Part 1 of Pumped Up and Ready for Love: Sex and Fluid Physics

Much of what you experience during arousal and sex results from changes in the way your circulatory system moves your blood around. Your heart races and your cheeks flush as excitement builds. Sooner or later, if all goes well, swelling of the erectile tissue in your groin will lead to the erection of your clitoris or your penis, as the case may be. Generally, your body handles all the blood flow issues automatically. Still, a look at the physics of fluids reveals that there are many things you can do to take control of your blood flow during arousal.

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Despite the fact that male and female genitalia look very different from the outside, they operate in essentially the same way, from the point of view of fluid physics. Both the penis and the clitoris are built of erectile tissue that becomes engorged with blood during sexual arousal. The chief difference between the two is that while most of the engorged portion of the penis is visible on the exterior of the male body, the exposed clitoris is only a small part of the erectile structure in a woman's genitals. In fact, women have roughly the same amount of erectile tissue as men. It's just hidden from view behind the clitoris and extends down on either side of the vaginal opening.

In either case, a physicist could describe genitals in terms of a simplified model consisting of a reservoir connected to an incoming tube, which supplies fluid, and an outgoing tube, which drains the fluid away. Your body adjusts the amount of fluid in the reservoir by regulating the relative flow, in through the source and out through the drain. It's similar to the way you can control the amount of water in your kitchen basin by turning the faucet handle.

Arteries are the blood vessel analogue of the kitchen tap. Instead of water, of course, they deliver blood provided by the pumping of your heart. They are stretchy tubes wrapped in muscle tissue. The muscles control blood flow through arteries by contracting and closing them down somewhat to slow the flow, or relaxing to let the vessels dilate and pass lots of blood.

Veins, like arteries, are also stretchy tubes. Although they don't have the sort of muscle layers that surround arteries. Instead, veins passively expand or contract as blood pressure rises or falls.

Most of the time, the muscles wrapping the genital arteries squeeze down to restrict blood flow, and the erectile tissue is relatively empty. During arousal, the arterial muscles relax to open up the tap and let lots of blood through to begin the process that leads to an erection. If there were enough blood flow, this would be sufficient to do the job without making any changes in the outward flow of blood through your veins.

In your genitals, however, the situation is a little more complicated. The erectile tissue reservoir is really more like a sponge than a kitchen basin. The large arteries that supply blood to your groin branch off into smaller and smaller vessels that are embedded in the spongy erectile tissue. Veins also fan out through the tissue, collecting blood and returning it to the large veins that lead back out.

When the arteries dilate during arousal, the swelling erectile tissue puts pressure on the veins. This forces them to collapse slightly and reduces the rate of the blood flowing out. The whole process amounts to opening the taps while simultaneously closing off the drain somewhat.

If things were to continue in this way, you would eventually have a problem as increasing pressure shut off the veins altogether and the blood flow stopped. Fortunately, there's a natural safety mechanism. Erectile tissue can only expand so far before it's fully engorged. At that point the blood still has room to force its way out through the veins.

In physics terms, the change in your erectile tissue as you become aroused is called a transient state. When you are not aroused, your genitals are in an unchanging steady state. Your genitals are also in a steady state when they are fully engorged. During a transient period, the flow of blood into your groin is different from the flow out as the erectile tissue becomes engorged. During a steady state, fluid physics requires the flow and out to be the same. In fact, the total flow of blood through your groin in either the aroused or relaxed state is just about the same, only the amount of blood loitering in the erectile reservoir is different between the two states.

It's a bit like using a damn to turn a valley into a lake. Diverting a river into the valley begins filling the reservoir. Closing the flood gates in the dam stops the flow of water out of the valley and downstream. Once the valley is filled up, the total amount of water flowing into the lake and spilling over the dam at the other end is the same that always flowed down the river. However, a lot more water is stored in the dammed up reservoir, just as more blood is stored in your erectile tissue when it's engorged.

You could rely solely on your body's natural processes to achieve a penile or clitoral erection, but many people enhance their experience with various sexual devices and techniques. In terms of the lake metaphor, most methods either widen the river flowing into the reservoir or raise the dam higher at the reservoir's outlet.

One of the most common sexual aids is the cock ring. It is a constriction placed at the base of the penis that effectively raises the dam holding more blood in your groin reservoir.

