<span>Penile shaft
</span>During erection, blood<span> flows into the spaces, causing distention and elevation of the penis. The amount of blood entering the penis can be increased by physical or psychological stimulation. As blood enters, there is a temporary reduction in the rate and volume of blood leaving the penis. The arteries carrying blood to the penis dilate; this, in turn, causes tissue expansion. The veins leading from the penis have funnel-shaped valves that reduce the outflow of blood. As the erectile tissue begins to enlarge, the additional pressure causes the veins to be squeezed against the surrounding fibrous tissue, and this further diminishes the outflow of blood. Essentially, blood becomes temporarily trapped in the organ.</span>
can be replenished over months, years, or decades.
<span>Error bars within treatments illustrate confidence intervals, standard errors, standard deviations or other quantities. However, different types of error bars exist, so figure legends must be sure to distinguish which error bar is being represented. </span>
Mass is measured by using a balance comparing a known amount of matter to an unknown amount of matter. Weight is measured on a scale.
