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The cellular junctions and receptors help an organism maintain homeostasis is when <span>they carry signals throughout the body to maintain body functions</span>
To avoid this u need to drink LOTS of water
Excreting urea helps in maintaining internal homeostasis by retaining water.
The mammals like humans excrete urea, while reptiles and birds, and some terrestrial invertebrates generate uric acid as waste. Generating uric acid in place of urea is beneficial as it is less toxic and diminishes water loss and the subsequent requirement for water.
This retention of water helps birds and reptiles to sustain internal homeostasis mainly in hot weather.
This picture represents humoral immunity, please read through it, and if you dont understand which option is your answer then i would recommend you read more about this topic, if you want it to be done fast, you can read this http://www.s-cool.co.uk/a-level/biology/immunity/revise-it/the-third-line-of-defence
I hope you can come up with the answer on your own as it will be a good thing for you in your future exams :)
Surely i can help you if you dont understand something so just ask :D