Muscles, heart rate and ventilation

Sample questions

Original practice questions written in the style of the exam.

1. Question 14 marks

   Explain how heart rate increases at the start of exercise.

   Mark scheme

   • Increased respiration raises the carbon dioxide concentration / lowers the pH of the blood (1)
   • This is detected by chemoreceptors (in the medulla, aortic and carotid bodies) (1)
   • Impulses are sent to the cardiovascular control centre in the medulla oblongata (1)
   • More impulses travel along sympathetic nerves to the SAN, increasing its rate of discharge (1)
2. Question 23 marks

   At rest, an athlete has a cardiac output of $5.6\,\text{dm}^3\,\text{min}^{-1}$ and a heart rate of 64 beats per minute. During exercise her heart rate is 160 beats per minute and her stroke volume is $120\,\text{cm}^3$.

   (a) Calculate her stroke volume at rest.

   (b) Calculate the percentage increase in cardiac output during exercise.

   Mark scheme

   • (a) Stroke volume $= 5600 \div 64 = 87.5\,\text{cm}^3$ (1)
   • (b) Cardiac output during exercise $= 160 \times 120 = 19\,200\,\text{cm}^3\,\text{min}^{-1}$ (1)
3. Question 35 marks

   Describe the roles of calcium ions and ATP in the contraction of a skeletal muscle fibre.

   Mark scheme

   • Calcium ions are released from the sarcoplasmic reticulum when an impulse arrives (1)
   • Calcium ions bind to troponin, which moves tropomyosin away from the myosin-binding sites on actin (1)
   • Myosin heads bind to actin, forming cross-bridges, and the power stroke pulls actin towards the centre of the sarcomere (1)
   • ATP binds to the myosin head, causing it to detach from actin (1)
   • Hydrolysis of ATP (by ATPase) re-cocks the myosin head so it can bind further along the actin (1)