Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 44.3 Diverse Excretory Systems Are Variations on a Tubular Theme
Excretory systems maintain homeostasis by removing metabolic wastes and regulating fluid composition. Across the animal kingdom, these systems vary in form but share a common structure: networks of tubules that filter, reabsorb, secrete, and excrete substances.
1. General Process of Excretion
- Filtration: blood or body fluid is filtered through a transport epithelium; large molecules stay behind
- Reabsorption: valuable solutes like glucose, salts, and amino acids are reclaimed
- Secretion: nonessential solutes and toxins are added to the filtrate
- Excretion: processed filtrate (urine) is eliminated from the body
2. Protonephridia (Flatworms)
- Network of dead-end tubules capped by flame bulbs with beating cilia
- Cilia draw in interstitial fluid; filtrate exits via body openings
- Functions mainly in osmoregulation in freshwater species
- Found in planarians, rotifers, some annelids, mollusc larvae, and lancelets
3. Metanephridia (Earthworms)
- Each body segment has a pair of tubules collecting coelomic fluid
- Fluid enters through ciliated funnel, moves through a tubule, and exits the body
- Reabsorbs useful solutes and water, while nitrogenous waste is excreted
- Functions in both excretion and osmoregulation
4. Malpighian Tubules (Insects)
- Extend from hemolymph into digestive tract
- No filtration step—tubules secrete solutes and nitrogenous waste into lumen
- Water follows by osmosis; useful solutes and water are reabsorbed in rectum
- Waste (mainly uric acid) is excreted as dry paste with feces
- Highly efficient at conserving water—a key to insect success on land
5. Kidneys (Vertebrates)
- Compact organs with tubules and associated capillaries
- Perform both osmoregulation and excretion
- Nephrons are the functional units—about 1 million per human kidney
- Cortical nephrons stay in the outer cortex; juxtamedullary nephrons extend deep into the medulla and are crucial for producing concentrated urine
6. Kidney Structure Overview
- Blood enters via renal artery, exits via renal vein
- Filtrate forms in Bowman’s capsule from pressure-filtered blood
- It passes through proximal tubule, loop of Henle, distal tubule, and into a collecting duct
- Filtrate is modified through reabsorption and secretion
- Final urine exits via the ureter → bladder → urethra
In a Nutshell
All excretory systems share a core design: tubular structures that filter body fluids and adjust their composition. Whether it's flame bulbs in flatworms or nephrons in mammals, these systems balance solutes and water while removing metabolic waste. Vertebrate kidneys add complexity and efficiency, enabling precise regulation of body fluid chemistry.