The seminiferous tubules are connected at both ends to a tubular network called the rete testis (see fig. 20.12). Spermatozoa and tubular secretions are moved to this area of the testis and are drained via the efferent ductules into the epididymis (the plural is epididymides). The epididymis is a tightly coiled structure,
Acrosome Head Midpiece Tail
- Acrosome -Head
■ Figure 20.20 A human spermatozoon. (a) A diagrammatic representation and (b) a scanning electron micrograph in which a spermatozoon is seen in contact with an ovum.
about 5 meters (16 feet) long if stretched out, that receives the tubular products. Spermatozoa enter at the "head" of the epi-didymis and are drained from its "tail" by a single tube, the duc-tus, or vas, deferens.
Spermatozoa that enter the head of the epididymis are non-motile. This is partially due to the low pH of the fluid in the epi-didymis and ductus deferens, produced by the secretion of H+ by active transport ATPase pumps. During their passage through the epididymis, the sperm undergo maturational changes that make them more resistant to changes in pH and temperature. The pH is neutralized by the alkaline prostatic fluid during ejaculation, so that the sperm are fully motile and become capable of fertilizing an ovum once they spend some time in the female reproductive tract. Sperm obtained from the seminiferous tubules, by contrast, cannot fertilize an ovum. The epididymis serves as a site for sperm maturation and for the storage of sperm between ejaculations.
The ductus deferens carries sperm from the epididymis out of the scrotum into the pelvic cavity. The seminal vesicles then add secretions that pass through their ducts; at this point, the duc-tus deferens becomes an ejaculatory duct. The ejaculatory duct is short (about 2 cm), however, because it enters the prostate and soon merges with the prostatic urethra. The prostate adds its secretions through numerous pores in the walls of the prostatic urethra, forming a fluid known as semen (fig. 20.21).
The seminal vesicles and prostate are androgen-dependent accessory sex organs—they will atrophy if androgen is withdrawn by castration. The seminal vesicles secrete fluid containing fructose, which serves as an energy source for the spermatozoa. This fluid secretion accounts for about 60% of the volume of the semen. The fluid contributed by the prostate contains citric acid, calcium, and coagulation proteins. Clotting proteins cause the semen to coagulate after ejaculation, but the hydrolytic action of fibrinolysin later causes the coagulated semen to again assume a more liquid form, thereby freeing the sperm.
An immunoassay for prostate-specific antigen (PSA) is a common laboratory test for prostate disorders, including prostate cancer. A more common disorder, affecting most men over 60 to a greater or lesser degree, is benign prostatic hyperplasia (BPH). This disorder is responsible for most symptoms of bladder outlet obstruction, where there is difficulty in urination. BPH treatment may involve a surgical procedure called transurethral resection (TUR), or the use of drugs. Drugs used to treat BPH include ai-adrenergic receptor blockers, which decrease the muscle tone of the prostate and bladder neck (making urination easier), and 5a-reductase inhibitors, which inhibit the enzyme needed to convert testosterone into di-hydrotestosterone (DHT). As previously described, DHT is required to maintain the structure of the prostate; therefore, a reduction in DHT may help to reduce the size of the prostate.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.