Excretory System
There’s probably way more than needed in this... Oh well, Juyeon makes bad notes. 
2 key homeostatic processes: osmoregulation and excretion
Osmoregulation : how animals regulate solute concentrations and balance gain and loss of water
Excretion : how animals get rid of nitrogen-containing waste products of metabolism
Osmoregulation
balances uptake and loss of water and solutes to regulate chemical composition of its body fluids
water enters and leaves cells by osmosis
organisms in diff environments face diff problems maintaining proper composition of body fluids o Marine Vertebrates: ocean = dehydrating environment since it is hypertonic to organisms living in it (greater solute concentration)

Fish lose water through gills and skin to environment

To counteract problem, they produce little urine and drink a lot of water = excess salt taken in with seawater is transported out through gills o Freshwater organisms: their osmoregulation problems are opposite of those in salt water

Environment = hypotonic to organisms, so they constantly lose salt and gain water

Freshwater fish excrete a lot of dilute urine and uptake salts by active transport o Terrestrial organisms: need to rid metabolic wastes but retain as much water as possible

Hence, diff excretory mechanisms evolved in various organisms for purpose of osmoregulation and removal of metabolic wastes
Excretion
Removal of metabolic wastes, including CO2 and water from cell respiration and nitrogenous wastes from protein metabolism o Proteins and nucleic acids are broken apart for energy or converted to carbohydrates or fats when enzymes remove nitrogen in form of ammonia (NH3) o
Some animals excrete ammonia directly, but many convert it to other less toxic compounds that require energy (ATP) to produce
3 nitrogenous wastes: ammonia, urea, and uric acid o Ammonia: very water soluble, but highly toxic

Excreted by most aquatic animals, including most bony fishes o Urea: not as toxic as urea

Mammals, most amphibians, sharks, and some bony fish excrete it

In mammals, it is formed in liver from ammonia

Animals need to use up energy to form Urea o Uric acid: paste like substance that isn’t soluble in water, hence not very toxic

Excreted by insects, many reptiles, and birds

Minimum of water loss by excreting uric acid
Excretory processes o Excretory systems are diverse, but nearly all produce fluid waste urine in process involving several steps

Filtration : Body fluid (blood, coelomic fluid, or hemolymph) is collected involving filtration through selectively permeable membranes. Hydrostatic pressure (blood pressure) forces water and small solutes into excretory system.
Fluid = filtrate.

Selective Reabsorption : excretory systems use active transport to reabsorb valuable solutes from the filtrate and returns them to body fluids.

Secretion : Other substances like toxins or excess ions are extracted from body fluids and added to filtrate using active transport

Excretion : filtrate leaves system and body as urine
Non-human excretory systems… o Platyhelminthes (planaria <3): protonephridium (flame cells)


Network of dead-end tubules lacking internal openings

Functions mainly for osmoregulation o Earthworms: metanephridia (nephridia)

Has internal openings that collect body fluids

Have excretory and osmoregulatory functions o Insects: Malpighian tubules

Remove nitrogenous wastes and function in osmoregulation
-
Human excretory system (aka the important one…) o Centers on kidneys = principal site of water balance and salt regulation

Supplied with blood by renal artery and drained by renal vein

Urine
 ureter
 urinary bladder

urination
 urethra

2 distinct regions: renal cortex and renal medulla

Nephron : functional unit of vertebrate kidney

Consists of single long tubule and a ball of capillaries ( glomerulus )

End of tubule forms a cup-shaped swelling =
Bowman’s capsule
, which surrounds the glomerulus o Basic pathway: blood in glomerulus

bowman’s capsule

filtrate

proximal tubule

loop of Henle

distal tubule

collecting tubule

renal pelvis o NOW MOR INDEPTH…. ALALALALLA…. o After everything is filtered in the bowman’s capsule filtrate goes to….

Proximal tubule

Secretion and reabsorption in proximal tubule substantially alter volume and composition of filtrate

Most of the NaCl and water from initial filtrate volume is reabsorbed into body fluid here

Descending limb of loop of Henle

Reabsorption of water continues

Freely permeable to water but not very permeable to salt and other small solutes

Makes filtrate increasingly more concentrated

Ascending limb of loop of Henle

Permeable to salt but not water

2 specialized regions o A thin segment near loop tip

NaCl diffuses out into interstitial fluid o A thick segment adjacent to distal tubule

NaCl is ACTIVELY transported into interstitial fluid

Filtrate becomes progressively diluted

Distal Tubule

Like proximal tube, important for secretion and reabsorption

Collecting duct


Collecting duct carries filtrate through medulla to renal pelvis
Tissue surrounding it is high in salt concentration

Additional water removal is dependent on presence of antidiuretic hormone (ADH) o If present: walls of collecting duct become permeable to water, and filtrate becomes hypertonic o If not, walls remain impermeable to water and urine remains hypotonic o ADH is released into blood when salt concentration becomes too high in blood o
If blood concentration is too dilute, ADH production is cut down and more urine is excreted o Alcohol blocks release of ADH, so more urination and dehydration o
ADH is produced in hypothalamus of brain and is stored and released by posterior pituitary