mucus is secreted by goblet cells by the process of endocytosis.
C  Carbohydrate chains are added to protein in the Golgi apparatus of goblet cells.
D  Mucus is moved over the surface of the airways by the action of ciliated cells.

6 A carbon dioxide molecule dissociates from haemoglobin and diffuses along the shortest path into an alveolus.

Assuming that the molecule diffuses through a gap in a capillary wall, how many phospholipid bilayers did the molecule pass through?

A  2
B  3
C  4
D  5

7 What maintains the diffusion gradient for the diffusion of oxygen out of an alveolus?

1  binding of oxygen with haemoglobin to form oxyhaemoglobin
2  blood flow bringing a new supply of red blood cells
3  increased surface area of red blood cells as they are forced through narrow capillaries
4  complete replacement of the air in the alveolus with each breath

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

8 What are the adaptations of an alveolus for its role in gas exchange?

1  very thin epithelial walls
2  close contact of walls and capillaries
3  walls with elastic fibres which recoil after stretching, to help force air out
4  stiff walls to prevent collapse of the alveolus when breathing out

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

9 Which event occurring at an alveolus does not require a red blood cell? 

A  carbon dioxide dissociates from carbaminohaemoglobin
B  carbon dioxide is formed from hydrogencarbonate ions
C  carbon dioxide diffuses from the blood into an air space
D  oxygen binds with haemoglobin to form oxyhaemoglobin

10 Measurements of an adult’s breathing show that at rest:

• the volume of air in a single breath is 500cm3
• 350 cm3  of each breath reaches the alveoli
• 2500 cm3 of air remain in the lungs after breathing out.

Which is not a correct conclusion from these measurements?

A  About one sixth of the air in the lungs is replaced by breathing out and then in.
B  Almost one third of each breath does not reach a gas exchange surface.
C Large changes in the composition of the air in the alveoli do not occur.

D The volume of air in the alveoli after breathing in is 3.0dm3

Answers to Multiple choice test

1. A

2. B

3. C

4. C

5. B

6. B

7. A

8. A

9. C

10. D

2. End-of-chapter questions

1   The following structures  are found  in the walls of the gas exchange system.

1   capillaries

2   cilia

3   elastic fibres

4   goblet cells

5   smooth  muscle cells

Which  would  be found  in the lining of an alveolus?

A   1 and 3

B   1,2  and 3

C   2 and 5

D  4 and 5

2  Cartilage is found   in which   structure?

A  alveolus

B  bronchiole

C  capillary

D  trachea

3  Which of the  following   is not  a role  of elastic  fibres  in  the  gas exchange   system?

A   contract   to  decrease   the  volume   of the  alveoli  during   expiration

B   recoil  to force  air out  of the  alveoli  during   expiration

C  stretch  to accommodate     more  air in  the  alveoli  during   deep  breathing

D stretch  to increase   the  surface   area  of the  alveoli  for  gas exchange

4  Which  of the  following   best  describes   the  process   of gas exchange   in  the  lungs?

    A   Air moves  in  and  out  of the  alveoli  during   breathing.
    B   CO₂  diffuses  from  deoxygenated  blood in  capillaries   into  the  alveolar  air.
    C   O₂   and  CO₂   diffuse  down   their  concentrations gradients between blood and alveolar  air.
    D  O₂  diffuses  from  alveolar   air into  deoxygenated  blood.

