Depending on the
species, spermatozoa can fertilize ova externally or internally. In
external fertilization, the spermatozoa fertilize the ova directly, outside of the female's sexual organs. Female
fish, for example,
spawn ova into their aquatic environment, where they are fertilized by the semen of the male fish.
internal fertilization, however, fertilization occurs inside the female's sexual organs. Internal fertilization takes place after
insemination of a female by a male through
copulation. In most
monotreme mammals, copulation is achieved through the physical mating of the
cloaca of the male and female. In
placental mammals, copulation occurs through the
Male Sex Organs and Ducts
During the process of
ejaculation, sperm passes through the
ejaculatory ducts and mixes with fluids from the
seminal vesicles, the
prostate, and the
bulbourethral glands to form the semen. The seminal vesicles produce a yellowish viscous fluid rich in fructose and other substances that makes up about 70% of human semen.
 The prostatic secretion, influenced by dihydrotestosterone, is a whitish (sometimes clear), thin fluid containing proteolytic enzymes, citric acid, acid phosphatase and lipids.
 The bulbourethral glands secrete a clear secretion into the lumen of the
urethra to lubricate it.
Sertoli cells, which nurture and support developing
spermatocytes, secrete a fluid into seminiferous tubules that helps transport sperm to the genital ducts. The ductuli efferentes possess cuboidal cells with
lysosomal granules that modify the ductal fluid by reabsorbing some fluid. Once the semen enters the ductus epididymis the principal cells, which contain
pinocytotic vessels indicating fluid reabsorption, secrete glycerophosphocholine which most likely inhibits premature
capacitation. The accessory genital ducts, the
prostate glands, and the
bulbourethral glands, produce most of the seminal fluid.
Seminal plasma of humans contains a complex range of
The seminal plasma provides a nutritive and protective medium for the spermatozoa during their journey through the female reproductive tract. The normal environment of the
vagina is a hostile one for
sperm cells, as it is very
acidic (from the native microflora producing
lactic acid), viscous, and patrolled by immune cells. The components in the seminal plasma attempt to compensate for this hostile environment. Basic
amines such as
cadaverine are responsible for the smell and flavor of semen. These alkaline bases counteract and buffer the acidic environment of the vaginal canal, and protect
DNA inside the
sperm from acidic denaturation.
The components and contributions of semen are as follows:
||Approximately 200- to 500-million spermatozoa (also called sperm or spermatozoans), produced in the
testes, are released per ejaculation. If a man has undergone a
vasectomy, he will have no sperm in the ejaculation.
fructose (2–5 mg per mL semen,
 the main energy source of sperm cells, which rely entirely on sugars from the seminal plasma for energy),
prostaglandins (involved in suppressing an immune response by the female against the foreign semen),
prostate specific antigen,
zinc (the zinc level is about 135±40 micrograms/ml for healthy men.
 Zinc serves to help to stabilize the DNA-containing
chromatin in the sperm cells. A zinc deficiency may result in lowered fertility because of increased sperm fragility. Zinc deficiency can also adversely affect
mucus (serve to increase the mobility of sperm cells in the vagina and cervix by creating a less viscous channel for the sperm cells to swim through, and preventing their diffusion out of the semen. Contributes to the cohesive jelly-like texture of semen.),
World Health Organization report described normal human semen as having a volume of 2 ml or greater,
pH of 7.2 to 8.0, sperm concentration of 20×106 spermatozoa/ml or more, sperm count of 40×106 spermatozoa per ejaculate or more, and motility of 50% or more with forward progression (categories a and b) of 25% or more with rapid progression (category a) within 60 minutes of ejaculation.
A 2005 review of the literature found that the average reported physical and chemical properties of human semen were as follows:
Appearance and consistency
Human semen in a petri dish
Semen is typically translucent with white, grey or even yellowish tint. Blood in the semen can cause a pink or reddish colour, known as
hematospermia, and may indicate a medical problem which should be evaluated by a doctor if the symptom persists.
After ejaculation, the latter part of the ejaculated semen
 forming globules,
 while the earlier part of the ejaculate typically does not.
 After a period typically ranging from 15 – 30 minutes,
prostate-specific antigen present in the semen causes the decoagulation of the seminal coagulum.
 It is postulated that the initial clotting helps keep the semen in the vagina,
liquefaction frees the sperm to make their journey to the ova.
A 2005 review found that the average reported viscosity of human semen in the literature was 3–7 cP.
Semen quality is a measure of the ability of semen to accomplish fertilization. Thus, it is a measure of fertility in a man. It is the sperm in the semen that is the fertile component, and therefore semen quality involves both sperm quantity and sperm quality.
The volume of semen ejaculate varies but is generally about 1 teaspoonful or less. A review of 30 studies concluded that the average was around 3.4 milliliters (ml), with some studies finding amounts as high as 5.0 ml or as low as 2.3 ml.
 In a study with Swedish and Danish men, a prolonged interval between
ejaculations caused an increase of the sperm count in the semen but not an increase of its amount.
Increasing semen volume
dietary supplements have been marketed with claims to increase seminal volume. Like other supplements, including so-called
herbal viagra, these are not approved or regulated by the
Food and Drug Administration (as licensed medications would be), and none of the claims have been scientifically verified. Similar claims are made about traditional
aphrodisiac foods, with an equal lack of verification.
Semen can be stored in diluents such as the Illini Variable Temperature (IVT) diluent, which have been reported to be able to preserve high fertility of semen for over seven days.
 The IVT diluent is composed of several salts, sugars and antibacterial agents and gassed with
Semen cryopreservation can be used for far longer storage durations. For human sperm, the longest reported successful storage with this method is 21 years.