Biology: Homeostasis and excretion

Topics on test:


·      Reflex arc

·      Structure of the eye

·      Accommodation 

·      How the kidney works

·      How the body controls body temperature and what (and how) different factors affect how much we sweat. 


Reflex arcs


A reflex is something your body does without conscious thought and it’s to help protect you. They are very quick because an organism’s life may depend on how quickly the reflex works. This is known as innate behaviour. A reflex arc, in the most basic of explanations, works like this:

·      There is a stimulus (eg heat)

·      This stimulates a receptor (eg pain receptor)

·      Signal is sent to the sensory neurone

·      Signal is passed along the relay neurone

·      Message is sent to brain, which decides how to produce a response

·      Signal is sent along the motor neurone

·      Effector produces a response (eg muscle contracts)


Note that effectors are usually muscles or glands.


Synapses are the gaps between the neurones. The neurones don’t touch directly. Instead, a substance diffuses across the gaps, which is known as a neurotransmitter. It works as follows:

·      Electrical impulse travels along an axon

·      Triggers nerve ending of a neuron to produce neurotransmitters (chemical messengers)

·      Chemical diffuses across the synapse, which is the gap

·      Neurotransmitters bind with receptor molecules on the membrane of the next neuron

The receptor molecules only bind to the specific chemicals released from the first neurone and stimulates the second neuron to transmit the electrical impulse.


Structure of the eye

(google: eye song)


·      Suspensory ligaments hold the lens in place and help control its shape

·      Ciliary muscles control the shape of the lens

·      Vitreous humor (jelly inner bit) keeps eye shape intact

·      Conjunctiva keeps the front of the eye moist with mucus

·      Optic nerve sends signals to the brain to be interpreted

·      Retina is where the light is refracted onto and has photoreceptors to capture light rays and turn them into electrical signals and contains cones and rods

·      Fovea is responsible for sharp vision

·      The cornea is clear and refracts the light


·      Circular and radial muscles are found in the iris and control the amount of light let in


IMPORTANT: Light is electromagnetic radiation travelling in waves



How the kidney works



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