5    The  figure  shows  an alveolus.

a   Name:

i  cells A, B and  C                                                                                                         [3]

ii  the  fluid  at  D.                                                                                                         [1]

b  Calculate   the  actual  distance   indicated  by X- Y. Show  your  working.          [2]


c   Explain   how  alveoli  are adapted   for  the  exchange   of gases.                        [4]
                                                                                                                                    Total  [10]

6    The  figure  shows  two  cells from  the  lining   of the  trachea.

a    Name:

i  cells  P and  R                                                                                                                      [2]

ii  structures  S, T, U  and  W                                                                                                [4]

b    Explain:

i  why  cell  P contains  many   of the  structures  labelled  T                                          [2]

ii  the  role  of structure U  in cell  R.                                                                                   [2]

c    Calculate  the  actual   length   of cell  P.  Show  your  working.                                      [2]

d    Describe   the  roles  of cell P and  cell R in  the  gas exchange  system.                       [4]

                                                                                                                                          [Total:   16]

7    a    Copy   and  complete   the  table  to  compare   the  trachea   with  a respiratory  bronchiole.   Use  a tick  (v)   to indicate   that  the  structure    is present   and  a cross  (x)  to  indicate   that  it is not.

[5]

     b    Describe   how  the  alveoli  are protected    against   infection.                                                   [5] 

                                                                                                                                                             [Total:   10]

8  The composition  of alveolar   air  remains   fairly  constant    even  though   gases  are exchanged   with   the  blood   in  the capillaries  that  surround    the  alveoli.

a   Describe   the  process   of gas exchange   between   alveolar  air and  blood.                          [4]

b   Explain  why  the  composition     of alveolar   air remains   fairly  constant.                              [3]

c   Suggest  3 ways  in which  the  gas exchange system responds  to the demands of exercise. [3]

                                                                                                                                                                [Total:   10]

3. End-of-chapter answers

 1 A
 2 D
 3 A
 4 C

5 a   i A squamous epithelial cell;
          B red blood cell;
          C endothelial cell; [3]
 
      ii) D plasma;             [1]

  b distance of scale bar is 15mm 15mm = 10μm X–Y distance measured on page is 47mm 47 ÷ 15 ×      10 = 31μm                     [2]

  c  (very) large number forming a large surface area;
       squamous epithelial cells are very thin to give short diff usion distance;
       surrounded by capillaries so well supplied with blood;
       capillaries are very close so short diff usion distance; 
      well ventilated so air constantly refreshed;
      maintains concentration gradients for oxygen and carbon dioxide; [max. 4]
                                                                                                             [Total: 10]

6 a i   P ciliated epithelial cell;
         R goblet cell; [2]
      ii S cilium/cilia;
        T mitochondrion;
        U Golgi apparatus;
       W nucleolus; [4]

 b i  T/mitochondria provide energy/ATP;
         for movement of cilia; [2]

   ii  U/Golgi apparatus packages proteins into vesicles;
       for secretion; [2]

c   length of cell P on page is 80mm 80 ÷ 750 × 0.107mm or 107μm [2]

d  cell P: cilia beat/move back and forth;
              move mucus;
              upwards/towards throat;

     cell R: secretes mucus;
                mucus traps dust/bacteria/viruses/pollen;
                prevents entry to alveoli/gas exchange surface; [max. 4]
                                                                                         [Total: 16]

b   mucus secreted; 
     by mucous glands (in the trachea)/goblet cells (in trachea and bronchi);
     bacteria/viruses/pathogens, stick to mucus;
     cilia move mucus, upwards/towards throat;
     mucus and pathogens swallowed;
     destroyed by acid in stomach;
     macrophages/phagocytes, in the alveoli;
    engulf and digest any pathogens;                                              [max. 5]
                                                                                                      [Total: 10]

8  a  oxygen diff uses down its concentration gradient;
        from alveolar air into red blood cell; 
        carbon dioxide diff uses down its concentration gradient;
       from red blood cells/plasma to alveolar air;
       across epithelial cells of alveolus and endothelium of capillary; [max. 4] 

   b  breathing/ventilation;
      introduces fresh air/atmospheric air;
      removes stale air/air rich in carbon dioxide;                                       [3]

   c  increase in:
       • depth of breathing;
       • rate of breathing;
       • expansion of alveoli to give a larger surface area;
      • diameter of airways;                                                                     [max. 3]
                                                                                                        [Total: 10]