HUMAN BIOLOGY

314 Chapter 16

What kinds of physiological changes

occur during sexual intercourse?

• Sexual intercourse, or coitus, typically involves a series of
  physiological changes in both partners.


• During arousal, blood vessels dilate so that more blood flows
  to the penis (males) and vulva (females). Orgasm involves
  muscular contractions (including those leading to ejaculation
  of semen into the vagina) and sensations of release, warmth,
  and relaxation.


• Intercourse may lead to a pregnancy even if the female is not
  sexually aroused or she does not experience orgasm.

taKe-hoMe Message

coitus Sexual intercourse.

orgasm The culmination of
the sex act.

sexual intercourse between Males and Females

Intercourse can produce a fertilized egg

If sperm enter the vagina a few days before or after

ovulation or anytime between, an ovulated egg may be

fertilized. Within 30 minutes after ejaculation, muscle

contractions in the uterus move the sperm deeper into the

female reproductive tract. Only a few hundred sperm will

actually reach the upper portion of the oviduct, which is

where fertilization usually takes place. The remarkable

image in Figure 16.9 shows living sperm around a second-

ary oocyte.

n The penis and vagina are mechanically compatible for

sexual intercourse, which may lead to pregnancy.

in sexual intercourse, both partners

experience physiological changes

Coitus and copulation are both technical terms for sexual

intercourse. The male sex act involves an erection, in which

the limp penis stiffens and lengthens. It also involves ejacu-

lation, the forceful expulsion of semen into the urethra and

out from the penis. As shown in Figure 16.4, the penis has

lengthwise cylinders of spongy tissue. The outer cylinder

has a mushroom-shaped tip (the glans penis). Inside it is

a dense array of sensory receptors that are activated by

friction. In a male who is not sexually aroused, the large

blood vessels leading into the cylinders are constricted.

In aroused males, these blood vessels vasodilate, so blood

flows into the cylinders faster than it flows out. Blood

collects in the spongy tissue, and the organ stiffens and

lengthens—a mechanism that helps

the penis penetrate into the female’s

vagina.

In a female, arousal includes

vasodilation of blood vessels in her

genital area. This causes vulvar tis-

sues to engorge with blood and swell. Mucus-rich secre-

tions flow from the cervix, lubricating the vagina.

During coitus, pelvic thrusts stimulate the penis as

well as the female’s clitoris and vaginal wall. The stimula-

tion triggers rhythmic, involuntary contractions in smooth

muscle in the male reproductive tract, especially the

vas deferens and the prostate. The contractions rapidly

force sperm out of each epididymis. They also force the

contents of seminal vesicles and the prostate gland into the

urethra. The resulting mixture, semen, is ejaculated into

the vagina.

During ejaculation, a sphincter closes off the neck of

the male’s bladder and prevents urine from being excreted.

Ejaculation is a reflex response. This means that once it

begins, it cannot be stopped.

Emotional intensity, heavy breathing, and heart

pounding, as well as generalized contractions of skeletal

muscles, accompany the rhythmic throbbing of the pelvic

muscles. For both partners, orgasm—the culmination of

the sex act—typically is accompanied by strong sensations

of release, warmth, and relaxation.

Some people mistakenly believe that unless a woman

experiences orgasm, she cannot become pregnant. This is

not true, however. A female can become pregnant from

intercourse regardless of whether she experiences orgasm,

and even if she is not sexually aroused. All that is required

is that a sperm meet up with a secondary oocyte that is

traveling down one of her oviducts.

Figure 16.9 This color-enhanced image shows a secondary

oocyte surrounded by sperm. If fertilization occurs, it will set

the stage for a new individual to develop, continuing the human

life cycle.

David M. Phillips/Science Source

16.5

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