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Overview of the Urinary System

A city needs both a clean water supply and an efficient waste-disposal network to function properly; your body relies on the urinary system for a similar waste-management role.
The urinary system has two main jobs: excretion (removing metabolic waste like urea) and osmoregulation (balancing the water and salt levels in your blood).
Macroscopically, the human urinary system is made up of four key components: two kidneys, two ureters, one bladder, and one urethra.
The system works sequentially to clean the blood. First, blood is filtered in the kidneys, then the resulting waste liquid (urine) travels down the ureters to be stored in the bladder before finally exiting the body via the urethra.

The Kidneys and Blood Supply

Despite being only the size of a computer mouse, your kidneys filter your entire blood volume dozens of times a day.
The kidneys are bean-shaped organs located at the back of the abdominal cavity, sitting just below the ribcage on either side of the spine.
Unfiltered, oxygenated blood is delivered directly from the aorta to the kidneys via the renal artery at very high pressure.
If you cut a kidney open, you would see three distinct internal regions:
- Cortex: The light-coloured outer layer where blood filtration begins.
- Medulla: The darker inner region containing pyramid-like structures.
- Renal pelvis: A central, funnel-like cavity that collects the newly formed urine.
Deep inside the cortex and medulla are roughly one million microscopic nephrons (kidney tubules). These are the functional units of the kidney where filtration and selective reabsorption actually occur.
Once the blood has been cleaned, it is now deoxygenated and leaves the kidney through the , traveling back to the vena cava.

Students frequently confuse the ureter and urethra. Remember this mnemonic: 'Ureter' has two 'e's (like the two tubes connecting the kidneys to the bladder), while 'Urethra' has an 'a' for 'away from the body'.

When asked to describe the blood in the renal vein, you must state that it is both 'deoxygenated' AND 'filtered' (or 'cleaned') to secure all the marks on an Edexcel mark scheme.

If you have to identify blood vessels on a diagram, look for the thickness of the walls; the renal artery will have thicker walls because it branches directly from the aorta and carries blood at a much higher pressure than the renal vein.

Remember that while glucose is present in the renal artery, it should be completely absent from the urine in a healthy individual because it is 100% selectively reabsorbed in the nephrons.

The removal of waste products of metabolism (e.g., urea, carbon dioxide) and substances in excess of requirements from the body.

The control of the water and salt (ion) balance in the blood and body fluids.

A bean-shaped organ responsible for excretion and osmoregulation by filtering the blood.

The muscular tube that transports urine from the kidney to the bladder.

A muscular bag that stores urine until it is excreted.

The tube through which urine passes from the bladder to the outside of the body.

The blood vessel carrying unfiltered, oxygenated blood from the aorta to the kidney at high pressure.

The blood vessel carrying filtered, deoxygenated blood from the kidney toward the vena cava.

The light-coloured outermost region of the kidney where ultrafiltration begins.

The darker inner section of the kidney containing pyramid structures.

A funnel-like cavity in the centre of the kidney that collects urine before it drains into the ureter.

The microscopic functional unit of the kidney where filtration and selective reabsorption occur.

A toxic nitrogenous waste product formed in the liver from the breakdown of excess amino acids.

Ring-shaped muscles at the base of the bladder that regulate the release of urine.