Urinary system

The Urinary system comprises all the organs involved in the production and discharge of urine.

The urinary system filters soluble waste products from the blood. It consists of two kidneys, two ureters, the urinary bladder and the urethra.

The kidneys are about 11 centimeters long and about 6 centimeters wide. Each kidney weighs about 140 grams in the average adult. The kidneys lie in the upper part of the abdomen at the back. They filter out water and also unwanted substances in the blood. These substances are produced by the normal working of the body. They are excreted by the kidneys in the form of urine. The kidneys also keep the salt and water of the body in correct balance.

The ureters are tubes about 25cm long that drain urine from the core of the kidneys, the renal pelvis, to the bladder. The bladder lies in the lower abdomen at the front of the body. The urethra is the tube through which urine passes to the outside.

The renal pelvis is funnel-shaped; its widest part is in the center of the kidney. As the renal pelvis narrows, it projects from the kidney and becomes the top of the ureter. The widest part of the renal pelvis is surrounded by the renal cortex. The renal cortex is covered by tough fibrous tissue that forms the protective outer layer of each kidney.

Blood is brought to the kidneys by the renal arteries and, after filtration, is returned to circulation through the renal veins.

The functional units of each kidney are microscopic filters. The renal cortex contains about one million of these filters. Each filter consists of a cup-shaped capsule that encloses a knot of blood capillaries. Blood from the renal artery is pumped into the glomerulus by blood pressure. Water, sugar, salts, urea and other small molecules pass through the capillary walls into the Bowman’s capsule. Blood cells and large molecules, such as whole proteins and fats, remain in the blood.

Filtered fluids pass from the Bowman’s capsule into a coiled tube, the nephron, that leads through the renal medulla to the renal pelvis. Each nephron has a U-shaped loop halfway along its length. The whole tube is closely surrounded by blood capillaries. As the filtered fluid passes through the tube, substances that the body needs, especially water, essential salts and sugar are reabsorbed from the fluid into the blood capillaries.

Urine that results from the filtration and reabsorption processes collects in the renal pelvis before passing through the ureter to the bladder. A circular ring of muscle at the top of the urethra keeps urine in the bladder until voluntary relaxed.

The most common disorders of the urinary system are urinary tract infections caused by bacteria. Urinary tract infections are more common in women than in men. Symptoms include frequency of urination, painful urination (dysuria), and in severe cases blood and pus in the urine (hematuria).

Disorders of the urinary tract include cystitis, pyelonephritis, ureteritis and urethritis.