Capillaries are extremely fine (about one cell layer thick) blood vessels whose main function is to carry oxygen and nutrients to various parts of our body and remove carbon dioxide and water (by products of respiration). The exchange of material between capillaries and tissues takes place by diffusion, and the fineness of capillaries enables this process. This material exchange is also aided by selective permeability of capillaries.
There are a number of sensors in human body, most common of which are temperature sensors, motion sensors, auditory sensors. Our eyes capture external light and our nerve cells relay those as images to our brain. Our skin has sensors everywhere and that is how we know when any other surface is in contact with our skin. Temperature sensors inform the hypothalamus when internal body temperature is rising so that defense mechanisms can be activated to decrease it.
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