Excretory system

*Excretory system

The body’s waste‑removal network is centered on the kidneys, but it also includes the urinary tract, skin, lungs and liver. Together they keep the internal environment stable by getting rid of excess water, salts, nitrogenous waste and carbon dioxide.

*Main organs

Kidneys – two bean‑shaped organs that filter blood. Each kidney contains about a million nephrons, the tiny functional units where filtration, reabsorption and secretion happen. They regulate blood volume, electrolyte balance, pH and produce erythropoietin (stimulates red‑cell production) and renin (helps control blood pressure).

Ureters – narrow tubes that carry urine from the renal pelvis to the bladder. Peristaltic waves push the urine along.

Bladder – a muscular sac that stores urine until it’s convenient to void. The detrusor muscle contracts during urination, while the internal and external urethral sphincters control the flow.

Urethra – the duct that carries urine from the bladder out of the body. In males it also transports semen.



Key processes

1. Filtration – blood pressure forces plasma through the glomerular capillaries into Bowman’s capsule, producing a filtrate that contains water, ions, glucose, amino acids and waste.

2. Reabsorption – most of the filtrate (water, glucose, needed ions) is taken back into the blood via the proximal tubule, loop of Henle and distal tubule.

3. Secretion – additional waste products and drugs are actively moved from the blood into the tubular fluid for removal.

4. Concentration – the loop of Henle and collecting duct adjust water reabsorption under the influence of antidiuretic hormone (ADH), producing urine that is more or less concentrated as needed.

A few important terms

Nephron – the functional unit of the kidney (glomerulus + renal tubule).
Glomerulus – a tuft of capillaries where filtration occurs.
Bowman’s capsule – the cup‑shaped structure that collects the filtrate.
Proximal convoluted tubule (PCT) – reabsorbs the bulk of nutrients and water.
Loop of Henle – creates a concentration gradient for water reabsorption.
Distal convoluted tubule (DCT) – fine‑tunes electrolyte balance.
Collecting duct – final adjustment of water and electrolytes under hormonal control.
Urea – the main nitrogenous waste excreted in urine.

Kidney – main organ that filters blood and forms urine
Nephron – functional unit of the kidney (glomerulus + renal tubule)
Glomerulus – capillary tuft where blood filtration begins
Bowman’s capsule – cup‑shaped structure that collects filtrate from the glomerulus
Renal tubule – series of ducts (PCT, Loop of Henle, DCT, collecting duct) that modify filtrate
Proximal convoluted tubule (PCT) – reabsorbs most nutrients, water, and ions
Loop of Henle – creates osmotic gradient for water reabsorption
Distal convoluted tubule (DCT) – fine‑tunes electrolyte and pH balance
Collecting duct – final water‑reabsorption under ADH control; drains into renal pelvis
Ureter – muscular tube that carries urine from kidney to bladder
Bladder – storage reservoir for urine
Urethra – duct that expels urine from bladder to outside
Urine – aqueous solution of waste (urea, creatinine, salts) and excess water
Filtration – pressure‑driven movement of plasma into Bowman’s capsule
Reabsorption – selective transport of needed substances back into blood
Secretion – active movement of additional wastes/drugs from blood into tubule fluid
Excretion – elimination of urine from the body
Osmoregulation – control of water‑salt balance
Antidiuretic hormone (ADH) – hormone that increases water reabsorption in collecting duct
Aldosterone – hormone that boosts Na⁺ reabsorption and K⁺ excretion
Renin‑angiotensin‑aldosterone system (RAAS) – pathway that regulates blood pressure and fluid balance
Erythropoietin – kidney‑derived hormone that stimulates red‑blood‑cell production
Glomerular filtration rate (GFR) – measure of kidney filtration efficiency
Urea – major nitrogenous waste product excreted in urine
Creatinine – by‑product of muscle metabolism used to assess kidney function
Sweat glands – excrete water, salts, small amounts of urea through skin
Lungs – excrete carbon dioxide and water vapor
Liver – converts ammonia to urea for renal excretion