Role of Skin and Lungs in Excretion

Excretion is not the kidney's job alone; two other organs make a real, if smaller, contribution.

Skin. In many animals the skin is thin and permeable enough that waste products such as ammonia can simply diffuse straight through it. Human skin, by contrast, is thick and largely impermeable, so it instead relies on two kinds of glands. Sweat glands — simple, unbranched, coiled tubular glands distributed over the whole body but especially dense on the palms and face — open onto the skin surface through sweat pores. Sweat is produced mainly for thermoregulation (cooling the body by evaporation), but in the process it also carries out water, sodium chloride, lactic acid and urea, making sweating a genuine (if secondary) excretory route. Sebaceous glands, found at the neck of hair follicles, secrete an oily substance called sebum that lubricates and softens the skin surface and helps protect it from infection and injury; sebum is not itself a waste product, but the gland sits in the same integument that also does excretory work. …