Excretory Products and their Elimination

The One Thing

Excretion removes metabolic wastes while maintaining homeostasis. The nephron forms urine through filtration, selective reabsorption, and tubular secretion, with hormonal and counter-current mechanisms regulating its final composition.

Reactions, Processes and Experiments

Key Terms

• Excretion: The removal of metabolic waste products such as nitrogenous wastes, excess salts, water, and carbon dioxide from the body.
• Nitrogenous wastes: Waste substances formed during protein and nucleic acid metabolism, mainly ammonia, urea, and uric acid.
• Ammonotelism: Elimination of nitrogenous waste mainly as ammonia; it requires a large amount of water and occurs in many aquatic animals.
• Ureotelism: Elimination of nitrogenous waste mainly as urea; humans and most terrestrial mammals are ureotelic.
• Uricotelism: Elimination of nitrogenous waste mainly as uric acid; it conserves water and is common in birds, reptiles, and insects.
• Human urinary system: Two kidneys, two ureters, a urinary bladder, and a urethra.
• Kidney: A paired bean-shaped organ that filters blood, forms urine, and maintains water, electrolyte, acid-base, and osmotic balance.
• Nephron: The structural and functional unit of the kidney; each kidney contains about one million nephrons.
• Renal corpuscle: The filtration unit made of the glomerulus and Bowman's capsule.
• Glomerulus: A network of capillaries supplied by the afferent arteriole where blood filtration begins.
• Bowman's capsule: A cup-shaped structure surrounding the glomerulus that collects the glomerular filtrate.
• Renal tubule: The tubular part of a nephron, consisting of the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.
• Ultrafiltration: Pressure-driven filtration of water and small dissolved substances from glomerular blood into Bowman's capsule.
• Selective reabsorption: Movement of useful substances such as glucose, amino acids, ions, and water from the filtrate back into the blood.
• Tubular secretion: Active addition of substances such as hydrogen ions, potassium ions, ammonia, and certain drugs from blood into the renal tubule.
• Glomerular filtration rate: The volume of filtrate formed by both kidneys per minute; normally approximately 125 mL per minute or about 180 litres per day.
• Loop of Henle: A U-shaped nephron segment that helps create the concentration gradient needed for water conservation and concentrated urine formation.
• Counter-current mechanism: Coordinated functioning of the loop of Henle and vasa recta that maintains a medullary osmotic gradient and supports urine concentration.
• Antidiuretic hormone: A hormone released from the posterior pituitary that increases water reabsorption in the distal tubule and collecting duct.
• Renin-angiotensin-aldosterone system: A hormonal system that increases blood pressure and sodium reabsorption when kidney blood flow or blood pressure falls.
• Atrial natriuretic factor: A heart-released hormone that promotes sodium and water loss and helps reduce blood pressure.
• Micturition: Expulsion of stored urine from the urinary bladder through the urethra.
• Osmoregulation: Maintenance of the proper water and dissolved-salt balance in body fluids.
• Dialysis: An artificial method of removing wastes and excess fluid from the blood when the kidneys fail.
• Uremia: Accumulation of urea and other nitrogenous wastes in the blood because of severe kidney dysfunction.

Easily Confused

• Excretion and micturition: Excretion is the removal of metabolic wastes from the body; micturition is specifically the expulsion of stored urine through the urethra.
• Ultrafiltration and selective reabsorption: Ultrafiltration moves water and small solutes from glomerular blood into Bowman's capsule; selective reabsorption returns useful substances from the filtrate to the blood.
• Selective reabsorption and tubular secretion: Selective reabsorption removes useful substances from the filtrate; tubular secretion adds substances from the blood to the renal tubule.
• Ammonotelism, ureotelism, and uricotelism: These processes differ according to whether the principal nitrogenous waste is ammonia, urea, or uric acid.
• Afferent and efferent arterioles: The afferent arteriole supplies the glomerulus, whereas the efferent arteriole carries blood away; the afferent arteriole is generally wider, helping maintain high glomerular pressure.
• Descending and ascending limbs of the loop of Henle: The descending limb is more permeable to water; the ascending limb is relatively impermeable to water and transports ions.
• Antidiuretic hormone and atrial natriuretic factor: Antidiuretic hormone promotes water reabsorption and concentrated urine, whereas atrial natriuretic factor promotes sodium and water loss and reduces blood pressure.
• Uremia and renal failure: Renal failure is severe loss of kidney function; uremia is the resulting accumulation of urea and other nitrogenous wastes in the blood.
• Urine and glomerular filtrate: Glomerular filtrate is the initial fluid formed by filtration; urine is the final fluid after reabsorption and secretion.

What Gets Asked

• Identify the components and functions of the human urinary system: state that it consists of two kidneys, two ureters, a urinary bladder, and a urethra, and trace urine from the kidney to the bladder through the ureters.
• Describe nephron structure and urine formation: include the renal corpuscle, renal tubule, glomerular filtration, selective reabsorption, and tubular secretion; omitting any of these processes loses marks.
• Explain the pressure conditions for filtration: state that the afferent arteriole is generally wider than the efferent arteriole, producing high pressure in the glomerulus, and identify the filtration membrane as the capillary endothelium, basement membrane, and inner lining of Bowman's capsule.
• Compare filtrate and urine: glomerular filtrate normally lacks blood cells and significant plasma proteins, whereas normal urine contains water, urea, uric acid, creatinine, ions, and other dissolved wastes but normally lacks glucose and significant proteins.
• Explain concentration of urine: distinguish the water-permeable descending limb from the relatively water-impermeable, ion-transporting ascending limb, and relate the loop of Henle and vasa recta to the counter-current mechanism and the approximately 1200 mOsmol per litre medullary gradient.
• Explain hormonal and clinical regulation: describe antidiuretic hormone, the renin-angiotensin-aldosterone system, atrial natriuretic factor, dialysis, and uremia; a common error is to state that dialysis replaces selective reabsorption rather than removing urea, excess salts, and excess water through a selectively permeable membrane.