Excretory Products and Their EliminationNEET Zoology · Class 11 · NCERT Chapter 16

Introduction

Every metabolic reaction in the body produces some waste. The most dangerous waste in animals is nitrogenous waste, formed mainly from the breakdown of amino acids. Allowing it to build up would poison the body, so animals get rid of it through excretion.

Expect 1 to 2 NEET questions every year from this chapter. The most reliable scoring areas are: modes of nitrogenous excretion, parts of the nephron, the three steps of urine formation, the counter-current mechanism, and the role of ADH and aldosterone.

Modes of Nitrogenous Excretion

Animals get rid of nitrogen in three different chemical forms. Each suits a different environment.

• Ammonotelic (ammonia): ammonia is very toxic but also very soluble. Animals that excrete it need a lot of water. Examples: most bony fish, aquatic amphibians (tadpoles), aquatic invertebrates.
• Ureotelic (urea): urea is less toxic than ammonia and needs less water. Examples: mammals (including humans), adult amphibians, sharks.
• Uricotelic (uric acid): uric acid is almost insoluble and is excreted as a near-solid paste. Needs almost no water. Ideal for animals that lay shelled eggs. Examples: birds, reptiles (most), insects, land snails.

Human Excretory System

The human excretory system has four parts:

• Kidneys (pair): bean-shaped, lie on the back wall of the abdomen, one on each side of the spine, between the last thoracic and third lumbar vertebrae. Each adult kidney is about 10 to 12 cm long, 5 to 7 cm wide, weighs about 120 to 170 g.
• Ureters (pair): tubes that carry urine from each kidney down to the urinary bladder.
• Urinary bladder: stores urine until it can be released. Smooth muscle wall.
• Urethra: tube that carries urine from the bladder to outside the body.

Structure of the Kidney

Cut a kidney lengthwise and you see two main regions:

• Cortex: outer region. Mostly nephrons with their glomeruli and convoluted tubules.
• Medulla: inner region. Has cone-shaped medullary pyramids (renal pyramids) that project into the calyces. Contains the Loop of Henle and collecting ducts.

Each pyramid empties through a minor calyx into a major calyx and then into the renal pelvis, which narrows into the ureter. The kidney also has a deep notch called the hilum where the renal artery enters and the renal vein and ureter leave.

Nephron: The Functional Unit

Each kidney contains about 1 to 1.5 million nephrons. The nephron is the structural and functional unit of the kidney. There are two types:

• Cortical nephrons (about 85%): short Loop of Henle that barely enters the medulla. Found mainly in the cortex.
• Juxtamedullary nephrons (about 15%): long Loop of Henle that goes deep into the medulla. Wrapped in capillaries called vasa recta. These nephrons concentrate the urine.

Parts of the Nephron

1. Glomerulus: a tuft of capillaries inside the Bowman capsule. Blood comes in via the afferent arteriole and leaves via the efferent arteriole. The afferent arteriole is wider than the efferent, which keeps glomerular pressure high enough for filtration.
2. Bowman capsule: double-walled cup that catches the filtrate. Glomerulus + Bowman capsule together = renal corpuscle (or Malpighian body).
3. Proximal Convoluted Tubule (PCT): heavily coiled tubule in the cortex. Cells have a brush border of microvilli for absorption. Reabsorbs about 70 to 80% of the filtrate volume, all the glucose and amino acids, most of the Na+ and water.
4. Loop of Henle: U-shaped loop with a descending limb (into medulla) and ascending limb (back to cortex). Sets up the medullary salt gradient via the counter-current mechanism.
5. Distal Convoluted Tubule (DCT): short coiled tubule back in the cortex. Fine control of ions. Also secretes H+, K+ and ammonia. Under hormonal control (aldosterone, PTH).
6. Collecting Duct: several nephrons drain into one collecting duct, which passes through the medulla and opens at the tip of a pyramid. Final water reabsorption happens here under ADH control.

