Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 44.4 The Nephron is Organized for Stepwise Processing of Blood Filtrate
The nephron is the functional unit of the kidney, processing blood to form urine through filtration, reabsorption, secretion, and excretion. The kidney’s highly organized structure and osmotic gradients enable it to conserve water while eliminating waste, playing a key role in homeostasis.
1. Nephron Structure and Blood Flow
- Each nephron consists of a tubule and a glomerulus (ball of capillaries)
- Bowman’s capsule surrounds the glomerulus and collects the filtrate
- Blood enters via the afferent arteriole, exits via the efferent arteriole, which forms peritubular capillaries and the vasa recta
- About 180 L of filtrate is formed daily; only ~1.5 L of urine is excreted after reabsorption
2. Filtrate Processing
- Filtrate passes through the proximal tubule, loop of Henle, distal tubule, and collecting duct
- Proximal tubule reabsorbs water, NaCl, glucose, amino acids, and HCO₃⁻; secretes H⁺, NH₃, drugs
- Loop of Henle:
- Descending limb: permeable to water → filtrate becomes concentrated
- Ascending limb: impermeable to water; salt is passively and actively removed → filtrate becomes dilute
3. Distal Tubule and Collecting Duct
- Distal tubule regulates Na⁺, K⁺, HCO₃⁻, and H⁺ to help balance ions and pH
- Collecting duct concentrates urine by adjusting permeability to water (via aquaporins) and urea
- Urea reabsorbed in inner medulla contributes to high osmolarity, aiding water reabsorption
4. Osmotic Gradient and Countercurrent Systems
- The loop of Henle and vasa recta create a countercurrent multiplier system, using active salt transport to generate a steep osmotic gradient
- This allows water to be reabsorbed in the collecting duct as filtrate passes through increasing osmolarity in the medulla
- NaCl and urea are the primary contributors to medullary osmolarity
- Kidney expends energy to maintain this gradient, especially in the thick ascending limb
5. Adaptations Across Vertebrates
- Desert mammals have many long-looped juxtamedullary nephrons → very concentrated urine
- Freshwater fish and amphibians: mostly cortical nephrons → dilute urine
- Marine bony fish: few small nephrons, reduced glomeruli, excrete divalent ions, conserve water
- Birds have some juxtamedullary nephrons but rely on uric acid for water conservation
- Reptiles: only cortical nephrons; reabsorb water from urine in cloaca
6. Vampire Bat Case Study
- While feeding, excretes large volumes of dilute urine to reduce weight
- In roost, produces very concentrated urine (up to 4,600 mOsm/L) to eliminate nitrogenous waste with minimal water loss
- This flexibility supports its unique high-protein, liquid diet and lifestyle
In a Nutshell
The mammalian nephron filters blood and reclaims useful solutes and water while concentrating waste into urine. Through specialized tubule regions, countercurrent exchange, and hormonal regulation, the kidney maintains water balance across diverse environments. Its structural variations reflect adaptations to freshwater, marine, and terrestrial challenges.