Responding to exercise

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Definition: the maintenance of a constant internal environment

eg: Core temperature control to allow enzymes to work efficiently

Levels of oxygen and glucose maintained within narrow limits to allow aerobic respiration (anaerobic respiration is for emergencies)

This topic focuses on the control of the heart rate to maintain oxygen and glucose levels

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Types of feedback

Negative feedback

  • Change detected by sensors or receptors
  • Message sent to effector
  • Effector works to reverse the change

Most common feedback in the body, as it maintains things within a very narrow range

Positive feedback

  • Change detected by sensors or receptors
  • Message sent to effector
  • Effector works to increase the change

Does occur, for example during labour, where the first few smooth muscle contractions trigger more, stronger contractions as birth progresses

If communication is hormonal, effectors are the target organs. If communication is nervous, the effectors are muscles or glands

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Negative feedback


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Cardiac output

Cardiac output = cardiac volume x heart rate

Cardiac output = amount of blood pumped per minute

Cardiac volume = amount of blood pumped in each heartbeat

Heart rate = beats per minute

Therefore the amount of blood flowing to the muscles (and so the amount of oxygen and glucose being provided) can be increased by raising the heart rate and by raising the cardiac volume (more efficient contraction of ventricles)

Cardiac output needs to increase when we exercise because muscles are respiring more to produce more ATP, and therefore need more oxygen and glucose. But at rest we need to return it to normal or risk exhausting our cardiac muscle.

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Sensors and triggers


When exercise first starts large, contracting muscles in the limbs squeeze more blood along veins. Stretch receptors in the cardiac muscle recognise more blood entering the atria and send more nerve receptors to the cardiovascular centre of the brain.

The increased stretching also directly increases the strength of contraction, without involving the brain.

Stress, or anticipation of exercise

Hormonal changes can also alter cardiac output; even in the absence of exercise stress can cause release of adrenaline which acts on the SAN, speeding the heart rate up.


Baroreceptors in the carotid artery respond to blood pressure changes, and send impulses to the cardiovascular centre to affect heart rate or change width of blood vessels to reverse the change detected

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Cardiovascular centre

Found in the medulla

The heart is mostly controlled by the autonomic (involuntary) nervous system, divided into 2 parts:

  • Sympathetic nervous system - excitatory, speeds things up
  • Parasympathetic nervous system - inhibitory, slows things down

Nerve impulses from both these systems stimulate the SAN to increase or decrease the heart rate as needed.

We do have a certain amount of voluntary control over our heart rate

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Other effects

When the sympathetic nerve speeds up heart rate, a few impulses (also along the sympathetic nerve) causes contraction of smooth muscle in blood vessels, diverting blood from areas such as the gut. The brain's blood supply remains relatively constant.


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Nodes of the heart


Purkinje fibers = Purkyne fibres! Either name is fine

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