Semen is a biological fluid released from the male reproductive tract during ejaculation, playing a direct role in sexual reproduction. This complex fluid serves as the medium that transports male gametes, known as sperm, towards the female reproductive system. Its composition is carefully balanced to support the journey and viability of these reproductive cells.
What is Semen?
Semen is the fluid that carries sperm, the male reproductive cells, out of the body to facilitate fertilization of an egg. While often confused, semen is distinct from sperm; sperm are the microscopic cells responsible for carrying genetic material, while semen is the entire fluid mixture containing these cells.
The fluid environment within semen provides a protective and nourishing medium for sperm, allowing them to survive the journey through the male and female reproductive tracts. Without this specialized fluid, sperm viability would be significantly reduced.
Major Components of Semen
Semen is primarily composed of two main categories: sperm and seminal plasma. Sperm are the male gametes, representing only a small fraction of the total volume, typically less than 5%. These microscopic cells possess a head containing genetic material, a midpiece for energy, and a tail for propulsion.
The vast majority of semen (95% or more) consists of seminal plasma, a complex mixture of secretions from several accessory glands. This fluid provides the necessary environment for sperm motility, survival, and protection.
Contributions from Glands
The seminal plasma is produced by several accessory glands, each contributing specific components to the overall fluid. The seminal vesicles, located behind the bladder, contribute the largest volume to seminal fluid, typically around 60-70% of the total ejaculate. Their secretions are rich in fructose, a sugar that serves as the primary energy source for sperm motility.
The prostate gland, situated below the bladder, adds a milky, slightly acidic fluid that accounts for about 20-30% of semen volume. This prostatic fluid contains enzymes like prostate-specific antigen (PSA), which helps to liquefy the ejaculate, and citric acid, a nutrient for sperm.
The bulbourethral glands, also known as Cowper’s glands, secrete a clear, slippery pre-ejaculate fluid before emission. This pre-ejaculate neutralizes any acidic urine residue in the urethra and lubricates the passageway for semen.
Key Substances and Their Functions
Seminal plasma contains a variety of specific chemical substances, each serving a distinct function to support sperm viability and fertilization. Fructose, secreted by the seminal vesicles, provides metabolic energy for sperm to power their flagellar movement, enabling them to swim effectively. Prostaglandins, also from the seminal vesicles, are lipid compounds that can induce contractions in the smooth muscle of the female reproductive tract, potentially aiding sperm transport towards the egg.
Zinc, predominantly from the prostate gland, contributes to sperm stability and helps protect against bacterial growth within the semen. Citric acid, another prostatic secretion, acts as a nutrient source for sperm and plays a role in regulating the pH of the seminal fluid.
Various enzymes, including proteolytic enzymes from the prostate, are responsible for the initial coagulation and subsequent liquefaction of the ejaculate, allowing sperm to become motile after ejaculation. Buffering agents, such as bicarbonate and phosphate, help neutralize the naturally acidic environment of the vagina, protecting sperm from damage and optimizing conditions for their survival.
What Semen Does Not Typically Contain
Healthy semen does not typically contain certain substances or cells; their presence can indicate an underlying health issue. Red blood cells, for instance, are not normally found; their appearance (hematospermia) can signal inflammation, infection, or injury. Similarly, an excessive amount of white blood cells often points to infection or inflammation.
Healthy semen also does not inherently contain common pathogens unless an individual has a sexually transmitted infection (STI). The presence of such microorganisms is an abnormal finding, prompting further medical investigation.