Anatomy of the urinary system

The urinary system consists of two kidneys, two ureters, a bladder and a urethra. Each kidney, surrounded by a thick layer of fat, is located high up on the back wall of the abdomen, one on each side of the spine. Each measures 10cm long by 5cm wide and is 3cm thick. The normal adult kidney weights approximately lOOgm. The kidneys are supplied with blood (which they filter) from the aorta – the body’s major artery – reaching the kidney via the renal artery. Blood returns to the circulation in the renal veins, which drain into the inferior vena cava. The ureters are narrow muscular tubes 25cm long which take urine from the kidneys to the bladder. In the abdomen, each ureter descends on a large sheet of muscle on each side of the vertebral column. The second halves of the ureters run in the pelvis, the lower ends entering the bladder, one on each side. The bladder is a hollow muscular reservoir in the centre of the pelvis behind the pubic bone. The empty bladder is pyramid-shaped and lies entirely within the pelvis. When full it is egg-shaped and about the size of a large grapefruit. It can sometimes be felt above the pubic bone when very full, especially in children. There are two small inlets into the bladder from the ureters and one larger outlet, the bladder neck, or internal urinary meatus. At this point special muscular fibres, called spincters, clamp the outlet shut. The urethra is the tube through which the bladder empties. Its structure differs in the male and female, but it is essentially a fibrous, muscular tube passing from the bladder neck, under the pubic bone, to the external opening, or external urinary ostium, from which the urine emerges. In the female the urethra is 3cm long and opens between the labia minora below the clitoris, just above the vaginal opening. Along its short course small glands, called urethral glands, open into it secreting lubricating fluid. The male urethra is a more complex structure being 20 cm in length and extending from the bladder neck through the prostate gland down the shaft of the penis to the external urinary ostium which opens at the glans penis. The first 3cm of a male’s urethra passes through the prostate gland. In this section there are 20 or 30 small prostatic ducts open through which secretions from this gland enter the urethra. A pair of ejaculatory ducts which convey semen from the seminal vesicals to the urethra also open into this part of the urethra. The penile urethra is 16cm in length and urethral glands open into this section as in the female. At its termination at the glans penis it opens as a slit. This shape produces a controllable narrow stream of urine as the bladder empties. In the uncircumcised male the delicate membrane of the glans penis is covered with a double fold of retratile skin known as the foreskin.