Cock rings squeeze down on the outside of a penis. They compress both arteries and veins, which causes them to collapse a little, restricting blood flow through the penis. This might lead you to wonder why they should enhance an erection. After all, if they squeeze down on both the arteries and veins you could imagine that a ring would reduce the blood flowing in as much as it reduces blood flowing out.

Fluid physics provides the solution to the cock ring puzzle. Here's how -

Whenever a liquid flows through a tube, it experiences some friction that opposes its motion. The amount of friction depends in part on how thick the fluid is. Physicists call this the viscosity. Water flows easily because its viscosity is low. Slow moving molasses is highly viscous, and blood has a viscosity somewhere between water and molasses.

When your heart pumps blood into your arteries, the resistance to the flowing blood causes the pressure to decrease the farther it moves away from your heart. (Other things affect the pressure as well, but we will worry about those later.) Narrow tubes, such as capillaries, resist flow more than wide arteries and veins.

The blood that flows through your groin passes through many tiny capillaries. If you were to measure the blood pressure on its travels from the arteries to the veins , you would find a significant pressure drop. Because the pressure in the out-going veins is so much lower than in the in-coming arteries, they are easier to pinch closed and they collapse more than arteries do under the same force.

You don't need a cock ring to test the effect. If you wrap a piece of string around one of your fingers, you will find that the finger tip swells and turns purplish-red as the string squeezes the veins down but the arteries remain more open.

Of course, a ring that's too tight could potentially shut down your veins entirely, leading to a stagnant pool of blood in the penis. This can cause permanent tissue damage as the cells run out of the oxygen that your blood provides. Most cock rings, however, include snaps or other fasteners that allow you to release them if things get out of hand.

You can also enhance an erection by pinching the base of the penis - with your thumb on the upper side, and your fingers pressing under the testicles just in front of the perineum. The pressure compresses the veins leading out of the penis just as a cock ring does. Although your hand will be in the way for some activities, and the effect will subside immediately upon releasing pressure, it can temporarily enhance girth and pleasure for a man, particularly during oral sex.

Unfortunately, the structure of a woman's genitals means that there are no feminine equivalents of cock rings, although the clamps that some people apply to their clittoris alters blood flow and often causes engorgement in a small region.

The penile pinch, however, can be adapted to women. If you spread your fingers in a V shape and place them on either side of the clitoris, you can restrict venous blood flow, and enhance clitoral size and sensation by pressing and gently squeezing your fingers together. Just as is the case with the penile pinch, this can be particularly pleasant for your female partner when you are performing oral sex.

Tensed muscles put pressure on blood vessels running through muscle tissue, much as a ring squeezes veins in a penis. When weightlifters and other athletes strain to perform an exercise, the veins near the surface of their skin stand out as the muscles squeeze the blood out of deep muscle veins. The extra engorgement of the labia, clitoris and penis just before orgasm comes in part from similar muscular tension, primarily due to straining in the abdomen, buttocks, thighs, and calves.

Vacuum pumps, including both penis pumps and clitoral pumps, also aid in erectile tissue engorgement, but they operate on an entirely different principle from cock rings. They reduce the air pressure outside of the organ rather than compressing veins inside. Normally, the atmosphere pushes on us from all directions with a force of about fifteen pounds per square inch. You don't notice the pressure because the fluids inside your body press outward and balance the air pressure on your skin. If you could selectively reduce the pressure around part of your body it would bulge outward because the pressure inside of you pushing out would no longer be balanced by the atmospheric pressure pushing back in.

A vacuum pump allows you to reduce the air pressure surrounding your penis or clitoris by surrounding it with a chamber that seals to the skin of your groin. You can then reduce the pressure by sucking the air out of the chamber with a pump.

Because the air pressure on the rest of your body remains unchanged, it's effectively the atmosphere pushing on the rest of you that forces blood into your genitals.

Unlike cock rings, which require least some arousal to function, pumps can inflate a penis or clitoris even in the absence of any natural erectile response. Once a penis is engorged with a pump, a cock ring can help maintain the erection.

Pumps can cause injuries if things are taken too far. The high pressure difference between the inside of the blood vessels and the vacuum in the pump may lead to ruptured blood vessels and permanent vein damage. They are generally safe, however, if you follow the manufacturer's instructions.

There are many other things that can affect blood flow in your genitals, including drugs, diet, and even your choice of sexual positions. We'll address several of them in next week's episode - Part two of Pumped Up and Ready for Love: Sex and Fluid Physics.