Excretory Products and their Elimination

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 16.

Where do 180 litres go?

Your kidneys form about 180 litres of filtrate every day. That is many times the total volume of your blood. Yet on an average day you pass only 1 to 1.5 litres of urine. So where does the rest go? Almost all of it is taken back into the blood. The kidney first filters nearly everything out, then carefully reclaims what the body still needs and keeps what it does not.

This chapter is the story of that filtering, reclaiming and fine tuning, and of what other animals do instead.

Three ways to get rid of nitrogen

Metabolism leaves behind ammonia, urea, uric acid, CO₂, water and ions (Na⁺, K⁺, Cl⁻, phosphate, sulphate). The nitrogenous wastes are the headline, because they come from the breakdown of proteins and nucleic acids and some of them are poisonous. The three forms trade off two things: how toxic the waste is, and how much water is needed to carry it away.

Why it works this way. Ammonia is very soluble and very poisonous, so it is only safe if it is flushed away at once, and that is easy only when water is limitless. This is why it suits aquatic animals. A land animal cannot afford that much water, so the liver converts ammonia into urea, which can be stored briefly and sent out in a smaller volume. Birds and reptiles go one step further and make uric acid, which leaves with almost no water.

Beyond the textbook. For the exam: marine fishes are ureotelic, and bony fishes are ammonotelic. In reality: the marine fishes that keep urea are the cartilaginous ones, such as sharks, which hold urea in their blood so that their body fluids match the salt concentration of sea water and they don't lose water. Most marine bony fishes get rid of their nitrogen mainly as ammonia across the gills. Uric acid also explains the egg of a bird or reptile: inside a sealed shell, the embryo can't flush wastes away, and insoluble uric acid crystals sit harmlessly. Why it matters: the pattern in the table follows water availability, which is the easiest way to remember it.

Excretory structures across animals

The human excretory system

It consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.

Each kidney is reddish brown and bean shaped. It lies between the levels of the last thoracic and third lumbar vertebra, against the dorsal wall of the abdomen. It measures 10–12 cm long, 5–7 cm wide and 2–3 cm thick, and weighs 120–170 g.

On the inner concave surface is the hilum, where the ureter, blood vessels and nerves enter and leave. Beyond the hilum is the funnel shaped renal pelvis, whose projections are the calyces.

The nephron: the working unit

Each kidney contains nearly one million nephrons. A nephron has two parts, the glomerulus and the renal tubule.

The glomerulus is a tuft of capillaries. Blood enters through the afferent arteriole (a branch of the renal artery) and leaves through the efferent arteriole. The tubule starts as a double walled cup, Bowman's capsule, which encloses the glomerulus. Together they form the Malpighian body (renal corpuscle).

The tubule in one line: Bowman's capsule → PCT (proximal convoluted tubule) → Henle's loop (descending, then ascending limb) → DCT (distal convoluted tubule) → collecting duct → renal pelvis

The Malpighian body, PCT and DCT lie in the cortex. The loop of Henle dips into the medulla. The efferent arteriole does not rejoin a vein directly. It breaks into the peritubular capillaries around the tubule, and a thin branch forms a U shaped vasa recta that runs parallel to the loop of Henle.

Nephrons come in two types, and the vasa recta is the key difference.

Making urine: three processes

Urine formation has three steps: glomerular filtration, reabsorption and secretion.

Filtration. The kidneys filter 1100–1200 mL of blood per minute, about one fifth of what each ventricle pumps out. Filtration happens through three layers: the endothelium of the glomerular capillaries, the basement membrane, and the epithelium of Bowman's capsule. Cells of that epithelium, the podocytes, are arranged to leave minute filtration slits (slit pores). Everything in plasma except the proteins passes through, so the process is called ultrafiltration.

The volume of filtrate formed per minute is the glomerular filtration rate (GFR), about 125 mL per minute, which is 180 litres per day.

Why it works this way. The efferent arteriole is narrower than the afferent one, so the pressure inside the glomerulus stays high. That pressure is what forces the fluid out across the filter. When the pressure and flow fall, the body must act quickly, and the juxtaglomerular apparatus (JGA) does exactly that. It is a sensitive region formed where the DCT touches the afferent arteriole. A fall in GFR activates its JG cells to release renin, which helps bring blood flow and GFR back to normal.

Worked example. GFR is 125 mL per minute, so the daily filtrate is 125 × 60 × 24 = 1,80,000 mL = 180 L. If you pass 1.5 L of urine, then 1.5 ÷ 180 is under 1% of the filtrate. That is why nearly 99% of the filtrate is reabsorbed. Note the two numbers that are easy to mix up: 1100–1200 mL per minute is the blood that is filtered, and 125 mL per minute is the filtrate that is formed.

Reabsorption. Tubular cells take back what the body needs. Glucose, amino acids and Na⁺ are reabsorbed actively. Nitrogenous wastes are reabsorbed passively, and water is also taken back passively in the initial segments.

Secretion. Tubular cells add H⁺, K⁺ and ammonia to the filtrate. This keeps the ionic and acid base balance of body fluids.