Blood Supply to the Nephron

The path of blood through one nephron:

Renal artery → afferent arteriole → glomerulus (filtration happens) → efferent arteriole → peritubular capillaries around the convoluted tubules → vasa recta around the Loop of Henle (juxtamedullary nephrons only) → renal vein.

Urine Formation

Urine formation has three main steps in order:

1. Glomerular Filtration

Blood enters the glomerulus at high pressure (about 55 mm Hg). Water and small solutes (electrolytes, glucose, amino acids, urea, vitamins) are pushed through the filtration membrane into the Bowman capsule. Blood cells and plasma proteins are too large; they stay in the blood.

The filtration membrane has three layers: endothelium of the glomerular capillary, basement membrane, and the epithelial cells of the Bowman capsule (podocytes with slit pores).

GFR (Glomerular Filtration Rate) in a healthy adult is about 125 mL per minute, which works out to about 180 litres per day. Almost all of this is reabsorbed; only about 1 to 1.5 litres leaves the body as urine.

GFR is held nearly constant by the Juxtaglomerular Apparatus (JGA), a small group of modified cells where the DCT touches the afferent arteriole. If GFR drops, the JGA releases renin, which starts the renin-angiotensin-aldosterone pathway that restores blood pressure.

2. Tubular Reabsorption

Useful substances are reabsorbed from the filtrate back into the blood (peritubular capillaries). About 99% of the water and most of the useful solutes are reabsorbed.

• PCT: reabsorbs almost all glucose and amino acids (by active transport with Na+), about 70% of Na+, water (passively, follows Na+), and bicarbonate.
• Descending limb of Loop of Henle: only water leaves passively into the salty medulla.
• Ascending limb: Na+ and Cl- are actively pumped out. Water cannot follow.
• DCT and collecting duct: fine-tuned reabsorption of Na+ (under aldosterone) and water (under ADH).

3. Tubular Secretion

Some substances are actively moved from the blood into the filtrate. This adds extra waste to the urine and helps regulate blood pH. Mainly done in the DCT and collecting duct.

• H+: excreted in exchange for Na+. Important for acid-base balance.
• K+: excreted under aldosterone control.
• NH3 (ammonia): buffers H+ and is excreted as ammonium (NH4+).
• Some drugs and toxins are also secreted into the filtrate this way.

Counter-Current Mechanism

The counter-current mechanism is how the kidney concentrates urine. It depends on the long Loop of Henle and the vasa recta of juxtamedullary nephrons.

• Descending limb of Loop of Henle: permeable to water, NOT to salts. Water leaves into the salty medulla. Filtrate becomes more concentrated as it goes down.
• Ascending limb: permeable to salts, NOT to water. Na+ and Cl- pumped out. Filtrate becomes more dilute as it goes up.
• The result is a steep osmolarity gradient in the medulla: about 300 mOsm in the cortex and up to 1200 mOsm at the medullary tip.
• The vasa recta run parallel to the loop in a counter-current arrangement. They carry away water without washing out the gradient.
• The collecting duct passes back through this medullary gradient. If ADH is present, water moves out of the duct into the medulla. The urine becomes concentrated and its volume drops.

Regulation of Kidney Function

The kidney is controlled by four main hormonal signals:

• ADH (vasopressin, from posterior pituitary): makes the collecting duct more permeable to water. More water is reabsorbed; urine becomes concentrated. ADH deficiency causes diabetes insipidus (huge volumes of dilute urine).
• Aldosterone (from adrenal cortex): acts on DCT and collecting duct. Reabsorbs Na+ (water follows) and excretes K+. Raises blood volume and blood pressure.
• Renin-Angiotensin pathway: JGA releases renin when blood pressure drops. Renin converts angiotensinogen to angiotensin I, then ACE in the lungs converts it to angiotensin II. Angiotensin II constricts blood vessels (raising BP directly) and stimulates aldosterone release.
• ANF (Atrial Natriuretic Factor, from heart): released when blood pressure is high. Increases sodium and water excretion. Lowers blood pressure. Opposes the renin-angiotensin-aldosterone system.

