Nematode's exhibit Renette glands near their pharynx that control salt excretion/water levels. This is a more sophisticated system than that found in
Platyhelminthes. As such, nematodes are active osmoregulators. Since nematodes are aquatic animals, they continually excrete ammonia via cellular diffusion into the environment as it is produced.

Echinoderms have no excretory organs.
​metabolic waste is eliminated through their tube feet

Mollusks also eliminate metabolic wastes via nephridia. The waste is then passed out through the mantle cavity.

pathologies

how is excretion different from digestion/egestion?

tubular secretion of K+ determined by:
K+ concentration in ECF
low pH...H+ is secreted instead of K+ in Na+ pump
amount of aldosterone released (affects NA+/K+ pump 

pH (acid/base) balance

annelida

unicellular organisms expel metabolic waste through diffusion and exocytosis

excretion and osmoregulation

flame cells = protonephridia

Malpighian tubules of insects/terrestrial arthropods remove nitrogenous wastes and other solutes from the animals body fluids. Wastes are actively transported into the lumen of the tubules causing water then enters the tubules via osmosis, forming urine. The urine passes through the intestine and is eliminated with the feces. 
​Terrestrial arthropods excrete waste that's nearly dry which is an excellent adaptation to life on land.
​Crustaceans and arachnids possess paired excretory organs (maxillary, antennal, or coxal glands) that open at the bases of certain appendages.

let's look at how different organisms accomplish this feat...

Annelids excrete their nitrogenous waste in the form of ammonia. In Annelida small coiled tubes called nephridia are found in each segment. Nephridia open to the coelom. The cells of the Nephridia filter out ammonia from the coelomic fluid.
Nephridia also remove excess of water from the body thereby regulating the osmotic pressure. The excretory fluid flows through a central canal, collected in 
a terminal vesicle and released from the body via a nephridiopore. 

echinodermata

All chordates do not possess kidneys as excretory organs.

Urinary Tract Infections (UTIs) are pretty common infections that happen when bacteria (likely from the skin or rectum) enter the urethra and infect the urinary tract. The most common type is a bladder infection (cystitis). Kidney infection (pyelonephritis) is another type of UTI. They’re less common, but more serious than bladder infections.

arthropods

protein catabolism:
nitrogenous wastes 

multicellular organisms rely on specialized cells/tissues/organs

mollusks

Urinary Tract Infections (UTI)

platyhelminthes

 H2O!!

-> H2O!!

Nephridia are more evolved than a flame cell of a flatworm in that it has a system for reabsorption of some useful waste products, such as metabolites and ions, by a capillary network before excretion. 

Dialysis

the small pores allows nitrogenous wastes to exit the body with a
​flush of water. 

                                                     Homeostasis requires fluid/electrolyte/pH balance

Evolution ​of
Excretion

nitrogen wastes are a product of protein metabolism. Amino (NH2) groups are removed from amino acids. The NH2 combines with H to form NH3 = Ammonia.   

excretion
​ occurs when waste products produced during metabolic processes are eliminated
from the body

buffer systems:  
carbonic acid
amino acid
hemoglobin
​phosphate 

Polycystic kidney disease (PKD) is a genetic disease. PKD causes cysts to grow inside the kidneys. These cysts make the kidneys much larger than they should be and damage the tissue that the kidneys are made of. PKD causes chronic kidney disease (CKD) , which can lead to kidney failure and end-stage renal disease (ESRD). PKD causes about 2 percent (2 out of every 100) of the cases of kidney failure in the United States each year.

There are two types of PKD: autosomal dominant PKD and autosomal recessive PKD. Autosomal dominant PKD causes cysts only in the kidneys. It is often called “adult PKD,” because people with this type of PKD might not notice any symptoms until they are between 30 and 50 years old. Autosomal recessive PKD causes cysts to grow in both the kidneys and the liver. Autosomal recessive PKD is often called infantile PKD because babies can show signs of the disease in their first few months of life.

transplanation

porifera

      Calcium

Chronic Kidney Disease

what are
​metabolic wastes?

nematodes

planarians exhibit a highly-branched duct system that releases wastes via 
pores located all along the sides of the worm's body. Flame cells remove waste products via filtration. Cilia move the waste along tubules to the excretory pores.
flame cells get their name because their
cilia looks like a flickering flame under the microscope .

buffer systems

dehydration synthesis:
 H2O 

chordates

      and the rest...

its a balancing act...

As animals increased in size and complexity, the logistics of waste removal proved problematic. Relatively simple nephridia seen in invertebrate animals morphed into the complex vertebrate kidney but the purpose remained the same...excretion of metabolic wastes and osmoregulation. Animal excretory organs are mostly
variations on a common theme.

porifera exhibits  intracellular digestion 

The excretion of wastes and osmoregulation are for the most part carried out by by kidneys. Tunicates and cephalochordates body fluids have a NaCl concentration that's pretty much the same as seawater. Vertebrates have body fluids with a lower salt content (except that pesky hagfish!) which suggests  that
vertebrates evolved in fresh water.

the mammalian kidney's ability to conserve water is a key evolutionary adaptation 

                                                        Water
 average production via cellular respiration = 300 ml H2O/day
 average loss of water via excretion/perspiration = 2500 ml H2O/day
​    (one degree of fever increases H2O loss by 200 ml/day...
                           drink plenty of water when you are sick)
                                     Dehydration = too little
                               Hyperhydration  = too much
   Fluid shifts occur between ICF and ECF via osmosis to maintain                                                           homeostasis

nephrós = kidney

                                                   Electrolytes:
 concentration measured in Equivalents = amount of +/- ions that produces                                                                                         1 mole of electric charge
                                                                             1 Eq = 1000 milli eq (mEq)
 most common problem = imbalance in gain/loss of Na+
 imbalance of K+ less common but more dangerous
​ other electrolytes: Ca2+, Mg2+,PO4 3-, Cl- 

PKD

the human excretory/urinary system

     Sodium

cellular respiration:
CO2 and H2O 

cnidarians

                      Some (kinda common sense) key points:
*chemoreceptors monitor the ECF for changes in plasma volume or 
          composition/osmotic concentration of ECF, not the ICF
*water travels via osmosis following the ion gradient (water follows salt)
*excess water/electrolytes must be released and insufficient water/electrolytes
          must be ingested to maintain homeostasis 

    Potassium

Kidney stones and oxalates:  
Oxalates are found in many foods. They bind to calcium during digestion in the stomach and intestines and leave the body in stool. Oxalate that is not bound to calcium travels as a waste product from the blood to the kidneys where it leaves the body in the urine. There are many different types of kidney stones, but 8 out of 10 stones are calcium oxalate stones. If there is too much oxalate and too little liquid in the urine, calcium oxalate fragments create crystals. As the crystals begin to increase in number, they stick to one another to form a larger crystal known as a kidney stone.