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:

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

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

• 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)

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