EXCRETORY SYSTEM

I. BASIC STRUCTURE

A. Origin = intermediate mesoderm in dorsal/posterior body wall.

1. Nephric ridge protrudes from body wall into coelom; runs from cranial to caudal.
2. Beginning at cranial end of ridge, paired nephric tubules develop segmentally:

a. Nephrostome = ciliated opening of tubule into coelom.
b. Tubule collects coelomic fluid, transports to lateral nephric duct, which extends caudally and ultimately connects to cloaca.
c. Glomerulus = knot of capillaries; fluids extruded through and filtered by walls of capillaries.

(1) External - protrudes into coelom.
(2) Internal - capillaries surrounded by expanded portion of tubule called renal capsule.

d. Tubule may modify fluid by selective reabsorption into peritubular capillaries, eg. glucose, Na+.
e. In higher vertebrates, nephric tubule may never establish direct opening (nephrostome) to coelom.
f. Differences in structure and development of tubules within nephric ridge help identify distinct anterior, middle, and posterior regions, which vary in position and types of connections to coelom.

B. Archinephros (holonephros) = hypothetical ancestral vertebrate kidney.

1. Derived from entire nephric ridge.
2. Segmentally arranged throughout length.
3. Large glomeruli (internal/external???); nephrostomes probably present.
4. Similar to form found in hagfishes & caecilians.

C. Pronephros - usually transient embryonic stage in vertebrates.

1. Anterior-most section of nephric ridge; first to appear.
2. Segmentally arranged.
3. Nephrostome present, except in mammals & birds.
4. Functional only in free-living larvae of bony fishes & amphibians.
5. Functional significance: pronephric duct extends and persists caudally to give rise to mesonephric duct.

D. Mesonephros - may persist as functional kidney in lower vertebrates.

1. Arises in middle region of nephric ridge; second to appear.
2. Begins to lose segmental character, due to branching of tubules.
3. Nephrostomes usually lost, may be retained in some sharks.
4. Mesonephric duct = caudal extension of pronephric duct.
5. Glomeruli usually internal.
6. Functional embryonic kidney in amphibians and amniotes.
7. Opisthonephros = extended mesonephric kidney with additional posterior tubules, functional in most adult fishes and amphibians.

E. Metanephros - kidney of adult amniotes.

1. Arises posterior to mesonephros, last to develop embryonically and phylogenetically.
2. More compact than mesonephros; no segmental character at all.
3. Glomeruli always internal, nephrostomes absent.
4. Dual origin:

a. Ureteric diverticulum = outgrowth of base of mesonephric duct; grows dorsally and cranially, stimulates growth of metanephric tubules in metanephric blastema. Outgrowth extends cranially to form metanephric duct (= ureter).
b. Metanephric blastema = posterior portion of nephric ridge, gives rise to body of kidney, consisting of tubules, glomerular capsules, etc.

III. TRENDS IN EVOLUTION

A. Loss of nephrostomes and external glomeruli.
B. Occurrence of more numerous and non-segmental nephrons.
C. Development from progressively more posterior and shorter part of nephric ridge.
D. Amniotes - long metanephric tubules with well-differentiated regions for reabsorption and concentration of urine. eg. Loop of Henle
E. Mammals and some birds: kidneys can produce urine with solutes more concentrated than in blood; corresponds to presence of loops in nephrons.

IV. FUNCTIONS

A. Osmoregulation = elimination of controlled amounts of H2O and salts.
B. Elimination of nitrogenous waste products of protein metabolism and other harmful substances; especially important for terrestrial vertebrates, not primary function in fishes.

1. Ammonotelism = direct excretion of ammonia. eg. fresh water teleosts, most marine fishes amphibians
2. Uricotelism = excretion of nitrogen in form of uric acid (poor solubility, combines with ions in cloaca to form precipitate, water can diffuse through walls of cloaca back into blood.
3. Ureotelism = excretion of nitrogen in form of urea; eliminated as concentrated urine, allowing elimination of waste and conservation of H2O. eg. mammals, elasmobranchs.