Excretory Products and their Elimination

Ammonotelism, ureotelism and uricotelism

When cells break down proteins, they make nitrogen waste. Animals remove it as ammonia, urea or uric acid.

Ammonia dissolves easily and leaves water animals through the body surface or gills. In mammals, the liver turns ammonia into urea (urea cycle), and the kidney removes it. Some urea is kept in the kidney medulla to keep it salty.

Excretory organs in other animals

• Flatworms (Planaria): protonephridia with flame cells.
• Earthworm: nephridia.
• Insects (cockroach): Malpighian tubules.
• Prawns: antennal or green glands.
• Vertebrates: kidneys.

Human excretory system and kidney structure

Humans have two kidneys, two ureters, one urinary bladder and one urethra. Kidneys are reddish-brown, bean-shaped, about 10 to 12 cm long, lying on each side of the backbone between the last thoracic and third lumbar vertebrae.

• The inner curve has a notch, the hilum, where the ureter, blood vessels and nerves enter or leave.
• Inside the hilum is a funnel, the renal pelvis, with branches called calyces.
• A tough capsule covers the kidney. Inside are the outer cortex and inner medulla. The medulla has cone-shaped medullary pyramids that open into calyces. Strips of cortex between the pyramids are columns of Bertini.

Nephron: the working unit

Each kidney has about one million nephrons. A nephron has two parts:

• Glomerulus: a tuft of capillaries formed by the afferent arteriole (wider, brings blood) and drained by the efferent arteriole (narrower, takes blood away).
• Renal tubule: starts as a cup, Bowman's capsule, around the glomerulus (together: Malpighian or renal corpuscle). Then comes the coiled proximal convoluted tubule (PCT), the hairpin loop of Henle (descending and ascending limbs), the distal convoluted tubule (DCT) and the collecting duct.

Most nephrons are cortical (short loop). A few are juxtamedullary (long loop dipping deep into the medulla). The efferent arteriole forms a capillary net around the tubule; a thin U-shaped vessel along the loop is the vasa recta.

Urine formation: filtration, reabsorption, secretion

1. Glomerular filtration

Blood pressure in the glomerulus pushes fluid through three layers (capillary wall, basement membrane, Bowman's capsule cells called podocytes with tiny slits). Almost everything small passes, but blood cells and plasma proteins stay. This is ultrafiltration.

The kidneys filter about 1100 to 1200 mL of blood per minute (about one-fifth of cardiac output). The glomerular filtration rate (GFR) is about 125 mL/min, or 180 L per day.

The juxtaglomerular apparatus (JGA), where the DCT touches the afferent arteriole, releases renin when GFR falls.

2. Reabsorption

Urine made per day is only about 1.5 L, so nearly 99% of the filtrate is taken back. Glucose, amino acids and Na⁺ are taken back by active transport; water and nitrogen waste by passive movement.

3. Tubular secretion

Tubule cells push H⁺, K⁺ and ammonia into the filtrate. This keeps the pH and ion balance of body fluids.

What each part does

• PCT: takes back nearly all glucose, amino acids, 70 to 80% of salts and water; secretes H⁺, ammonia, K⁺; takes back HCO₃⁻.
• Descending limb of Henle: lets water out but not salts, so filtrate becomes more concentrated.
• Ascending limb: impermeable to water; pushes out salts, so filtrate becomes dilute.
• DCT: takes back Na⁺ and water when needed, secretes H⁺, K⁺, NH₃.
• Collecting duct: takes back large amounts of water (with ADH) to make concentrated urine; lets a little urea into the medulla.

Concentrating urine: the counter-current mechanism

Mammals can make urine much more concentrated than blood. The trick is two U-shaped flows side by side in opposite directions: the loop of Henle and the vasa recta. This is called counter-current.

