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Anatomy and physiology of the prostate
The prostate gland is part of a man's reproductive and urinary systems. The prostate is oval shaped with a rounded tip. It is approximately 4 cm wide and 3 cm thick. The actual size of the prostate varies from man to man. It can range from the size of a walnut to a small apple.
The prostate surrounds the base (or neck) of the bladder. It has 2 lobes that surround the urethra. The urethra carries urine from the bladder, through the prostate, and out the penis.
The prostate gland is covered in a layer of connective tissue called the prostatic capsule.
The prostate gland is made up of different types of cells:
- gland cells that produce the fluid portion of semen
- muscle cells that control urine flow and ejaculation
- fibrous cells that provide the supportive structure of the gland
Some structures around the prostate are:
- seminal vesicles – These glands produce semen and are found on both sides of the prostate.
- vas deferens – These tubes carry sperm from the testicles to the seminal vesicles.
- nerve bundles – These nerves control bladder and erectile function and are found on both sides of the prostate.
- muscles – These muscles control urination.
Zones of the prostate
The prostate gland is divided into 3 zones:
The peripheral zone is the area of the prostate that is closest to the rectum. It can easily be felt by the doctor during a digital rectal examination (DRE). It is the largest zone of the prostate gland.
The majority of prostate tumours (approximately 75%) are found in the peripheral zone.
The transition zone is the middle area of the prostate, between the peripheral and central zones. It surrounds the urethra as it passes through the prostate. This zone makes up about 20% of the prostate gland until the age of 40.
As men age, the transition zone begins to enlarge, until it becomes the largest area of the prostate. This is called benign prostatic hyperplasia (BPH). When the transition zone enlarges, it pushes the peripheral zone of the prostate toward the rectum.
The central zone is in front of the transition zone. It is the part of the prostate that is farthest from the rectum. Because of this, prostate tumours in this zone cannot be felt by the doctor during a DRE.
The main function of the prostate is to produce the fluid portion of semen. The gland cells within the prostate produce a thin fluid rich in proteins and minerals that maintain and nourish sperm. This fluid is made continuously. The excess passes from the body in the urine. When a man is sexually aroused, the prostate produces larger amounts of this fluid. It then mixes with sperm and is ejaculated as semen.
The prostate also plays a part in controlling the flow of urine. The urethra runs from the bladder, through the prostate, and out through the penis. The muscle fibres of the prostate are wrapped around the urethra and are under involuntary nervous system control. These fibres contract to slow and stop the flow of urine.