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

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)

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

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

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

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

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

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

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

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

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

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

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

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)

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

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