1. 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

2. 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)

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

11. 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

13. 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

15. Formation of Urine Occurs in three steps:
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

16. 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

18. Secretion – last step of filtration
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
Secretion – last step of filtration
Substances move from capillaries surrounding nephron into urine (H+, K+, ammonia, certain drugs)

20. 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

21. 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