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Semen, a white to slightly yellowish opaque fluid emitted from the male reproductive tract that contains sperm cells (see sperm) that are capable of fertilizing the female eggs, and other liquids known as seminal plasma that help to keep the sperm cells alive. Sperm cells are produced by the testes; they constitute only about 2 to 5 percent of the total semen volume. As sperm travel through the male reproductive tract, they are bathed in fluids produced and secreted by the various tubules and glands of the reproductive system. From the testes, sperm are stored in the epididymis, in which secretions of potassium, sodium, and glycerylphosphorylcholine (an energy source for sperm) are contributed to the sperm cells. Sperm mature in the epididymis;
they then pass through a long tube called the ductus deferens to another storage area, the ampulla. The ampulla secretes a yellowish fluid, ergothioneine, a substance that reduces (removes oxygen from) chemical compounds; and fructose, a sugar that nourishes the sperm. During the process of semen ejaculation, liquids from the prostate gland and seminal vesicles are added, which help dilute the concentration of sperm and provide a suitable environment for them. Fluids contributed by the seminal vesicles are 40 to 80 percent of the total semen volume; these fluids contain fructose, amino acids, citric acid, phosphorus, and potassium. The prostate gland contributes 15 to 30 percent of the seminal plasma; the constituents of its secretions are mainly citric acid, acid phosphatase, calcium, sodium, zinc, potassium, protein-splitting enzymes, and fibrolysin (an enzyme that reduces blood and tissue fibres). The final addition to semen is fluid secreted by the bulbourethral and urethral glands; this is a thick, clear, lubricating protein commonly known as mucus.
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The seminal plasma serves to dilute the sperm so that they become mobile. Too much or too little fluid will cause the sperm to be nonmotile. Essential also to sperm motility are small quantities of potassium and magnesium, the presence of adequate amounts of oxygen in the plasma, proper temperature, and a slight amount of acidity. Sulfate chemicals in semen help prevent the sperm cells from swelling; and fructose is the main nutrient to sperm cells.
In man, the total volume of semen for each ejaculation is only around 3 to 6 millilitres (0.2—0.4 cubic inch); in stallions the average ejaculate is about 125 millilitres. In human beings, each ejaculation contains normally 200,000,000—300,000,000 sperm. Semen frequently contains degenerated cells sloughed off from the network of tubules and ducts through which the semen has passed. Occasionally, there are also fat, protein, pigment, and crystalline granules.
Semen is probably ejaculated in a sequence of steps. First, the urethra is flushed and lubricated with the mucous secretions of the bulbourethral and urethral glands. Then, secretions from the prostate are emitted, followed by the sperm and fluids from the ductus deferens and ductus epididymidis. The final part of the ejaculate is probably the secretion of the seminal vesicles; this portion again flushes the urethra so that all of the sperm are transported. In some animals, such as the mouse, the fluid from the seminal vesicles contains a gelatin-like substance that coagulates in the female reproductive tract, forming a plug that prevents sperm from escaping from the vagina.
