Kidneys and Excretion (chapters 28 and 29)
I. Basic mechanisms of kidney function 716
1. Regulate the composition of the body fluids - water balance, electrolyte & acid-base balance,
blood pressure, pH of blood also affected by the kidney
2. Rids the body of the wastes of metabolism and removal of foreign chemicals, drugs, and food additives.
3. A minor endocrine organs – erythropoietin
C. Types of nitrogenous wastes
A. Protozoans and sponges- lack complex excretory organ
1. Contractile vacuoles excrete water and solutes
2. Depends on ambient osmotic conditions- contracts more frequently and expels more water in fresh H2O
B. Flatworms- branched longitudinal tubules in a hollow bulb
Cilia inside bulb create currents within cell (flame cells)- fluid and waste carried out through excretory pores.
Mollusks- have protonephridia or
1. Freshwater animals- nephridia produce a copious hypoosmotic urine- excrete water, conserve ions
2. In marine animals- urine- iso-osmotic with body fluid, vol. low to conserve water
D. Crustaceans- rely primarily on antennal and maxillary glands for solute excretion.
Urine is formed by filtration at the terminal coelom-sac, which has an arterial; blood supply.
E. Earthworms- closed circulatory system- excretion is carried out by nephridia.
1. Body fluid enter nephridium through the membrane of the bulb-like nephrostome, which is ciliated opening into the nephridium
2. The nephridium gives rise to a coiled tubule, which is closely associated with blood capillaries,
3. This allows reabsorption of material; the nephridium terminates in a large bladder that opens to the outside by nephridiopore
F. Insects- excretory structure- Malpighian tubules forms urine by active K+ secretion into the tubules,
water and solutes follow passively. 740
1. Tubules are outpocketing of the gut at the junction of the midgut and the hindgut.
2. These sacs are washed by the blood, fluids and salts are reabsorbed.
3. The urine formed moves into the hindgut and out of body through the rectum.
4 Both feces and urine exit through the anus where water is reabsorbed
III. Excretion in vertebrates
1. Freshwater fish have nephrons- large glomeruli-excrete excess water but conserve ions.
2. Marine fish- nephrons with very few glomeruli- may be absent
Amphibians- similar structure and
function to freshwater fish nephrons. 719-723
They produce copious dilute urine stored in large bladder.
1. Patterns of N excretion- diverse; some are ammonotelic when in water, ureotelic on land
2. Tadpole liver usually does not have high levels
of the urea cycle enzymes and the levels do not increase until metamorphosis
3. Two genera of tree frogs in Africa and S. America excrete uric acid
C. Reptiles- diverse - most are to some extent uricotelic
1. Nephrons adapted to minimize urinary water loss.
2. GFR - relatively high; most water, nutrients and solutes are reabsorbed
3. Do not have a bladder; urine drains from their ureters into the cloaca and is refluxed into the hindgut.
4. Urine is modified in the colon/cloaca by active solute uptake and passive water reabsorption
5. Nephrons are unable to form hyperosmotic urine
D. Birds- similar to reptiles
1. Kidneys not evolved to the effectiveness of the mammal's
2. Uric acid excreted
3. Salt glands in the head
A. Mechanisms: filtration, reabsorption, and secretion
B. Regulations: neural, hormonal, and autoregulation
C. The ability of the kidneys to concentrate urine: The counter-current multiplier system 729
V. Urine formation in other vertebrates 738
VI. Nitrogen disposition and excretion 744
VII. The dramatic adaptations of particular species 757
Body size and water costs. 750
Desert survival. 758