Overview of the Urinary System

The major functions of the urinary system are to remove waste products and toxins from the body, and to regulate the level of fluids, electrolytes and other substances in the body.

The urinary system includes the two kidneys, the two ureters, the bladder and the urethra.

The urethra is long in a male as it passes through the penis. The urethra is short in a female 

The kidneys are located below the rib cage, and towards the back. The right kidney is typically located slightly lower than the left, due to the large liver. The adrenal gland is situated at the top of each kidney.

Blood enters the kidneys via the renal arteries, which branch off from the aorta. The blood exits the kidneys via the renal veins, which then empty into the inferior vena cava.

Anatomy of the Kidney and the Nephron
Urine is drained into the calyces and then into the renal pelvis, and then into the ureter. 

There are about 1 million nephrons in each kidney. The nephrons filter the blood to produce urine. The filtration takes place in the glomerulus. 

The Blood Supply to the Glomerulus
Blood enters the kidney via the renal artery. As the renal artery enters into the kidney, it branches into smaller arteries, which then branch into smaller and smaller arteries, until they become arterioles.

The arterioles take blood to the glomeruli. The arteriole that enters the glomerulus is called the afferent arteriole. 

Blood Filtered in the Glomerulus

The glomerulus is a tangled cluster of capillaries. Much of the blood that passes through these capillaries gets filtered. This is due to the presence of numerous pores in the capillary walls. The substances in the blood that are small enough to pass through these pores are pushed through, because of high pressures. This is the first stage of filtration.

Further filtration mechanisms then occurs in the glomerulus. The substances that successfully pass through these filtration mechanisms are called filtrate. The filtrate includes water, electrolytes, waste products and other substances.

The filtrate travels along the nephron. The nephron is surrounded by a network of capillaries. 

The capillaries surrounding the nephron reabsorb the useful substances in the filtrate, as well as most of the water. These useful substances and water therefore enter back into the blood circulation, instead of being excreted into the urine.

Furthermore, the capillaries surrounding the nephron also secrete unwanted substances into the filtrate. Any unwanted substances that failed to be filtered at the glomerulus, are now given another chance to be excreted into the urine.

The filtrate then flows out of the nephron and into the collecting ducts. The collecting ducts then empty into the calyces, which drain urine into the renal pelvis. The renal pelvis empties into the ureter. Since no further processing of the filtrate shall occur, it can now be called urine.

The ureters take the urine to the bladder. In the bladder, the urine is stored temporarily. The bladder wall is composed of smooth muscle, which expands as it fills.

When the bladder empties, urine travels down the urethra, and expelled out into the environment. 

Blood Not Filtered in the Glomerulus
Some substances in the blood are too large to be filtered through the pores in the capillary walls. Examples of substances that are too large to be filtered are red blood cells and most proteins. These substances therefore continue to travel through the capillaries, without being filtered out.

This unfiltered blood then exits the glomerulus through the efferent arteriole. The efferent arteriole then divides into a network of capillaries that surround the nephron. As previously mentioned, these capillaries surrounding the nephron enable reabsorption and secretion to take place.

The capillaries surrounding the nephron then join to venules, which then join to veins, which then join to larger veins. Eventually, the blood exits the kidney via the renal vein.