JOHN BAVOSI / SCIENCE PHOTO LIBRARY JOHN BAVOSI / SCIENCE PHOTO LIBRARY
Menstruation. Artwork of the lining of the uterus (endometrium) during menstruation, the shedding of the uterine lining during the monthly menstrual cycle in women. At the start of the cycle the endometrium thickens, increasing the number of blood vessels (narrow, blue and red) that supply it. Later, the cells (dark yellow) of the endometrium become swollen with retained fluid in preparation for the implantation of a fertilised egg. This swelling occurs at ovulation (egg release from the ovary). If the egg is not fertilised, the endometrium breaks down and is shed as a bloody discharge (red, top left to bottom right) through the vagina.
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