July 16, 2007

Urinary System

The urinary system is the organ system that produces, stores, and eliminates urine. In humans it includes two kidneys, two ureters, the bladder, and the urethra. The analogous organ in invertebrates is the nephridium.


Typically, every human has two kidneys. The kidneys are bean-shaped organs about the size of a bar of soap. The kidneys lie in the abdomen, posterior or retroperitoneal to the organs of digestion, around or just below the ribcage and close to the lumbar spine. The kidneys are surrounded by what is called peri-nephric fat, and situated on the superior pole of each kidney is an adrenal gland. The kidneys receive their blood supply of 1.25 L/min (25% of the cardiac output) from the renal arteries which are fed by the Abdominal aorta. This is important because the kidneys' main role is to filter water soluble waste products from the blood. The other attatchment of the kidneys are at their functional endpoints the ureters, which lies more medial and runs down to the trigone of the bladder.

Functionally the kidney performs a number of tasks. In its role in the urinary system it concentrates urine, plays a crucial role in regulating electrolytes, and maintains acid-base homeostasis. The kidney excretes and re-absorbs electrolytes (e.g. sodium, potassium and calcium) under the influence of local and systemic hormones. pH balance is regulated by the excretion of bound acids and ammonium ions. In addition, they remove urea, a nitrogenous waste product from the metabolism of proteins from amino acids. The end point is a hyperosmolar solution carrying waste for storage in the bladder prior to urination.

Humans produce about 1.5 liters of urine over 24 hours, although this amount may vary according to circumstances. Because the rate of filtration at the kidney is proportional to the glomerular filtration rate, which is in turn related to the blood flow through the kidney, changes in body fluid status can affect kidney function. Hormones exogenous and endogenous to the kidney alter the amount of blood flowing through the glomerulus. Some medications interfere directly or indirectly with urine production. Diuretics achieve this by altering the amount of absorbed or excreted electrolytes or osmalites, which causes a diuresis.


The ureters are the ducts that carry urine from the kidneys to the urinary bladder, passing anterior to the Psoas major. The ureters are muscular tubes that can propel urine along by the motions of peristalsis. In the adult, the ureters are usually 25-30cm long.

In humans, the ureters enter the bladder through the back, running within the wall of the bladder for a few centimetres. There are no valves in the ureters, backflow being prevented by pressure from the filling of the bladder, as well as the tone of the muscle in the bladder wall.

In the female, the ureters pass through the mesometrium on the way to the urinary bladder.

The ureter has a diameter of about 3 millimeters, and the lumen is star-shaped. Like the bladder, it is lined with transitional epithelium, and contains layers of smooth muscle.

The epithelial cells of the ureter are stratified (in many layers), are normally round in shape but become squamous (flat) when stretched. The lamina propria is thick and elastic (as it is important that it is impermeable).

There are two spiral layers of smooth muscle in the ureter wall, an inner loose spiral, and an outer tight spiral. The inner loose spiral is sometimes described as longitudinal, and the outer as circular, (this is the opposite to the situation in the gastrointestinal tract). The distal third of the ureter contains another layer of outer longitudinal muscle.

The adventitia of the ureter, like elsewhere is composed of fibrous connective tissue, that binds it to adjacent tissues.


The urinary bladder is a hollow, muscular, and distensible (or elastic) organ that sits on the pelvic floor in mammals. It is the organ that collects urine excreted by the kidneys prior to disposal by urination. Urine enters the bladder via the ureters and exits via the urethra.

It swells into a round shape when it is full and gets smaller when empty. In the absence of bladder disease, it can hold up to 500 mL (17 fl. oz.) of urine comfortably for two to five hours. The epithelial tissue associated with the bladder is called transitional epithelium. It allows the bladder to stretch to accommodate urine without rupturing the tissue.

In males, the bladder is superior to the prostate, and separated from the rectum by the rectovesical excavation.

In females, the bladder is separated from the rectum by the rectouterine excavation, and it is separated from the uterus by the vesicouterine excavation.

The wall of the urinary bladder consists of three layers:

Mucosa: in this instance transitional epithelium & lamina propria
Detrusor muscle: consists of an inner and outer longitudinal layer and a middle circular layer of smooth muscle
a fibrous adventitia and the visceral peritoneum: lie on superior surface

The detrusor muscle is a layer of the urinary bladder wall, made up of smooth muscle fibers. When the bladder is stretched, this signals the parasympathetic nervous system to cause contraction of the detrusor muscle. This then encourages the bladder to expel urine through the urethra.

For the urine to finally exit the bladder, both the autonomically controlled internal sphincter and the voluntarily controlled external sphincter must be opened. Problems with these muscles can lead to incontinence.

The urinary bladder usually holds up to 400 - 620 mls of urine, but it can hold double of this without rupturing, for example in urinary outflow obstruction.

The desire to micturate usually begins to be experienced when the bladder contains around 75% of its usual working volume. If a person is distracted the desire to micturate disappears for a period, but will return with more urgency later, depending on degree of bladder fullness.


The urethra is a tube which connects the urinary bladder to the outside of the body. The urethra has an excretory function in both genders to pass urine to the outside, and also a reproductive function in the male, as a passage for sperm.
Female Urinary system

In the human female, the urethra is about 1-2 inches long and opens in the vulva between the clitoris and the vaginal opening.

Men have a longer urethra than women. This means that women tend to be more susceptible to infections of the bladder (cystitis) and the urinary tract

In the human male, the urethra is about 8 inches (20 cm) long and opens at the end of the penis. The inside of the urethra has a spiral groove (like rifling in a gun barrel), which makes the urine flow in a narrow stream.

The endpoint of the urinary system is the urethra. Typically the urethra in humans is colonised by commensal bacteria below the external urethral sphincter. The urethra emerges from the end of the penis in males and between the clitoris and vulva in females

The urethra is divided into three parts in men, named after the location:

Prostatic Urethra: Crosses through the prostate gland. There are several openings: (1) a small opening where sperm from the vas deferens and ejaculatory duct enters, (2) the prostatic ducts where fluid from the prostate enters, (3) an opening for the prostatic utricle, but nothing is added from it. These openings are collectively called the verumontanum.

Membranous Urethra: A small (1 or 2 cm) portion passing through the external urethral sphincter. This is the narrowest part of the urethra. It is located in the deep perineal pouch. The ducts from the bulbourethral glands enter here

Spongy Urethra (or Penile Urethra): Runs along the length of the penis on its ventral (underneath) surface. It is about 15-16 cm in length, and travels through the corpus spongiosum. The ducts from the urethral gland enter here.

The length of a male's urethra, and the fact it contains a number of bends, makes catheterisation more difficult.

The epithelium of the urethra starts off as transitional cells as it exits the bladder. Further along the urethra there are stratified columnar cells, then stratified squamous cells near the external meatus (exit hole).

There are small mucus-secreting urethral glands, that help protect the epithelium from the corrosive urine.