What is the anatomy of the prostate?

• The prostate is part of the male reproductive system and increases in size with age.
• The prostate functions as a valve, preventing urine from flowing out during intercourse and thus allowing ejaculation.
• Overgrowth of the prostate can cause problems related to urination, due to its location.

The prostate is an organ of the male reproductive system, a glandular and fibromuscular mass surrounding the initial portion of the urethra. Until puberty it is underdeveloped. The prostate is very small during childhood, estimated to weigh about 1.4 g up to the age of 10 years. 

During puberty, due to hormone production, it begins to grow abruptly, with hyperplasia of the ductal epithelium, and by the age of 20 the gland weighs between 10 and 14 grams. Between 35 and 40 years of age, it achieves its final development, reaching an average weight of about 25-30 grams. In adults it is approximately the size of a walnut, measuring 4 x 2 x 3cm and weighing around 25-30 grams. 

From this age onwards, prostatic involution begins to develop and benign prostatic hyperplasia may appear around the prostatic urethra (transitional zone) or in the central zone (prostatic middle lobe).

The prostate is conical or chestnut-shaped, with the base towards the bladder and above and the apex towards the front and below. It is situated below the bladder, above the perineal floor, in front of the rectum and behind the symphysis pubis. It is supported anteriorly by the puboprostatic ligament and inferiorly by the urogenital diaphragm. Its position makes it possible to examine it by rectal examination.

The urethra passes through the centre of the prostate, so urine leaves the bladder through the prostate. For this reason, prostatic hyperplasia causes urinary symptoms such as urination problems. Just above and on either side of the prostate are the seminal vesicles, which are crossed by the vas deferens, forming the ejaculatory ducts which, once inside the prostate, lead to the prostatic urethra at the level of the verumontanum. Within the prostate, the ejaculatory ducts are accompanied by an invagination of extra-prostatic fat, constituting a weak zone of tumour expansion.

The prostatic arteries are branches of the inferior vesical artery and the middle haemorrhoidal artery. The veins end anteriorly in the seminal plexus and are thought to act as guides for extra-capsular tumour expansion, especially in the area of the apex. 

The lymphatic vessels form a network on the surface of the organ, the obturator nodes are considered a primary step in prostatic drainage.

The prostate is divided into several anatomical zones. Knowledge of these zones is important for the differential diagnosis of benign prostatic hyperplasia and prostate cancer.

• The peripheral zone corresponds to the tissue surrounding the distal urethra and forms part of the posterior, lateral and inferior sectors of the gland. It constitutes 70% of the gland and is the site of carcinomas (70%). Hyperplasia does not develop in this zone.
• The central zone constitutes 25% of the glandular tissue and forms the base of the prostate in relation to the proximal urethra. It is the site of 10% of carcinomas and does not develop benign prostatic hyperplasia.
• The transitional zone constitutes 5% of the glandular tissue and corresponds to the glandular tissue located on either side of the proximal urethra and periurethral sphincter. It is the site of benign prostatic hyperplasia and 20% of carcinomas.
• The periurethral gland area constitutes 1% of the glandular tissue and is located within the muscle forming the periurethral sphincter. It is the site of benign prostatic hyperplasia.
• The anterior fibromuscular area is made up of muscle and fibrous tissue. Its maximum thickness is 1cm and it tapers laterally to form the fibrous prostatic capsule that covers the gland in the lateral and posterior sectors.
• The proximal periurethral sphincter consists of smooth muscle and surrounds the urethra from the verumontanum to the bladder neck. Areas of fibromuscular structure are not the site of pathology.

The transitional zone develops benign prostatic hyperplasia and carcinoma. The external gland (peripheral and central zone) is the site of carcinoma (80%) and does not develop benign prostatic hyperplasia. 

The internal gland (transitional zone and periurethral glands) develops benign prostatic hyperplasia. In other words, the prostate has a peripheral part where malignant prostate tumours can grow and a central part that grows benignly.

The prostate is an exocrine gland. Its main function is to produce a fluid that binds with semen to improve sperm quality and increase fertility, but it has no bearing on sexual desire or lack of adequate erection. 

The fluid produced by the prostate together with sperm produced in the testes, fluid from the seminal vesicle and secretions released by another gland below the prostate (the bulbourethral gland), make up semen.

Prostatic fluid corresponds to about 30% of the volume of ejaculated sperm; it is the fluid that gives sperm their milky appearance. This fluid is alkaline and neutralises the acidity of the vaginal fluid and stimulates sperm motility.

All these fluids mix together in the urethra. Prostatic secretion is important for the proper functioning of spermatozoa, and therefore for male fertility. This gland produces antigens (specific to the prostate), citric acid, fibrinogen, spermine, zinc (which has bactericidal properties), magnesium (which gives the semen a milky appearance) and a number of enzymes.

The prostate has the function of a valve, preventing urine from leaking into the reproductive system and during sexual intercourse it closes the passage of urine while allowing sperm to pass through. 

During orgasm, sperm is transmitted from the vas deferens to the urethra, through the ejaculatory ducts that enter the prostate. Men are able to ejaculate by stimulating the prostate through prostate massage. Prostate massage is used in hospitals to obtain rapid semen samples without the need for sexual stimulation or masturbation.