EXCRETORY SYSTEM

1. EXCRETORY SYSTEM

2. Under the segmental somites therefore segmented themselves into NEPHROTOMES In each nephrotome is a NEPHRON

3. NEPHRON consists of two layered cup of epithelium = RENAL CAPSULE (Bowman’s capsule). Capsule envelops a knot of capillaries = GLOMERULUS with an afferent arteriole and an efferent arteriole. Whole thing = RENAL CORPUSCLE. Rest is simple epithelial tube which varies among vertebrates - proximal tubule = absorptive; variable length intermediate tubule; distal tubule; collecting tubules; excretory duct.

4. Filtration mechanism increased by 2 arterioles (rather than ateriole and veinule) and efferent smaller so pressure increases. Diffusion distance short, capillaries fenestrated, filtration slits on podocytes. Larger molecules held back (< 60K m. wt) everything else goes through. Amino acids, water, sugars recovered, rest eliminated.

5. Podocyte feet form outer diffusion barrier

6. Recovery takes place in the tubules through selective resorption, leaving urine to be excreted. Resorption into PERITUBULAR CAPILLARIES Blood supply varies: A = efferent renal + renal portal vein; B = renal portal vein; C = efferent renal

7. Proximal and distal convoluted tubules have microvilli for absorption

8. Most vertebrates have the nephron described = internal glomeruli. The first few nephrons at anterior end of ammocoetes (larval lampreys) and some larval amphibians = external glomeruli. Filtrate goes into coelom and then into duct by cilia = PRIMITIVE (present in many larvae and invertebrates)

9. Elasmobranchs, primitive actinopterygians, many amphibians have connection to coelom Suggests an evolutionary sequence

10. Nephrons differentiate in rostral to caudal sequence – PRONEPHROS, MESONEPHROS, METANEPHROS. Proposed that the primitive condition is all pronephric and they all work = HOLONEPHROS (hypothetical ancestral kidney) This resembles larval hagfish, but they don’t all work.

11. PRONEPHROS – forms at rostral end. Variable number – amniotes 1 – 3, non-functional; anamniotes 12+, functional in larvae of hagfish, lampreys, bony fish, amphibians, present in adult hagfish. But when non-functional it initiates formation of archinephric duct.

12. MESONEPHROS – forms after a gap, segmentation gradually lost due to secondary tubules developing. Functions in embryos and larvae of all vertebrates. Forms a separate unit in amniotes.

13. OPISTHONEPHROS – where mesonephros doesn’t form a separate unit and runs to caudal end e.g. adult hagfish Primitive opisthonephros Advanced opisthonephros Fish, amphibians Lost segmentation, caudal part enlarges, cranial part used for sperm

14. METANEPHROS – amniote invention, mesonephros functions in embryo only. Ureteric bud extends from caudal archinephric duct, grows into caudal end of nephric ridge, branches, induces renal tubules. Ureteric bud becomes collecting tubules and uterer.

15. Ureteric bud separates from archinephric duct. In males archinephric duct is involved with sperm movement, in females it degenerates. In reptiles and birds they enter the CLOACA independently. Mammals have a separate hindgut and u-g system and ureteric bud enters bladder (from allantois)

16. What is excreted? – nitrogenous waste from deamination of amino acids. Each NH2 group makes 1 NH3 = ammonia, very toxic. NH3 very soluble in water, flushed out (freshwater teleosts). Converted into urea (less toxic) or uric acid (low solubility in water, precipitates out, discharged as a paste (birds). OSMOREGULATION Hagfish & marine cartilaginous fish have cells with the same inorganic salt content as sea = ISO-OSMOTIC. All others, salt content is less = HYPO-OSMOTIC Maintaining this balance = OSMOREGULATION – evolved as an adaptation to life in freshwater

17. Salt concentration FRESHWATER FISH Lots of dilute hypo-osmotic urine (NH3) Minimise water intake cells water freshwater Gills – major excretion Kidney – major osmoregulator

18. 1.Large renal corpuscle means more water in raw filtrate 2.Thick intermediate segments mean many cilia to drive filtrate through tubule (fast moving filtrate), which means less water will be reabsorbed

19. Salt concentration SALTWATER FISH Minimise water loss, drinks water, excretes excess salts Sea water water cells

20. 1. Reduce size of renal corpuscle means more water in raw filtrate 2. Lose intermediate segments 3. Slow moving filtrate N.B. eels & salmon

21. Salt concentration MARINE CHONDRICHTHYANS water Minimise water intake Lots of dilute hypo-osmotic urine (NH3) Sea water cells + urea – evolved a system of retaining urea & raise osmotic pressure

22. freshwater teleosts and sharks - copius dilute urine

23. REPTILES – don’t lose water!! – no cutaneous respiration, no more respiration than needed, keratinised skin. Uric acid excreted, renal corpuscles v small to reduce water absorption. BIRDS and MAMMALS – extra problems of high metabolism and endothermy so more waste, more water lost during respiration. Solution = more renal tubules (2-4 million in human), higher filtration pressures to clear more waste, evolution of special tubules.

24. 1.New loop of Henle 2.More tubules

25. Cortex, medulla, pyramids, loop of Henle, vasa recta

26. Everything below 60K m. wt gets filtered, then useful things reabsorbed by proximal and distal tubules. Loop of Henle and vasa recta form a counter current, ADH regulates water resorption

27. Length of loop of Henle depends on how much water to be retained – desert rodents v. long • Other ways of getting rid of salt in birds and mammals • salt glands • whale kidneys (human 1L sw – 1.35L urine; whale 1L sw – 0.65L urine)