Segment by segment

The counter current trick: concentrating urine

Mammals can make urine that is more concentrated than their blood. The trick is a gradient. The fluid in the two limbs of Henle's loop flows in opposite directions, and so does the blood in the two limbs of the vasa recta. This is a counter current. Because the two sets of tubes lie side by side, they keep the surrounding medullary fluid increasingly salty, from about 300 mOsmol per litre in the cortex to about 1200 in the inner medulla. The gradient is mainly due to NaCl and urea.

NaCl is carried out of the ascending limb and exchanged with the descending limb of the vasa recta, and the ascending vasa recta then returns it to the medullary fluid. Similarly, a little urea enters the thin ascending limb and is returned to the fluid by the collecting duct. Water in the collecting duct then flows out into the salty surroundings, so the urine becomes concentrated. Human kidneys can make urine nearly four times as concentrated as the initial filtrate.

Pause and predict: as filtrate moves down the descending limb and then up the ascending limb, what happens to its concentration in each?

Check your answer: it gets more concentrated going down, because water leaves while salts stay. It gets more dilute going up, because the ascending limb is impermeable to water but lets electrolytes out into the medullary fluid.

Who controls the kidney?

The kidney is regulated by the hypothalamus, the JGA and, partly, the heart.

ADH. Osmoreceptors detect a loss of body fluid or a rise in ionic concentration and stimulate the hypothalamus to release antidiuretic hormone (ADH, or vasopressin) from the neurohypophysis. ADH helps water reabsorption from the later parts of the tubule, which prevents diuresis (excess urine). An increase in body fluid volume switches the osmoreceptors off and ADH release stops, completing a feedback loop. ADH also constricts blood vessels, which raises blood pressure.

Renin-angiotensin mechanism. A fall in glomerular blood flow, pressure or GFR makes the JG cells release renin.

The chain in one line: renin acts on angiotensinogen → angiotensin I → angiotensin II (powerful vasoconstrictor, raises glomerular blood pressure and GFR) → adrenal cortex releases aldosterone → Na⁺ and water are reabsorbed in the distal tubule → blood pressure and GFR rise

ANF. An increase in blood flow to the atria of the heart releases atrial natriuretic factor (ANF). It dilates blood vessels and lowers blood pressure, so it acts as a check on the renin-angiotensin mechanism.

Pause and predict: a person has bled heavily and blood pressure has dropped. Which of the above systems switches on, and which one waits?

Check your answer: the renin-angiotensin chain switches on, because glomerular flow and GFR have fallen, and ADH release rises too because blood volume is low. ANF waits, since it needs increased blood flow to the atria.

Beyond the textbook. For the exam: ADH promotes water reabsorption and reduces urine output. In reality: alcohol suppresses ADH release, which is why you pass large volumes of pale urine after a few drinks and end up dehydrated. Why it matters: it shows the negative feedback loop working in reverse, as diuresis when ADH is absent.

Storing and releasing urine: micturition

Urine from the nephrons drains to the renal pelvis, then through the ureters to the urinary bladder, where it is stored until the CNS gives a voluntary signal. Stretching of the bladder wall activates stretch receptors, which signal the CNS. The CNS sends motor messages that contract the smooth muscle of the bladder and relax the urethral sphincter, and urine is released. This is micturition, and the neural mechanism behind it is the micturition reflex.

An adult passes on an average 1–1.5 litres of urine a day. It is light yellow, watery, slightly acidic (pH about 6.0) and has a characteristic odour. About 25–30 g of urea is excreted per day. Urine analysis is a diagnostic tool. Glucose in urine is glycosuria and ketone bodies are ketonuria, and both point to diabetes mellitus.

Beyond the textbook. For the exam: glycosuria and ketonuria indicate diabetes mellitus. In reality: glucose gets into the urine because the tubule's glucose carriers can only handle so much. When blood glucose is very high, the filtered glucose exceeds their capacity and spills out, taking water with it. Why it matters: it shows that the "active reabsorption" of glucose has a ceiling.

Other organs that excrete

When kidneys fail

Haemodialysis cleans the blood of a uraemic patient. Blood from an artery is mixed with the anticoagulant heparin and pumped through a dialysing unit. The unit has a coiled cellophane tube surrounded by a dialysing fluid that has the same composition as plasma except that it lacks nitrogenous wastes. The tube is porous, so wastes move out along their concentration gradient. The cleared blood is pumped back through a vein after anti-heparin is added.

Kidney transplantation is the ultimate treatment of acute renal failure. A functioning kidney from a donor, preferably a close relative, lowers the chance of rejection by the host's immune system.

Exam traps

60-second revision

Where this chapter connects

• Before: Body Fluids and Circulation describes the blood that the kidney filters, a fifth of each ventricle's output per minute.
• Next: Locomotion and Movement covers gout, where uric acid crystals collect in the joints.
• Hormone link: Chemical Coordination and Integration covers ADH, aldosterone and ANF in the endocrine system.
• Nerve link: Neural Control and Coordination covers the reflex arc behind the micturition reflex.
• Comparative link: Structural Organisation in Animals describes the excretory organs of the earthworm, cockroach and frog.