• NaCl leaving the ascending limb and urea leaking from the collecting duct build up in the medulla.
• So the fluid around the loop rises from about 300 mOsm/L in the cortex to about 1200 mOsm/L deep in the medulla.
• The vasa recta carries salt in a loop so the gradient is not washed away.
• As the collecting duct runs down through this salty zone, water leaves it by osmosis, and urine becomes up to about four times as concentrated as the filtrate.

Osmoregulation means keeping the right amount of water and salts in the body. The kidney does this by changing how much water it keeps.

Regulation of kidney function: ADH, RAAS and ANF

ADH (antidiuretic hormone, vasopressin)

Receptors in the hypothalamus sense when blood becomes too concentrated (you lost water). The posterior pituitary releases ADH. ADH makes the DCT and collecting duct take back more water, so less urine is made (less diuresis). It also narrows blood vessels a little, raising BP. When you drink a lot, ADH falls and you pass more urine.

Renin-angiotensin-aldosterone system (RAAS)

1. When blood flow, BP or GFR falls, the JG cells release renin.
2. Renin turns angiotensinogen (from the liver) into angiotensin I, then into angiotensin II.
3. Angiotensin II narrows blood vessels (raises BP) and makes the adrenal cortex release aldosterone.
4. Aldosterone makes the DCT take back Na⁺ and water, raising BP and GFR.

ANF (atrial natriuretic factor)

When blood flow to the heart atria rises, their walls release ANF. It widens blood vessels and lowers BP. So ANF works as a check on the RAAS.

Micturition and other excretory organs

Urine is stored in the bladder. When it stretches, receptors send signals to the CNS, which makes the bladder muscles contract and the urethral sphincter relax, and urine leaves. This is micturition (a reflex). An adult passes 1 to 1.5 L of urine a day, containing about 25 to 30 g of urea. Normal urine is light yellow and slightly acidic (pH about 6).

Other organs that help

• Lungs: remove about 200 mL of CO₂ per minute and some water.
• Liver: puts bilirubin, biliverdin, cholesterol, old steroid hormones, vitamins and drugs into bile, which leaves with faeces.
• Skin: sweat glands give out water, NaCl, a little urea and lactic acid (mainly to cool the body); oil (sebaceous) glands give out sterols and waxes.
• Saliva can remove small amounts of nitrogen waste.

Disorders of the excretory system

• Uremia: urea builds up in blood because the kidneys fail. It is very harmful and may lead to kidney failure.
• Renal calculi (kidney stones): hard lumps of crystals (like calcium oxalate) in the kidney.
• Glomerulonephritis: swelling (inflammation) of the glomeruli.
• Glycosuria (glucose in urine) and ketonuria (ketone bodies in urine) are signs of diabetes mellitus.

Haemodialysis

Blood is taken from an artery, mixed with an anticoagulant like heparin, and pumped through a dialysing unit (artificial kidney). It has a thin cellophane tube in a fluid that is like plasma but with no nitrogen waste. Waste moves out of the blood by diffusion. Clean blood gets an anti-heparin and is returned through a vein.

Kidney transplant

The best final treatment for failed kidneys. A working kidney from a donor, preferably a close relative, is used to lower the chance of rejection by the immune system. The patient takes medicines to calm the immune system.

Key formulas and definitions

• GFR ≈ 125 mL/min ≈ 180 L/day
• Urine/day ≈ filtrate × (1 − fraction reabsorbed)
• Renal blood flow ≈ 1/5 of cardiac output ≈ 1100–1200 mL/min
• Medulla gradient: 300 → 1200 mOsm/L
• RAAS: renin → angiotensinogen → angiotensin I → angiotensin II → aldosterone

Worked examples

Common mistakes

• Saying the kidney filters only waste. It filters almost everything small (even glucose) and then takes the useful parts back.
• Thinking plasma proteins are in the filtrate. Proteins and blood cells are too big and stay in the glomerulus.
• Mixing up the limbs of Henle's loop: the descending limb lets water out; the ascending limb lets salt out but not water.
• Saying ADH makes you pass more urine. ADH makes the kidney keep water, so urine is less.