The human body produces three main types of waste that must be excreted:

• Carbon dioxide and water — produced during cellular respiration; carbon dioxide is removed by the lungs, and excess water is lost through urine and sweat
• Nitrogenous wastes — produced from the breakdown of amino acids and proteins; the main nitrogenous waste is urea, which is less toxic than ammonia
• Excess water and salts — taken in through food and drink; must be eliminated to maintain osmotic balance

Different organs specialize in removing specific waste products:

The kidneys are the main excretory organs responsible for removing nitrogenous wastes and regulating water and salt balance.

The urinary system consists of organs that work together to produce, store, and eliminate urine:

1. Kidneys — bean-shaped organs that filter blood and produce urine
2. Ureters — tubes that carry urine from each kidney to the bladder
3. Bladder — a muscular sac that stores urine until it is eliminated
4. Urethra — a tube that carries urine from the bladder out of the body

Each kidney has three main regions:

• Cortex — the outermost region; contains the glomeruli where filtration begins
• Medulla — the middle region; contains renal pyramids that concentrate urine
• Pelvis — the innermost region; collects urine before it enters the ureter

The nephron is the tiny filtering unit inside each kidney. Each kidney contains about one million nephrons. A nephron has four main parts:

1. Bowman's capsule — a cup-shaped structure that encloses the glomerulus (a ball of capillaries)
2. Proximal convoluted tubule — where most reabsorption of useful substances occurs
3. Loop of Henle — a U-shaped tube that concentrates urine by reabsorbing water
4. Distal convoluted tubule — further adjusts the composition of urine before it enters the collecting duct

Excretion in the kidneys occurs in three stages: filtration, reabsorption, and removal.

1. Filtration

Filtration takes place in the Bowman's capsule under high pressure:

• Blood enters the kidney through the renal artery, which branches into smaller vessels forming the glomerulus
• The afferent arteriole (bringing blood in) is wider than the efferent arteriole (taking blood out), creating high pressure
• Under this pressure, small molecules like water, glucose, amino acids, salts, and urea are forced out of the blood and into the Bowman's capsule — this is called ultrafiltration
• Large molecules like proteins and blood cells remain in the blood because they are too big to pass through the capillary walls
• The filtered liquid is called glomerular filtrate

2. Reabsorption

As the filtrate flows through the nephron tubule, useful substances are reabsorbed back into the blood:

• The proximal convoluted tubule reabsorbs most of the water, glucose, and amino acids through active transport
• This tubule has microvilli to increase surface area, mitochondria to provide energy for active transport, and a rich capillary network for efficient exchange
• The loop of Henle reabsorbs more water from the filtrate, concentrating the urine
• The distal convoluted tubule makes final adjustments to the salt and water content of urine

3. Removal

The remaining filtrate, now called urine, passes through the collecting duct and into the pelvis:

• Urine contains urea, excess water, and salts that the body does not need
• Urine flows from the kidneys through the ureters to the bladder
• When the bladder is full, nerve signals tell the brain to urinate
• The sphincter muscle relaxes, and urine is expelled through the urethra

Urine formation involves three processes working together:

In Tanzania, understanding how the kidneys work is important for everyday health. For example, when someone drinks plenty of water during a hot day in Dar es Salaam, the kidneys respond by producing more dilute urine to remove excess water while conserving salts. This is why health workers in local clinics advise people to drink enough water — it helps the kidneys filter waste effectively and prevents kidney stones, a common health problem that can be costly to treat at hospitals like Muhimbili National Hospital.