Excretory System

• Excretion is the removal of harmful nitrogen-containing (nitrogenous) metabolic wastes formed inside the body.
• These wastes are leftovers of metabolism (the non-stop chemical work of cells) and would poison the blood if not removed.
• Nitrogenous waste forms when the body breaks down extra proteins and amino acids it cannot store.
• Do not confuse it with egestion – throwing out undigested food as faeces, which was never part of body chemistry.
• Key exam point: only metabolic, nitrogenous waste counts as true excretion.

Excretion is the removal of harmful nitrogen-containing (nitrogenous) metabolic wastes formed inside the body.

These wastes are leftovers of metabolism (the non-stop chemical work of cells) and would poison the blood if not removed.

Nitrogenous waste forms when the body breaks down extra proteins and amino acids it cannot store.

Do not confuse it with egestion – throwing out undigested food as faeces, which was never part of body chemistry.

Key exam point: only metabolic, nitrogenous waste counts as true excretion.

• Ammonia (NH3) is the most toxic but most watersoluble; removed by aquatic animals (ammonotelism).
• Urea is made in the liver from ammonia, is moderately toxic, and is removed by the kidneys.
• Uric acid is least toxic and almost insoluble; thrown out as dry paste to save maximum water (uricotelism).
• Toxicity and water needed both follow the order ammonia > urea > uric acid.
• Humans and frogs are ureotelic (excrete mainly urea).
• Ammonia (ammonotelic) — Aquatic — Bony fish, tadpole
• Urea (ureotelic) — Land mammals — Human, frog
• Uric acid (uricotelic) — Dry-land — Birds, reptiles, insects

Ammonia (NH3) is the most toxic but most watersoluble; removed by aquatic animals (ammonotelism).

Urea is made in the liver from ammonia, is moderately toxic, and is removed by the kidneys.

Uric acid is least toxic and almost insoluble; thrown out as dry paste to save maximum water (uricotelism).

Toxicity and water needed both follow the order ammonia > urea > uric acid.

Humans and frogs are ureotelic (excrete mainly urea).

• Kidneys are the main organs, but they are not the only excretory organs.
• The lungs remove carbon dioxide (CO2) and water vapour.
• The skin removes water, salts and urea through sweat.
• The liver removes bile pigments (bilirubin and biliverdin) from broken-down haemoglobin.
• The large intestine can remove some salts like excess iron and calcium.
• Kidneys — Urea, salts, water (urine)
• Lungs — CO2, water vapour
• Skin — Sweat (water, salts, urea)
• Liver — Bile pigments

Kidneys are the main organs, but they are not the only excretory organs.

The lungs remove carbon dioxide (CO2) and water vapour.

The skin removes water, salts and urea through sweat.

The liver removes bile pigments (bilirubin and biliverdin) from broken-down haemoglobin.

The large intestine can remove some salts like excess iron and calcium.

• It has four parts: two kidneys, two ureters, one urinary bladder, one urethra.
• Kidneys are two bean-shaped, reddish-brown organs that filter and clean the blood.
• Ureters are thin tubes carrying urine from each kidney down to the bladder.
• The urinary bladder is a stretchy muscular bag that stores urine.
• The urethra is the tube through which urine finally leaves the body.

Kidneys are two bean-shaped, reddish-brown organs that filter and clean the blood.

Ureters are thin tubes carrying urine from each kidney down to the bladder.

The urinary bladder is a stretchy muscular bag that stores urine.

The urethra is the tube through which urine finally leaves the body.

• Each kidney lies between the last thoracic and third lumbar vertebra, against the back wall of the abdomen.
• An adult kidney is about 10-12 cm long, 5-7 cm wide, 2- 3 cm thick.
• Each kidney weighs roughly 120-170 g.
• The inner curved side has a notch, the hilum, where ureter, blood vessels and nerves enter or leave.
• Inside the hilum is a funnel-shaped renal pelvis with finger-like calyces that collect urine.
• Length — 10-12 cm
• Width — 5-7 cm
• Thickness — 2-3 cm
• Weight — 120-170 g

Each kidney lies between the last thoracic and third lumbar vertebra, against the back wall of the abdomen.

Each kidney weighs roughly 120-170 g.

The inner curved side has a notch, the hilum, where ureter, blood vessels and nerves enter or leave.

Inside the hilum is a funnel-shaped renal pelvis with finger-like calyces that collect urine.

• A tough protective covering, the renal capsule, wraps the kidney from outside.
• A length-wise cut shows two zones: an outer lighter cortex and an inner darker medulla.
• The medulla is divided into cone-shaped medullary pyramids whose tips drip urine into the calyces.
• Cortex tissue dips inward between pyramids as strips called renal columns (columns of Bertini).
• The renal pelvis and calyces collect urine before it enters the ureter.

A tough protective covering, the renal capsule, wraps the kidney from outside.

A length-wise cut shows two zones: an outer lighter cortex and an inner darker medulla.

The medulla is divided into cone-shaped medullary pyramids whose tips drip urine into the calyces.

Cortex tissue dips inward between pyramids as strips called renal columns (columns of Bertini).

The renal pelvis and calyces collect urine before it enters the ureter.

• Each kidney has over one million nephrons; the nephron is the structural and functional unit of the kidney.
• A nephron has two main parts: the glomerulus (filter) and the renal tubule (winding pipe).
• The glomerulus is a tuft of blood capillaries; blood enters via the afferent and leaves via the efferent arteriole.
• The cup-shaped Bowman's capsule wraps the glomerulus and catches the filtered liquid.
• Glomerulus plus Bowman's capsule form the Malpighian (renal) corpuscle.
• Tubule order: PCT -> Loop of Henle -> DCT -> collecting duct, which empties into the renal pelvis.

