WEEK 15 HANDOUT FOR VERTEBRATE ZOOLOGY: UROGENITAL SYSTEM 5/2/00
Text (7th ed.): Ch. 7 Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
possible web site: http://www.meddean.luc.edu/lumen/MedEd/urology/nlrendev.htm
(normal and abnormal kidney development in humans)
HVCC video material: VT 0937,"I am Joe's Kidney"
The primary function of the mammalian kidney is to regulate the fluid in the body by forming urine, a modified filtrate of plasma, through its functional unit, the nephron.
excretion: the elimination of metabolic waste products from an animal.
osmoregulation: maintainence of proper internal salt and water concentrations in an animal.
|
hyperosmotic: |
osmotic concentration of solution higher than that of cells, e.g., salt water |
|
isoosmotic: |
osmotic concentration of solution the same as that of cells, e.g., IV solution |
|
hypoosmotic: |
osmotic concentration of solution less than that of cells, e.g., distilled H2O |
Vertebrates for the most part are not osmoconforming with their environment and so either have to conserve water (e.g., living in desert, living in salt water) or eliminate excess water (living in fresh water). Sharks are osmoconforming by allowing the accumulation of urea in their tissues, but still have a rectal gland to get rid of accumulated salt ions.2
The kidney, as you remember, is a paired organ that is dorsal and retroperitoneal in the abdominal cavity. Kidneys have a direct connection to the dorsal aorta via renal arteries, which makes sense in light of the filtration mentioned above.
Hence, looking at a modern, metanephric kidney in a mammal, we see a bean-shaped organ in a protective renal capsule, with an outer cortex, an inner medulla, and interiorly a pelvis that drains into a ureter, which drains into the muscular, distensible bladder for urine storage.
The functional units or tubules of a kidney are called the nephrons. A glomerular capsule (Bowman's capsule) starts the process in the cortex of the kidney. A glomerulus of capillaries in the capsule are the actual site of a filtration process (note that the blood from the glomerulus continues into peritubular capillaries before returning back to the renal vein).
3 main functions of a kidney:
|
filtration: |
blood passed through a filter with some pressure, so that small molecules, including waste molecules, are passed through it. |
|
reabsorption: |
fluid certain ions and molecules are taken back into the bloodstream by the kidney. |
|
secretion: |
addition of certain (usually waste) products into the filtrate (forming urine). |
These all add up to excretion, or passing of the urine out of the body.
In the nephron tubule, filtration occurs from glomeruli into Bowman's capsule. The filtrate passes from Bowman's capsule through the PCT (proximal convoluted tubule), the loop of the nephron (loop of Henle), the DCT (distal convoluted tubule), before reaching a collecting duct. Now, just dumping out the filtrate would be a waste (literally) and would not address issues of osmoregulation.
Hence, water and selected ions and other items are reabsorbed before the urine collects in the pelvis of the kidney and off into the ureters. This reabsorption most typically involves the use of energy to reclaim sodium (and other items tagging along with it) via Na+/K+-ATPases (sodium pump)--remember this structure from our discussion of the nervous system? 2/3 of ion and fluid reabsorption occurs in the PCT.
The loop of Henle (loop of the nephron) dips into the medulla. In the descending loop, the walls are permeable to water, and in the ascending loop, the walls are permeable to salts. As the loop reclimbs up to the DCT, sodium is actively pumped out into extracellular space. This saltiness in the extracellular space of the medulla keeps increasing as more fluid goes through the loop, and hence, helps to draw water out of the filtrate in the loop. This water then can be retaken up by the peritubular capillaries and brought back into the general bloodstream. Meanwhile, the material in the loop of Henle makes it through the loop to the DCT.
In the DCT, the substance aldosterone, working through sodium pumps, enhances the reabsorption of sodium and the excretion of potassium. Finally, the fluid in the nephron reaches a collecting duct. The walls of the collecting duct are made permeable to water by ADH (anti-diuretic hormone), which helps to make the urine concentrated as it leaves the collecting duct into the ureters.
The length of the loop of Henle (and of the medulla) therefore correlates with the concentration of urine. A freshwater frog may have little or no loop, while a desert kangaroo rat's may be extra long.2
UROGENITAL DEVELOPMENT: INTRODUCTION TO REPRODUCTIVE STRUCTURES
Just as development of other organs hints at the past of vertebrates, looking at development of the kidney as well as the kinds of kidneys in a variety of extant vertebrate groups can give some insight into the evolution of the kidney. The ancient pattern was an archinephric kidney (archinephros), where renal tubules were connected to a long archinephric duct (this ancient pattern is still seen in hagfish embryos).2 Anterior to posterior, the vertebrate kidney we typically see can be thought of as three units in a developing nephric ridge of tissue (think of this as both developmental and evolutionary stages):2
In the anterior portion of the nephric ridge, the segmented pronephros is usually a short-lived stage in vertebrate development, where the filtering tubules are very exposed to the body cavity and drain into the archinephric duct. Only in such primitive fishes as the hagfish is the pronephros seen as an adult kidney.
In the middle of the developmental urogenital ridge, the less segmented (but longer) mesonephros incorporates additional tubules. In addition, the testis taps into the mesonephric duct. In many fishes and amphibians of today, this makes up the bulk of the adult kidney.
Finally, developing in the posterior of the ridge, the metanephros includes sophisticated tubules with their own new drainage duct or ureter. This is the setup seen in reptiles, birds, and mammals. The duct of the mesonephros (Wolffian duct) in males gets turned into the separate vas (ductus) deferens (does this help to reinforce the concept of the urogenital system?) The fallopian tubes form from another embryological set of tubing, the paramesonephric or Müllerian ducts.1
1 M Cartmill, et al., Human Structure (Harvard U. Press, Cambridge, MA, 1987), p. 164
2 CP Hickman Jr, et al., Biology of Animals, 7th ed., (WCB McGraw-Hill, Boston, 1998), pp. 170, 173, 174, 179.
|main page| |background| |03028: Physiology| |03048: Anatomy|
|03050: Invertebrate Zoology| |03051: Vertebrate Zoology| |03074: Economic Botany|
Please send comments and questions to: cronewil@hvcc.edu
Copyright 1999, 2000 by Wilson Crone
External and unofficial links are not endorsed by Hudson Valley Community College
This web page last updated on April 24, 2000