What is the anatomy of the urinary system and what are the related diseases?

The urinary system (urinary tract; renal system) plays a key role in the maintenance of the homeostasis of several internal mechanisms that help regulate the human organism. The main purpose of the urinary system is to filter the blood and eliminate waste from the body. Thus, the urinary system regulates the blood pressure, blood volume, and blood pH and controls the level of electrolytes and metabolites.
The main structures of the urinary system are the kidneys, the ureters, the bladder, and the urethra.
The kidneys are the most complicated organs of the urinary tract. They are located in the middle of the back, behind the abdominal cavity, and their shape resembles a bean. Their main function is to filter waste (urea, uric acid, water, and other metabolic wastes and fluids) from the bloodstream and dispose of it, in the form of urine.
The most important structures of the kidneys, which regulate the concentration of excess fluids and wastes in the body, filter the blood and produce urine, are the nephrons; each kidney has one million nephrons. The nephrons are composed of small blood capillaries called glomerulus and a small renal tube called the renal tubule; the renal tubule carries the urine to the renal pelvis, while around fifty percent of the urea is filtered and returned to the bloodstream, in order to create more waste and maintain balance.
The kidneys are composed of three main parts: outer renal cortex—this is where the blood is filtered; inner renal medulla—this is where urine is formed; and renal pelvis—this is where the urine passes from the renal pyramids to the ureters.
Some of the most common diseases, disorders, and pathologies of the kidneys are kidney stones, kidney cysts, kidney tumors, chronic kidney disease, nephrosis (non-inflammatory kidney disease), pyelonephritis (inflammation of the kidneys that is usually formed as a result of a bacterial infection), and glomerulonephritis (inflammatory disease that damages the kidney’s filtration units).
The ureters are renal tubes made of muscle fibers which transport the urine from the kidneys to the bladder. The muscles tighten and relax and transport the urine to the bladder every ten to fifteen seconds. There are two ureters in the human body, one attached to each kidney.
Some of the most common diseases, disorders, and pathologies of the ureters are infections (which can happen when the urine stands still or goes back up into the kidneys instead of going down to the bladder), ureteral obstruction (ureteropelvic obstruction is when the connection between the kidney and the ureter is blocked; ureterovesical obstruction is when the connection between the ureter and the bladder is blocked), ureteral duplication (a common disorder in which one kidney has two ureters instead of one), ureteral stones, swelling of the ureteral wall, retroperitoneal fibrosis (fibrous tissue growths that form in the back of the abdominal cavity), and ureterocele (a underdeveloped or narrow ureter can form a tiny bulge which can block the urine flow).
The bladder is a flexible, hollow organ located at the base of the pelvis, and its main function is to store the urine until it’s passed down to the urethra and out of the body. A normal, healthy bladder can store up to 400–600 ml of urine. An empty bladder is the same size and shape as a pear. The bladder is mostly composed of muscle tissue, that stretches to hold urine and contracts when the urine is passed down to the urethra; and nerves which alert the body when it’s time to urinate.
Some of the most common diseases, disorders, and pathologies of the bladder include bladder infections and inflammations, bladder stones, bladder tumors, cystitis, overactive bladder, urinary incontinence, and cystocele (common mainly after childbirth, in which the bladder presses on the vagina).
The urethra is a fibromuscular tube that allows the urine to pass out of the body. Located near the end of the bladder and urethra are the so-called urethral sphincters, which are muscles that control the release of urine; there are internal urethral sphincters, which provide involuntary control of urination, and external male and female urethral sphincters, which provide voluntary control of urination. The male and female renal system are very similar, and one of the main differences between the two is the length of the urethra: the female urethra is four centimeters long, while the male urethra is twenty centimeters long.
Some of the most common diseases, disorders, and pathologies of the urethra are urethritis (an inflammation of the urethra), urethral tumors, and urethral stricture (a narrow opening of the urethra).