17 December 2014

#48 Summary of The mammalian heart

 1 The human heart, like that of all mammals, has two atria and two ventricles. Blood enters the heart by the atria and leaves from the ventricles. A septum separates the right side of the heart, which contains deoxygenated blood, from the left side, which contains oxygenated blood.

 2 Semilunar valves at the entrances to the blood vessels that leave the heart (aorta and pulmonary
artery) prevent back flow of blood into the heart, and atrioventricular valves prevent backflow of blood from ventricles into the atria.

 3 The heart is made of cardiac muscle and is myogenic (the muscle is self-stimulating).

4 The sinoatrial node (SAN) sets the pace of contraction for the muscle in the heart. Excitation
waves spread from the SAN across the atria, causing their walls to contract. A non-conducting barrier prevents these excitation waves from spreading directly into the ventricles, thus delaying their
contraction. Th e excitation wave travels to the ventricles via the atrioventricular node (AVN) and
the Purkyne tissue, which runs down through the septum, before spreading out into the walls of the

 5 Both sides of the heart contract and relax at the same time. Th e contraction phase is called systole, and the relaxation phase is diastole. One complete cycle of contraction and relaxation is known as the cardiac cycle.

1. Multiple-choice test

1 Which of the following describes the mammalian circulation?

A open single circulation
B closed single circulation
C open double circulation
D closed double circulation

2 The diagram shows a vertical section through a human heart.

 Which row identifies the blood vessels W, X, Y and Z?

3 Which row describes the aorta?

4 The diagrams are vertical sections through the human heart.
Which pair of arrows shows blood flow through the heart?

5 The right ventricle has much less muscle in its wall than the left ventricle.
What are the consequences of this?

1 The right ventricle develops a much smaller pressure than the left ventricle.
2 The right ventricle delivers a smaller volume of blood than the left ventricle.
3 Blood from the right ventricle travels less far than blood from the left ventricle.

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

6 What are the positions of the valves on the left side of the heart when the pressure in the left ventricle is higher than the pressures in the left atrium and aorta?

7 Which of the following statements is not correct?

A Atrial muscles are connected to the ventricle muscles, except at the atrioventricular node (AVN).
B Both atria contract at the same time.
C Both ventricles contract at the same time.
D Contraction of the atria is complete before contraction of the ventricles begins.

8 Which is the correct sequence of events in a cardiac cycle, beginning with its initiation by the pacemaker?

1 A wave of electrical activity passes along Purkyne tissue.
2 A wave of electrical activity reaches the atrioventricular node (AVN).
3 A wave of electrical activity spreads from the sinoatrial node (SAN) across the atria.
4 Cardiac muscle of the walls of the atria contracts.
5 Cardiac muscle of the walls of the ventricles contracts.

A 1 → 5 → 3 → 4 → 2
B 2 → 1 → 5 → 3 → 4
C 3 → 4 → 2 → 1 → 5
D 4 → 2 → 1 → 5 → 3

9 When a heart is removed from a mammal and kept in well-oxygenated buffer solution at 37°C, it continues to beat rhythmically.
What may be concluded about the heart from this observation?

A It has an in-built mechanism for initiating contractions.
B It needs a blood supply to be able to contract.
C It needs a stimulus from a nerve to be able to contract.
D It needs a stimulus from a hormone to be able to contract.

10 The volume of blood pumped by the heart in a given period of time is called the cardiac output. It is calculated from the volume of blood pumped by one contraction of the heart (stroke volume) and the number of times the heart contracts per minute (heart rate).

cardiac output = stroke volume × heart rate

The cardiac output of a heart beating at 75 beats per minute was calculated to be 6.0dm3 per minute.
What was the stroke volume of the heart?

A 0.08cm3
B 12.5cm3
C 80cm3
D 125cm3

Answers to Multiple choice test

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

2. End-of-chapter questions

1 Where   is the  mammalian   heart   beat  initiated?

A   atrioventricular    node
B   left  atrium
C   Purkyne   tissue
D   sinoatrial    node

2    What   causes  the  bicuspid   valve  to  close  during   ventricular   systole?

A   a greater   blood   pressure   in  the  left  atrium   than   in  the  left ventricle
B   a greater   blood   pressure   in  the  left ventricle   than   in  the  left  atrium
C   contraction   of muscles   in  the  septum
D   contraction   of muscles   in  the  valve

3    Figure below shows  the  pressure   changes   in  the  left  atrium,   left ventricle   and  aorta  throughout   two cardiac   cycles.  Make   a copy  of this  diagram.

a    i How  long  does  one  heart   beat  (one  cardiac   cycle)  last?
     ii  What   is the  heart   rate  represented   on  this  graph,   in  beats  per  minute?
b    The  contraction   of muscles   in  the  ventricle   wall  causes  the  pressure   inside  the  ventricle   to  rise. When   the muscles   relax,  the  pressure   drops   again.  On  your  copy  of the  diagram,   mark  the  following   periods:
   i the  time  when   the  ventricle   is contracting   (ventricular  systole)
  ii the  time  when   the  ventricle   is relaxing   (ventricular  diastole).

c    The  contraction   of muscles   in  the  wall  of the  atrium   raises  the  pressure   inside  it. This  pressure   is also raised when   blood   flows  into  the  atrium   from  the  veins,  while  the  atrial  walls  are relaxed.   On  your  copy  of the diagram,  mark  the  following   periods:
   i the  time  when   the  atrium   is contracting   (atrial  systole)
  ii  the  time  when   the  atrium   is relaxing   (atrial  diastole).