Other Excretory Organs

The kidneys are the main excretory organs, but a few others share the load:

• Lungs: excrete CO2 (about 200 mL per minute at rest) and water vapour.
• Liver: the main site of urea synthesis (urea cycle). Also excretes bile pigments (bilirubin, biliverdin), cholesterol, and some drugs.
• Skin (sweat glands): sweat contains water, salts, lactic acid, urea (about 1 to 2% of total urea). Sebaceous glands excrete sterols, hydrocarbons, waxes through sebum.

Disorders and Treatments

• Uremia: build-up of urea in the blood when the kidneys cannot filter properly. Untreated, it leads to renal failure.
• Renal failure: the kidneys can no longer remove waste. Acute (sudden, sometimes reversible) or chronic (gradual, irreversible).
• Renal calculi (kidney stones): hard deposits in the kidney. About 80% are made of calcium oxalate. Cause severe pain when they move into the ureter. Drinking enough water lowers the risk.
• Glomerulonephritis: inflammation of the glomeruli, often after a streptococcal infection. The filtration membrane gets damaged.

Treatments for kidney failure:

• Hemodialysis: blood is pumped through a machine where it passes over a semi-permeable membrane against a clean dialysing solution. Wastes diffuse out. Usually 3 sessions per week in a hospital.
• Peritoneal dialysis: uses the patient own peritoneum as the filter. Dialysing fluid is put into the abdominal cavity and then drained. Can be done at home.
• Kidney transplantation: the long-term cure. The donor kidney is placed in the pelvic area of the recipient. Matching of donor and recipient is critical to avoid rejection.

Worked NEET Problems

Summary Cheat Sheet

• Ammonotelic: bony fish, aquatic amphibians (tadpole). Ureotelic: mammals, adult amphibians, sharks. Uricotelic: birds, reptiles, insects, land snails.
• Human excretory system: kidneys (pair), ureters (pair), urinary bladder, urethra.
• Kidney inside: cortex + medulla (pyramids, calyces, pelvis). Hilum = entry of renal artery, exit of renal vein and ureter.
• Nephron: structural and functional unit. 1 to 1.5 million per kidney. Cortical (85%) and juxtamedullary (15%).
• Nephron parts: Glomerulus → Bowman capsule → PCT → Loop of Henle → DCT → Collecting duct.
• Renal corpuscle (Malpighian body): glomerulus + Bowman capsule.
• Afferent arteriole wider than efferent = high glomerular pressure for filtration.
• Urine formation 3 steps: (1) Glomerular filtration, (2) Tubular reabsorption, (3) Tubular secretion.
• GFR: about 125 mL/min = 180 L/day. Final urine: 1 to 1.5 L/day. So about 99% is reabsorbed.
• PCT: reabsorbs 70 to 80% of filtrate, ALL glucose and amino acids, brush border for absorption.
• Loop of Henle: descending = water out (no salt); ascending = salt out (no water). Sets the medullary gradient.
• Counter-current mechanism: Loop of Henle + vasa recta build a salt gradient from 300 mOsm (cortex) to 1200 mOsm (medullary tip).
• DCT: fine ion control + tubular secretion (H+, K+, NH3).
• Collecting duct: ADH-controlled final water reabsorption. Concentrates urine.
• ADH: from posterior pituitary; more water reabsorbed; concentrated urine.
• Aldosterone: from adrenal cortex; more Na+ (and water) reabsorbed; more K+ excreted.
• Renin: from JGA when BP drops. Triggers angiotensin pathway, eventually raising BP.
• ANF: from heart atrium when BP high. Lowers BP by promoting Na+ and water excretion.
• Other excretory organs: lungs (CO2), liver (urea, bile pigments), skin (sweat).
• Disorders: uremia (urea in blood), renal calculi (calcium oxalate, 80%), glomerulonephritis.
• Treatments: hemodialysis, peritoneal dialysis, kidney transplant.

Next: use the interactive learning widgets to explore the nephron diagram, walk through the three steps of urine formation, and compare excretion modes across animal groups, or work through the 12+ NEET PYQs with full solutions. To time yourself, take the free 10-question mock test.