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EXCRETORY SYSTEM

EXCRETORY SYSTEM. Under the segmental somites therefore segmented themselves into NEPHROTOMES In each nephrotome is a NEPHRON ARCHINEPHRIC DUCT joins the nephrons together. NEPHRON consists of two layered cup of epithelium = RENAL CAPSULE (Bowman’s capsule).

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EXCRETORY SYSTEM

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  1. EXCRETORY SYSTEM

  2. Under the segmental somites therefore segmented themselves into NEPHROTOMES In each nephrotome is a NEPHRON ARCHINEPHRIC DUCT joins the nephrons together

  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. Proximal and distal convoluted tubules have microvilli for absorption

  7. 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

  8. 3 types of nephron arrangements 1. 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. 2. Elasmobranchs, primitive actinopterygians, many amphibians have connection to coelom 3. Most vertebrates have the nephron described = internal glomeruli 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)

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