Each kidney has over one million nephrons; the nephron is the structural and functional unit of the kidney.

A nephron has two main parts: the glomerulus (filter) and the renal tubule (winding pipe).

The glomerulus is a tuft of blood capillaries; blood enters via the afferent and leaves via the efferent arteriole.

The cup-shaped Bowman's capsule wraps the glomerulus and catches the filtered liquid.

Glomerulus plus Bowman's capsule form the Malpighian (renal) corpuscle.

• Cortical nephrons have a short loop of Henle that barely enters the medulla; they form the majority.
• Juxtamedullary nephrons have a long loop dipping deep into the medulla and make concentrated urine.
• A U-shaped capillary, the vasa recta, runs alongside the long loop and helps save water.
• The vasa recta is poorly developed or absent in cortical nephrons.
• Loop of Henle — Short — Long
• Number — Majority — Fewer
• Vasa recta — Reduced — Well developed
• Urine concentration — Less — More

Cortical nephrons have a short loop of Henle that barely enters the medulla; they form the majority.

Juxtamedullary nephrons have a long loop dipping deep into the medulla and make concentrated urine.

A U-shaped capillary, the vasa recta, runs alongside the long loop and helps save water.

The vasa recta is poorly developed or absent in cortical nephrons.

• Step 1 is glomerular filtration: high blood pressure pushes plasma into Bowman's capsule.
• Filtrate passes through endothelium, basement membrane and podocytes with filtration slits, so it is called ultrafiltration.
• Everything except large proteins and blood cells passes through into the capsule.
• Step 2 is reabsorption: useful substances like glucose, amino acids, salts and most water return to blood (mostly in the PCT).
• Step 3 is tubular secretion: extra ions like H+, K+ and ammonia are added into the tubule to keep blood pH balanced.
• Filtration — Plasma filtered into capsule
• Reabsorption — Useful items returned to blood
• Secretion — Extra wastes added to filtrate

Step 1 is glomerular filtration: high blood pressure pushes plasma into Bowman's capsule.

Filtrate passes through endothelium, basement membrane and podocytes with filtration slits, so it is called ultrafiltration.

Everything except large proteins and blood cells passes through into the capsule.

Step 2 is reabsorption: useful substances like glucose, amino acids, salts and most water return to blood (mostly in the PCT).

Step 3 is tubular secretion: extra ions like H+, K+ and ammonia are added into the tubule to keep blood pH balanced.

• About 1100-1200 ml of blood is filtered by the kidneys every minute.
• This is roughly one-fifth (1/5) of the blood the heart pumps each minute.
• GFR (Glomerular Filtration Rate) is about 125 ml of filtrate per minute.
• This adds up to nearly 180 litres of filtrate per day, but only about 1.5 litres becomes urine.
• GFR is regulated by the JGA (Juxtaglomerular Apparatus), which releases the enzyme renin when blood pressure falls.
• Blood filtered/min — 1100-1200 ml
• GFR — ~125 ml/min
• Filtrate/day — ~180 litres
• Urine/day — ~1.5 litres

About 1100-1200 ml of blood is filtered by the kidneys every minute.

This is roughly one-fifth (1/5) of the blood the heart pumps each minute.

This adds up to nearly 180 litres of filtrate per day, but only about 1.5 litres becomes urine.

GFR is regulated by the JGA (Juxtaglomerular Apparatus), which releases the enzyme renin when blood pressure falls.

• Normal urine is a light yellow, watery fluid, slightly acidic, with a faint smell.
• It is about 95% water with urea, uric acid, salts, creatinine and the pigment urochrome.
• Glucose, proteins or blood in urine signal disease such as diabetes (glucose) or kidney damage.
• Urine collects in the bladder; its stretching triggers the urge to urinate, called micturition.
• Micturition is controlled by the micturition reflex and relaxation of the urethral sphincters.

Normal urine is a light yellow, watery fluid, slightly acidic, with a faint smell.

It is about 95% water with urea, uric acid, salts, creatinine and the pigment urochrome.

Glucose, proteins or blood in urine signal disease such as diabetes (glucose) or kidney damage.

Urine collects in the bladder; its stretching triggers the urge to urinate, called micturition.

Micturition is controlled by the micturition reflex and relaxation of the urethral sphincters.

• Kidney stones (renal calculi) are hard crystals, mainly of calcium oxalate, formed in the kidney.
• Kidney failure means kidneys stop filtering, so urea builds up in blood (uraemia).
• Dialysis is an artificial method to remove wastes from blood when kidneys fail.
• A permanent cure for failure is kidney transplantation from a matching donor.
• Hormones controlling kidney function are ADH (vasopressin), which saves water, and aldosterone, which saves salt.
• Kidney stone — Calcium oxalate crystals
• Uraemia — Urea builds in blood
• Failure — Dialysis or transplant

Kidney stones (renal calculi) are hard crystals, mainly of calcium oxalate, formed in the kidney.

Kidney failure means kidneys stop filtering, so urea builds up in blood (uraemia).

Dialysis is an artificial method to remove wastes from blood when kidneys fail.

A permanent cure for failure is kidney transplantation from a matching donor.

Hormones controlling kidney function are ADH (vasopressin), which saves water, and aldosterone, which saves salt.