d  The atrioventricular   valves  open  when   the  pressure   of the  blood   in  the  atria  is greater  than  that  in the  ventricles. They snap shut  when   the  pressure   of the  blood   in  the  ventricles   is greater  than  that  in the  atria.  On  your diagram,mark  the  point   at which   these  valves  will  open  and  close.
e The opening and  closing  of the  semilunar  valves  in the  aorta  depends   in a similar  way  on  the  relative  pressures inthe aorta and  ventricles.   On  your  diagram,   mark  the  point   at which   these  valves will  open  and  close.
f   The right ventricle  has  much   less muscle   in its walls  than  the  left ventricle,   and  only  develops   about   one-quarter of  the pressure   developed   on  the  left  side  of the  heart.   On  your  diagram,   draw  a line  to represent   the probablepressure  inside  the  right  ventricle   over  the  1.3 seconds   shown.

The  diagram shows a normal   ECG.   The  paper  on  which   the  ECG   was  recorded   was  running    at a speed  of 25 mm s-1 

a    Calculatethe heart  rate  in beats  per  minute.
b   Thetime interval  between   Q and  T  is called  the  contraction     time.
   i  Suggest why it is given  this  name.
  ii Calculate the  contraction  time  from  this  ECG.
c    The time interval  between   T  and  Q is called  the  filling time.
   i Suggest why it is given  this  name.
  ii Calculate the  filling  time  from  this  ECG.
d An adult male recorded   his  ECG   at different   heart   rates.  The  contraction     time  and  filling  time  were  calculated from  the ECGs.  The  results  are  shown   in the  table.

i  Suggest   how  the  man  could   have  increased   his heart   rate  for  the  purposes   of the  experiment.
ii  Present   these  results  as a line  graph,   drawing   both   curves  on  the  same  pair  of axes.
iii Comment  on  these  results.

5    The  figure  below  shows  a cross-section   of the  heart   at the  level  of the  valves.

a    i Copy   and  complete    the  following   flow  chart   to show  the  pathway   of blood   through    the  heart.

  ii Explain   how  the  valves  P and  Q ensure   one-way   flow  of blood   through    the  heart.

The cardiac cycle describes  the  events  that  occur  during   one  heart   beat.  The  following   figure  shows  the  changes   in pressure that occur  within   the  left  atrium,   left ventricle   and  aorta  during   one  heart  beat.

Copy  and complete  the  table  below.  Match   up  each  event  during   the  cardiac  cycle with  an  appropriate number from 1 to  7 on  the  figure.  You should   put  only  one  number    in each  box.  You may  use  each  number once, more than once or not  at all.
The firstanswer has been  completed    for  you.

Explainthe roles of the  sinoatrial    node   (SAN),   atrioventricular     node   (AVN)  and  the  Purkyne   tissue  during   one     heart  beat.          [5]
                                                                                                    [Total:   13]                                                                                                                                                                                  

[Cambridge Intemational AS andA  Level Biology 9700  Paper 21,  Question 3, May - june  2010]

3. End-of-chapter answers

 1 D
 2 B
 3 a i about 0.75 seconds
        ii 60 ÷ 0.75 = 80 beats per minute
 For b, c, d, e and f, see figure below.

4 a 1 beat = about 20 mm on the grid. 25 mm on the grid represents 1 second
 so 20 mm represents 20÷25 seconds = 0.8 seconds. If one beat lasts 0.8 seconds, then in 1 second there are 1÷0.8 beats  so in 1 minute there are 60÷0.8 = 75 beats.
 b i this is the time during which the ventricles are contracting
    ii on the grid, the distance between Q and T is about 7 mm  this represents 7 ÷ 25 = 0.28 seconds

c i this is the time when the ventricles are relaxed, and are fi lling with blood
   ii on the grid, the distance between T and Q is about 13 mm
 this represents 13 ÷ 25 = 0.52 seconds
 A quicker way of working this out is to subtract the answer to b ii from 0.8 seconds.

d i by performing varying levels of exercise

iii As heart rate increases, contraction time remains constant, but fi lling time decreases.
This indicates that the increase in heart rate is produced by a shorter time interval between
ventricular contractions, rather than by a faster ventricular contraction.

The more frequent contractions increase the rate of circulation of blood around the body,
providing extra oxygen to exercising muscles.

If this was done by shortening the time over which the ventricles contract, much of the
advantage would be lost, as less blood would probably be forced out by each contraction.
By shortening the time between contractions, the amount of blood pumped out of the heart
per unit time is increased.

Exam-style questions

 5 a i right ventricle;
          pulmonary vein; [2]
     ii they open to allow blood to fl ow from atria to ventricles;
       they close during ventricular systole/when ventricles contract;
       reference to closure being caused by diff erences in pressure in atria and ventricles; [max. 2]

c SAN produces rhythmic pulses of electrical activity;
 which spread across the muscle in the atria;
 causes muscle in atria to contract;
 specialised tissue, in septum/near AVN, slows spread/delays transfer to ventricles;
 Purkyne tissue conducts impulses down through septum;
 impulses spread upwards through ventricle walls;
 causing ventricles to contract from bottom upwards;
 delay of 0.1 to 0.2 s after atrial walls; [max. 5]
 [Total: 13]


  1. I think you transposed questions 3 and 4's diagrams!

  2. Cont'd On the multiple choice question test

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