Editing Excretory system (section)

===Urinary system===
{{Main|Urinary system}}
The [[kidneys]] are large, bean-shaped organs which are present on each side of the [[vertebral column]] in the [[abdominal cavity]]. Humans have two kidneys that are supplied with blood from the [[renal artery]]. The kidneys remove from the blood the nitrogenous wastes such as urea, as well as salts and excess water, and excrete them in the form of urine. This is done with the help of millions of [[nephrons]] present in the kidney. The [[renal vein]] (or kidney vein) carries the filtrated blood away from the kidneys. Urine passes from the kidneys though the ureters to the urinary bladder and passes through the [[urethra]] during urination.

==== Kidneys ====
{{Main|Kidneys}}

The kidney's primary function is the elimination of waste from the bloodstream by production of urine. They perform several homeostatic functions such as:-

# Maintain volume of extracellular fluid
# Maintain ionic balance in extracellular fluid
# Maintain pH and osmotic concentration of the extracellular fluid.
# Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.

The way the kidneys do this is with nephrons. There are over 1 million nephrons in each kidney; these nephrons act as filters inside the kidneys. The kidneys filter needed materials and waste. Needed materials go back into the bloodstream; unneeded materials become urine and are expelled through the urethra.

In some cases, excess wastes crystallize as kidney stones. They grow and can become painful irritants that may require surgery or ultrasound treatments. Some stones are small enough to be forced into the urethra.

==== Ureter ====
{{Main|Ureters}}

The [[ureters]] are muscular ducts that propel urine from the kidneys to the [[urinary bladder]]. In the human adult, the ureters are usually {{convert|25|–|30|cm|in|round=0.5|abbr=on}} long. In humans, the ureters arise from the renal pelvis on the medial aspect of each kidney before descending towards the bladder on the front of the [[psoas major]] muscle. The ureters cross the pelvic brim near the bifurcation of the iliac arteries (which they run over). This "pelviureteric junction" is a common site for the impaction of [[kidney stones]] (the other being the uteterovesical valve). The ureters run posteriorly on the lateral walls of the pelvis. They then curve anteriormedially to enter the bladder through the back, at the vesicoureteric junction, running within the wall of the bladder for a few centimeters. The backflow of urine is prevented by valves known as ureterovesical valves. In the female, the ureters pass to the bladder through the mesometrium.

==== Urinary bladder ====
The [[urinary bladder]] is the organ that collects urine from the kidneys. It is a hollow [[muscular]], and distensible (or elastic) organ, and sits on the [[pelvic floor]]. Urine enters the bladder via the ureters and exits via the [[urethra]] during [[urination]].

Embryologically, the bladder is derived from the [[urogenital sinus]], and it is initially continuous with the [[allantois]]. In human males, the base of the bladder lies between the rectum and the pubic symphysis. It is superior to the [[prostate]], and separated from the [[rectum]] by the [[rectovesical excavation]]. In females, the bladder sits inferior to the [[uterus]] and anterior to the [[vagina]]. It is separated from the uterus by the [[vesicouterine excavation]]. In [[infants]] and young children, the urinary bladder is in the abdomen even when empty.

==== Urethra ====
The urethra is a tube which transports urine from the urinary bladder to the outside of the body through the [[penis]] or [[vulval vestibule]]. In placental mammals, the urethra also transports [[semen]] through the penis during [[ejaculation]].