the excretory system regulation of the osmotic and ionic composition of intracellular fluids and...
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The Excretory System • Regulation of the osmotic and ionic
composition of intracellular fluids and extracellular fluids is critical to maintain homeostasis
– Osmoregulation – active regulation of osmotic pressure of body fluids to keep them from becoming too dilute or too concentrated
– Excretion – process of ridding the body of metabolic wastes and excess water
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• Metabolic wastes include water, carbon dioxide, and nitrogenous wastes
– Carbon dioxide is excreted mostly by respiratory system
– Excretory organs (such as kidneys) remove most of nitrogenous wastes and excess water
– Nitrogenous wastes include ammonia, uric acid, and urea
• during the breakdown of amino acids, ammonia is produced (deamination – process of removing amino group – produces ammonia)
• ammonia is highly toxic – converted into less toxic urea (urea is produced in liver in mammals and amphibians) or uric acid (insects, many reptiles and birds)
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• Osmoregulation in marine invertebrates– most release wastes across general surface
membranes– water balance is not a problem since most are
isotonic with sea water (osmoconformers)– Organisms living in coastal environments
must survive fluctuating conditions (fresh water) – osmoregulators – maintain optimal salt concentrations regardless of changes in environment
• example: shore crab – body fluids are hypertonic to brackish water (mixture of salt and fresh water) – gills remove salts from water and put into blood while excretory organs excrete excess water that diffuses in
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• Freshwater Animals – body fluids are hypertonic to fresh water – must deal with constant influx of water and loss of salts
– Aquatic mammals prevent this by having an impermeable barrier (skin, fur) – possible because breathe air
– Fully aquatic animals must remain permeable for gas exchange – excess water is eliminated thru very dilute and copious urine produced by kidneys and special cells in gills absorb salts
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• Marine Vertebrates – hypotonic to seawater – results in excessive water loss, excessive salt intake
– Drink water continuously, special cells in gills remove excess salt
– Nitrogenous wastes removed through gills (ammonia) – very little urine is produced
– Sharks – solve problem by retaining urea in blood – keeps blood slightly higher conc. than sea – salts excreted by special cells in rectum
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• Terrestrial Animals – biggest problem is dessication
– replace water by drinking, food and products of cellular respiration
– metabolic wastes (ammonia) harder to get rid of – ammonia is quickly converted to urea (much less toxic)
– Urea is very soluble and must be released in a watery solution
– Reptiles, birds, and insects excrete uric acid instead (very insoluble) – conserves water
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• Excretory mechanisms in Animals– Contractile Vacuoles in protozoa – fill w/water
– contracts to eject water from cell – primarily used for elimination of excess water and some wastes
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– Flame cell systems – flatworms• beginning of tubular excretory system• tubules run length of body – open to outside of body
thru tiny pores• bulblike structures at end of tubules remove water
from tissue with help of cilia – travels down tubules to pores
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– Nephridia of earthworms – excretory organ• closed circulatory system – blood vessels
have become associated with excretory organs
• nephridium consists of nephrostome (open ciliated funnel), a coiled tubule connecting nephrostome to a bladder and a nephridiopore (mat’ls pass to outside)
blood capillaries surround tubule – mat’ls move into nephrostome – also picked up directly from blood by coiled tubule
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The Vertebrate Kidney• Closely associated with the circulatory system• Kidney structure:
– Cortex – outer part of kidney– Medulla – inner portion– Renal pelvis – center cavity
• Associated structures:– Ureters – carry urine from each renal pelvis to bladder– Urinary bladder – stores urine– Urethra – tube that carries urine from the body– Renal artery – carries blood to kidney from aorta– Renal vein – carries blood away from kidney to inferior
vena cava
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Kidneys are made up of microscopic nephrons – functional units
• Bowman’s capsule – cup shaped top of nephron• Glomerulus – network of blood capillaries tucked
into Bowman’s capsule• Proximal convoluted tubule – 1st segment of the
renal tubule closest to Bowman’s capsule• Loop of Henle – extension of tubule that reaches
down into medulla• Distal convoluted tubule – part of tubule distal (far)
from Bowman’s capsule• Collecting tubule (duct) – collects and sends urine
to renal pelvis
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Formation of Urine
Occurs in three steps:
1. Filtration – blood pressure forces fluid from blood in glomerulus into Bowman’s capsule
• Water and dissolved substances enter capsule
• Filtrate – contains salt, glucose, vitamins, nitrogenous wastes, and other small molecules
• Blood cells and plasma proteins are too big and stay behind in blood
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2. Reabsorption – substances move out of renal tubule back into blood in capillaries surrounding nephron
• Occurs in proximal conv. tubule, Loop of Henle, distal conv. tubule, and collecting duct
• 100% of glucose is reabsorbed in proximal conv. Tubule
• Descending branch of Loop of Henle is permeable to water but not very permeable to salts
• Ascending branch of Loop of Henle is NOT permeable to water - pumps actively transport salts into medullar tissue
• Tissue surrounding Loop of Henle becomes concentrated causing more water to diffuse out by osmosis at the descending portion – counter current multiplier system
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• Filtrate entering distal conv. tubule is actually less concentrated (amount of salts depends on salt intake)
• Collecting tubule is permeable to water and osmosis occurs into surrounding hypertonic medullar tissues – urine becomes more concentrated
• Amount of water leaving with urine depends on water levels in body
3. Secretion – last step of filtration• Substances move from capillaries surrounding
nephron into urine (H+, K+, ammonia, certain drugs)
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Hormonal control of Kidneys• Hypothalamus controls kidneys – monitors
amount of water in body fluids• Makes a hormone called antidiuretic hormone
(ADH) – stored in posterior pituitary• When water levels are too low, hypothalamus tells
pituitary to release stored ADH• ADH causes collecting duct to become more
permeable – more water passes out of collecting duct back into blood – conserves water, urine is more concentrated
• When water levels are too high, hypothalamus signals pituitary to release less ADH – collecting ducts become less permeable to water – more passes out with urine
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Release of Urine• Urine is carried by ureters to urinary bladder
– Elastic muscular bag – can greatly expand– Special stretch receptors send messages to
brain
Urethra carries urine away from bladder and out of body – closed off by sphincter muscle
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