Organisms that are mobile can use taxes and kinesis to ensure that the environment they’re in is favourable.

Kinesis refers to orienting behaviour that’s non-directional. This is when an organism slows down or speeds up its rate of movement in proportion to the intensity of the stimulus. This ensures that they keep in environments that are favourable and away from environments that aren’t. For example, when it’s dark, woodlice reduce their rate of movement and turn more.

Taxis refers to orienting behaviour that’s directional. So an organism will either turn towards or away from a stimulus. Taxis can be either positive or negative: positive taxis attracts the organism, for example bacteria towards glucose, whereas negative taxis repels, like bacteria from bleach. Common stimuli include:

• light (phototaxis)
• gravity (geotaxis)
• movement (rheotaxis)
• chemicals (chemotaxis)

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Control of Heart Rate

The heart rate refers to the frequency of the cardiac cycle. In a healthy adult resting heart the heart beats at around 70 beats per minute (bpm) in males and 75 bpm in females. This varies in children and can be much lower in athletes.

The heart rate is controlled by the autonomic nervous system. This system is composed of two parts:

• the sympathetic nervous system: this deals with the ‘fight or flight’ response
• the parasympathetic nervous system: this is concerned with the rest response

Blood pressure is maintained by baroreceptors located in the carotid arteries. If there’s a change in the pH and carbon dioxide levels in the blood this is detected by chemoreceptors which are based in the medulla oblongata. A decrease in blood pressure or pH, or an increase in carbon dioxide alerts the sympathetic nervous system which gets the heart to pump faster.