Human vaginas don't have the fantastical loops and blind alleys of a duck vagina, but they still have some pretty amazing shape-changing powers. Here's how they're put together, and how that anatomy lets them grow when they need to.
They certainly need to grow before they're put to use. The average vagina is only about two-and-a-half inches long and a little more than an inch wide when it isn't being used for sex or childbirth. The average erect penis is about twice as long as a resting vagina (and a baby's head nearly four times as wide!), so it's clear that something has to change when the organ's called into action.
An Organ Poised to Shape-Shift
The human vagina is a muscular tube. It opens to the outside world in the vulva and plunges through the muscles of the pelvic floor to meet the cervix.
Its wall is made up of four distinct tissue types, stacked like a layer cake from the vaginal cavity to the organ's outer edge.
The innermost layer of the vaginal wall is a thick mucous membrane (the white stuff, in the image to the left) that sits atop a network of blood vessels. A layer of smooth muscle fibres underlies the blood vessels, followed by a layer of tough collagen and elastic fibres that mark the outer edge of the vagina.
The wall is folded, forming three-dimensional pleats called rugae that make the inside of the vagina feel bumpy.
Like an accordion, the pleats let the vagina store the extra material it needs to expand -- all the tissue needs is a signal to unfold.
Making Room for Sex
When a women becomes sexually aroused, nerves inside the vaginal wall release two neurotransmitters: nitric oxide (NO) and vasoactive intestinal peptide (VIP). These molecules widen the arteries inside the vaginal wall and let more blood flow into the tissue. They also help relax the smooth muscle in the wall.
As vaginal tissues engorge with blood their now-relaxed folds extend and make the vagina both longer and wider. Engorgement makes the vagina the right shape to accommodate a penis, and also helps the vagina lubricate itself.
As the tissues fill with blood, plasma leaks from the congested capillary beds, overwhelming the mucous membrane's ability to reabsorb it. It seeps out from between the cells, forming a slick and slippery layer on the inner surface of the vagina that helps keep sex fun.
Making Room for Baby
Fitting over a penis is easy. The vagina has to do some serious remodeling to fit over a baby. The hormones a woman releases during pregnancy preps the tissue in advance.
Under their influence, the tough collagen supporting the vaginal wall relaxes and smooth muscle fibres disconnect from one another, making the tissue far far stretchier than normal.
During delivery, as the muscles in the uterus contract in concert, the muscles in the vaginal wall relax, and the super-elastic wall expands as the baby squeezes through the birth canal.
You'd think something that drastic would be permanent, but the vagina is back to normal within 6 to 12 weeks. Now that's amazing. [Levin 2002 [Traish et al. 2002 [Munarriz et al. 2003 [Goldstein et al. 2005 [Barnhart et al. 2006 [Farage and Maibach 2006 [Feola et al 2011]] Top image: Theodore Lee via Flickr [CC BY-ND 2.0 ; vaginal wall: Ed Uthman via Flickr [CC BY 2.0; female reproductive organs: Henry Grey (1918) Anatomy of the Human Body via Wikimedia; childbirth diagrams: Frank Netter via Wikimedia]