• Bladder: A hollow organ that stores urine.
• Bowman’s capsule: A cup-shaped structure around the glomerulus that collects the filtered substances; part of the nephron.
• Dialysis: A medical procedure in which blood is filtered with the help of a machine.
• Excretion:The process of removing wastes and excess water from the body.
• Filtration: The process of filtering substances from blood in the glomerulus.
• Glomerulus: Part of the nephron; a cluster of arteries that filters substances out of the blood.
• Homeostasis: The ability to maintain a stable internal environment despite external changes.
• Kidney: Organ that filters the blood and forms urine.
• Kidney stones: Crystals of dissolved minerals that form in urine inside the kidneys.
• Nephrons: The structural and functional units of the kidneys; includes the glomerulus, Bowman’s capsule, and renal tubule.
• Reabsorption: The return of needed substances in the filtrate back to the bloodstream.
• Renal tubule: A long, narrow tube surrounded by capillaries that reabsorbs many of the filtered substances and secretes other substances; part of the nephron.
• Urea: The main waste nitrogen in the urine of mammals. It is produced in the metabolism of proteins.
• Ureter: Tube-shaped structure that brings urine from the kidneys to the urinary bladder.
• Urethra: A muscular tube that carries urine out of the body.
• Urinary system: The organ system that makes, stores, and gets rid of urine.
• Urine: The liquid waste product of the body that is excreted by the urinary system.

Formation of Urine

In the last post about the nephron function we studied how urine is produced, but in this new post we are going to explain very carefully all the process. I hope you'll be able to understand it.
The process of urine formation is as follows:
1. Blood flows into the kidney through the renal artery. The renal artery branches into capillaries inside the kidney. Capillaries and the nephrons lie very close to each other in the kidney.
2. In the nephron, the blood pressure within the capillaries causes water and solutes (small soluble molecules) such as salts, sugars, and urea to leave the capillaries and move into the Bowman’s capsule. (GLOMERULAR FILTRATION).
3. The water and solutes move along through the tubules of nephrons. At this point most of the water and solutes are returned to the capillaries that surround the nephron (TUBULAR REABSORPTION). Some chemicals are secreted in the las part of the tubules (TUBULAR SECRETION).
4. The fluid that remains in the nephron at this point is called urine.
5. The blood that leaves the kidney in the renal vein has much less waste than the blood that entered the kidney.
6. The urine is collected in the ureters and is moved to the urinary bladder where it is stored.
Nephrons filter 125 ml (about ¼ cup) of body fluid per minute. In a 24-hour period nephrons produce about 180 liters of filtrate, of which 178.5 liters are reabsorbed. The remaining 1.5 liters of fluid forms urine.
Urine enters the bladder through the ureters. Similar to a balloon, the walls of the bladder are stretchy. The stretchy walls allow the bladder to hold a large amount of urine. The bladder can hold about 400 to 620 ml of urine, but may also hold more if the urine cannot be released immediately. Urination is the process of releasing urine from the body. Urine leaves the body through the urethra.
Nerves in the bladder tell you when it is time to urinate. As the bladder first fills with urine, you may notice a feeling that you need to urinate. The urge to urinate becomes stronger as the bladder continues to fill up.
In this table it is possible to compare plasma and urine composition.

	plasma g/100ml	urine g/100ml	concentration in urine
water	90-93	95
protein	7 - 8.5	0
urea	0.03	2	x60
uric acid	0.002	0.03	x15
glucose	0.1	0

Nephrons

A single kidney may have more than a million nephrons. Nephrons are the structural and functional units of the kidneys. The diagram represents an individual nephron and shows its main structures and functions. The structures include the glomerulus, Bowman’s capsule, and renal tubule.
Nephron structures.

• The renal corpuscle that consists of two structures:

• The glomerulus is a cluster of capillaries that filters substances out of the blood.
• Bowman’s capsule is a cup-shaped structure around the glomerulus that collects the filtered substances.

• The renal tubule is a long, narrow tube surrounded by capillaries that reabsorbs many of the filtered substances and secretes other substances. It is divided into three parts: the proximal tubule, the Loop of Henle, and the distal tubule.

Tubules of several nephrons join to form a single collecting tubule.
Nephron function.
The nephron function is to produce urine. In this process we can consider two stages
FILTRATION
Filtration is the process of filtering substances from blood in the glomerulus. The renal arteries, which carry blood into the kidneys, branch into the capillaries of the glomerulus of each nephron. The pressure of blood moving through these capillaries forces some of the water and dissolved substances in the blood through the capillary walls and into Bowman’s capsule.
The fluid that collects in Bowman’s space is called filtrate. It is composed of water, salts, glucose, amino acids, and urea. Larger structures in the blood—including protein molecules and blood cells—do not pass into Bowman’s space. Instead, they return to the main circulation.
REABSORPTION AND SECRETION.
From the space inside the Bowman’s capsule the filtrate passes into the renal tubule. The main function of the renal tubule is reabsorption. Reabsorption is the return of needed substances in the filtrate back to the bloodstream.
At the beginning, in the proximal tubule, salts, glucose and amino acids are picked up from the filtrate. In the rest of the tubule the main reabsorbed substance is water. Before the tubule arrives to the collecting tubule, some substances can be secreted in the distal tubule.
The collecting tubule reabsorbs water from tubular fluid and return it to the blood. The remaining fluid, called urine, has a smaller volume and a greater concentration than tubular fluid. From the collecting ducts, urine enters a ureter and is eventually excreted from the body.

The Urinary system

The urinary system is the organ system that makes, stores, and gets rid of urine. It includes two kidneys, two ureters, the bladder, and the urethra. The main function of the urinary system is to filter waste products and excess water from the blood and remove them from the body. Urine is the liquid waste product of the body that is excreted by the urinary system. Recall that in the excretion process, other organs as skin and lungs take part in it.Organs of the urinary system
Kidneys
The kidneys are a pair of bean-shaped, reddish brown organs about the size of a fist. They are located just above the waist at the back of the abdominal cavity, on either side of the spine. Located on top of each kidney is an adrenal gland.
The kidney has three layers, the outer layer is the renal cortex, and the middle layer is the renal medulla. The inner layer, the renal pelvis is urine is collected and is funnelled into the ureter
From the aorta, the renal arteries carry blood to the kidneys to be filtered, then the renal veins carry the filtered blood away from the kidneys to the inferior vena cava.
Ureters
From the kidneys, urine enters the ureters, which carry it to the bladder. Each ureter is a muscular tube about 25 centimeters long.
Bladder
The bladder is a hollow organ that stores urine. It can stretch to hold up to 500 milliliters. When the bladder is about half full, the stretching of the bladder sends a nerve impulse to the sphincter that controls the opening to the urethra. In response to the impulse, the sphincter relaxes and lets urine flow into the urethra.
The urethra is a muscular tube that carries urine out of the body. Urine leaves the body through another sphincter in the process of urination. This sphincter and the process of urination are normally under conscious control.
As in other organ systems here you have a video for kids but it is very clear.