E. URINARY SYSTEM
1. Review the overall function of kidneys.
The kidneys function in osmoregulation and, in tetrapods, in the elimination of nitrogenous waste.
2. Name the different nitrogenous wastes eliminated by the tetrapods.
Ammonia (in amphibians), urea (in mammals), uric acid (in birds and reptiles)
3. Characterize the nephron. Name its parts and discuss the function of each part.
The nephron is the functional unit of the kidneys. Nephrons make urine by filtering blood. Nephrons consist of a glomerulus (a network of blood capillaries including afferent glomerular arteriole and efferent glomerular arteriole), Bowman’s capsule (double-walled cup-shaped outgrowth surrounding the glomerulus that receives filtrate), renal corpuscle (the glomerulus and its Bowman capsule functioning as filtration unit), renal tubule (function in secretion directly from blood and reabsorption from tubules back to blood), peritubular capillaries (supplied with blood from efferent glomerular arterioles and the renal portal system), common collecting tubule (a channel through which fluids pass from the secreting cells), and a longitudinal kidney duct (duct that carries urine to cloaca in all but placental mammals).
4. Define renal corpuscle.
A mass of arterial capillaries (glomerulus) enveloped in a capsule (Bowman’s capsule) and attached to a tubule in the kidney; functions as filtration unit
5. Address the functions of ADH (antidiuretic hormone) and aldosterone.
ADH acts to increase water absorption in the kidneys. This puts more water back in the blood, increasing the concentration of the urine. When too much fluid is present in the blood, sensors in the heart signal the hypothalamus to cause a reduction of the amounts of ADH in the blood. This increases the amount of water absorbed by the kidneys, producing large quantities of a more dilute urine.
Aldosterone, a hormone secreted by the kidneys, regulates the transfer of sodium from the nephron to the blood. When sodium levels in the blood fall, aldosterone is released into the blood, causing more sodium to pass from the nephron to the blood. This causes water to flow into the blood by osmosis.
6. Compare the pronephros, mesonephros, and metanephros.
Pronephros – anterior portion of the nephogenic mesoderm; pronephric tubules drain into pronephric duct which opens into cloaca
Mesonephros – nephrogenic mesoderm just posterior to pronephros; once the pronephric kidney regresses, the pronephric duct becomes the mesonephric duct; a mesonephros forms the adult kidney in fishes and amphibians; it is the functional embryonic kidney of amniotes; the vasa efferentia carries sperm from testicles to mesonephric duct; the spermatic duct (vas deferens) is the mesonephric duct in males
Metanephros – adult kidney of amniotes, formed from the posterior part of the nephrogenic mesodermm which becomes displaced anteriorly and laterally during development; includes a cortex (outer region of kidney) and a medulla (inner region); pelvis is a slit like funnel collection urine from the medulla; includes a ureter, or metanephric duct
7. Describe the vasa efferentia and vas deferens.
The vasa efferentia is any of a number of small ducts that carry semen from the testis to the epididymis. The vas deferens (=mesonephric duct=spermatic duct) is the main duct through which semen is carried from the epididymis to the ejaculatory duct.
8. Identify the cortex, medulla, pyramids, papillae, pelvis and ureter.
The cortex is the outer region of the kidney that houses the glomerulim convoluted tubules and the upper ends of the loops of Henle in mammals. The medulla is the inner region of the kidney that consists of loops and collecting tubules in mammals. Pyramids are aggregates of loops of Henle, vasa recta, and collecting tubules. Renal papilla exist at the point where each pyramid tapers to a point. Renal pelvis with extensions (calyces) is responsible for collecting urine from common collecting tubules. The ureter is the long, narrow duct that conveys urine from the kidney to the urinary bladder or cloaca.
9. Explain the advantage of a countercurrent system.
With two flows moving in opposite directions, the countercurrent exchange system is able to maintain a constant concentration gradient between the two flows over their entire length, and can result in almost all of the substance being transferred. It allows for efficiency.
10. Describe the urinary bladder amongst vertebrate classes. Discuss its function.
Urinary bladders are adaptations to terrestriality. They arise from the evagination of ventral wall of embryonic cloaca, except in placental mammals where it arises from the allantois, an embryonic structure. The bladder serves as water reservoirs in all vertebrates except for placental mammals. In placental mammals, it serves as a holding tank for urine and for pheromones. In most fish, mesonephric ducts form the bladder. In tetrapods, the cloaca derivative and the urinary bladder empties into the cloaca, except in placental mammals. In placental mammals, the urinary bladder dumps into the urogenital sinus or directly to body’s exterior via the urethra.
11. Define urachus and allantois.
Urachus – a cord of fibrous tissue extending from the bladder to the navel and constituting the functionless remnant of a part of the duct of the allantois of the embryo
Allantois – a membranous sac that develops from the posterior part of the alimentary canal in the embryos of mammals, birds, and reptiles. It is important in the formation of the umbilical cord and placenta in mammals. It gives rise to the urinary bladder in placental mammals.