CHAPTER12 SUMMARY
•Excretion is crucial to internal fluid homeostasis in the following ways:
(1) Maintenance of proper internal levels of inorganic solutes (Na +, K+, Cl-, H+, CO2, etc.); (2)
Maintenance of proper water volume in body fluids, important for proper circulatory-fluid
pressure and cell hydration; (3) removal of non-nutritive and harmful substances resulting from
metabolism or ingestion without losing useful organic molecules; (4) maintenance of osmotic
balance (which results from a combination of water and dissolved solutes). Four types of organ
systems have evolved to regulate excretion:
•Respiratory systems (gills and lungs) regulate CO2. Gills can remove other solutes such as
ammonia, and can help regulate HCO3-.
•Digestive systems remove not only undigested food but also some end-products of internal
metabolism such as bilirubin from the vertebrate liver. Intestinal tracts may also help regulate
ions and water.
•Integument (skin) and glands may excrete some organic wastes as secondary functions (such
as sweat glands in humans), or as primary functions to excrete excess inorganic ions (salt
glands of brine shrimp and marine birds and reptiles).
•Renal organs (e.g., crustacean antennal glands, insect Malpighian tubules, and vertebrate
kidneys) have tubules that filter body fluids and regulate water, ions and many organic
substances, and then selectively reabsorb or secrete these molecules. The output of renal
organs is usually called urine.
•Nitrogen metabolism creates special stresses and produces three major end products:
•Ammonia is the primary nitrogenous product of protein breakdown. It is highly toxic and
requires considerable water to flush out. Thus it is the primary nitrogenous waste only in most
aquatic animals
•Urea (formed from two ammonias and one carbon dioxide) costs energy to make, but is less
toxic and requires less water to flush out. It is the primary nitrogenous waste in mammals,
adult amphibians, and some reptiles. It is also a major osmolyte in cartilaginous fishes.
•Uric acid (formed from purine metabolism, containing 4 nitrogens) is the most costly
nitrogenous waste, but is non-toxic and may be beneficial as an antioxidant. It is the primary
nitrogenous waste in birds, most reptiles, and insects.
•Renal excretory organs use up to four processes to produce urine: (1) Filtration, in which water
and small solutes pass through a sieve-like boundary. Filtration can be driven by either
hydrostatic or osmotic pressure differences across the boundary. Hydrostatically driven filtration
is also called ultrafiltration; (2) Secretion, in which specific solutes are transported into the tubule
lumen for excretion; (3), in which specific solutes and water may be transported back into the
body from the lumen; (4) Osmoconcentration, in which water is removed from the lumen fluid
while leaving solutes behind, producing an excretion more concentrated than body fluids and
thus saving water. Tubule functions are studied in isolated perfused tubules. Types are:
•Protonephridia (in rotifers, flatworms, larval annelids and larval mollusks), which use
ultrafiltration driven by cilia, secretion, and reabsorption
•Metanephridia (in adult mollusks, crustaceans, and vertebrates), which use ultrafiltration
typically driven by blood pressure, secretion, reabsorption, and sometimes osmoconcentration.
The mammalian kidney is a methanephridial organ.
•Malpighian tubules and hindguts (in terrestrial arthropods), which use filtration driven by
osmosis, secretion, reabsorption, and sometimes osmoconcentration
Mammalian Urinary System
The urine-forming functional unit of the kidneys is the nephron, which is composed of
interrelated vascular and tubular components.
•The vascular component consists of two capillary networks in series, the first being the
glomerulus, a tuft of capillaries that filters large volumes of protein-free plasma into the
tubular component.
•The second capillary network consists of the peritubular capillaries, which wind around the
tubular component. The peritubular capillaries nourish the renal tissue and participate in
exchanges between the tubular fluid and plasma.
•The tubular component begins with Bowman’s capsule, which cups around the glomerulus to
catch the filtrate, then continues a specific tortuous course to ultimately empty into the renal
pelvis. As the filtrate passes through the various regions of the tubule, the cells lining the
tubules modify it, adding some unwanted ECF constituents, and returning to the plasma only
those materials necessary for maintaining the proper extracellular fluid composition and
volume. What is left behind in the tubules is excreted as urine.
•The kidneys perform four basic processes in carrying out their regulatory and excretory
functions: (1) glomerular filtration, the nondiscriminating movement of protein-free plasma from
the blood into the tubules; (2) tubular reabsorption, the selective transfer of specific constituents
in the filtrate back into the blood of the peritubular capillaries; (3) tubular secretion, the highly
specific movement of selected substances from the peritubular capillary blood into the tubular
fluid; and (4) osmoconcentration, producing a concentrated extracellular fluid and urine.
Everything that is filtered or secreted but not reabsorbed is excreted as urine.
Glomerular Filtration
•Glomerular filtrate is produced as a portion of the plasma flowing through each glomerulus is
passively forced under pressure through the glomerular membrane into the lumen of the
underlying Bowman’s capsule. The net filtration pressure that induces filtration is caused by an
imbalance in the physical forces acting across the glomerular membrane. A high glomerular
capillary blood pressure favoring filtration outweighs the combined opposing forces of plasmacolloid osmotic pressure and Bowman’s capsule hydrostatic pressure.
•Of the plasma flowing through the kidneys, normally 20% is filtered through the glomeruli,
producing an average glomerular filtration rate (GFR) of 125 ml/min in humans. This filtrate is
identical in composition to plasma except for the plasma proteins retained by the glomerular
membrane.
•The GFR can be deliberately altered by changing the glomerular capillary blood pressure as a
result of sympathetic influence on the afferent arterioles. Afferent arteriolar vasoconstriction
decreases the flow of blood into the glomerulus, resulting in a reduction in glomerular blood
pressure and a fall in the GFR. Conversely, afferent arteriolar vasodilation leads to increased
glomerular blood flow and a rise in the GFR. Sympathetic control of the GFR is part of the
baroreceptor reflex response to compensate for a change in arterial blood pressure. As the GFR is
altered, the amount of fluid lost in the urine is changed correspondingly, providing a mechanism
to adjust plasma volume as needed to help restore blood pressure to normal on a longterm basis.
Tubular Reabsorption
•After a protein-free plasma is filtered through the glomerulus, each substance is handled
discretely by the tubules; the concentrations of different constituents are variously altered as the
filtered fluid flows through the tubular system.
•The reabsorptive capacity of the tubular system is tremendous. Over 99% of the filtered plasma
is returned to the blood through reabsorption.
•The major substances actively reabsorbed are Na+ (the principal extracellular fluid cation), most
other electrolytes, and organic nutrients such as glucose and amino acids. The most important
passively reabsorbed substances are Cl- H2O, HCO3-, and urea.
•The pivotal event to which most reabsorptive processes are linked in some way is the active
reabsorption of Na+. An energy-dependent Na+–K+ ATPase carrier located in the basolateral
membrane of each proximal tubular cell transports Na+ out of the cells into the lateral spaces
between adjacent cells. This transport of Na+ induces the net reabsorption of Na + from the
tubular lumen to the peritubular capillary plasma, most of which takes place in the proximal
tubules. The energy used to supply the Na+–K+ ATPase carrier is ultimately responsible for the
reabsorption from the proximal tubule of Na+ glucose, amino acids, Cl-, HCO3-, H2O, and urea.
•Specific cotransport carriers located at the luminal border of the proximal tubular cell are driven
by the Na+ concentration gradient to selectively transport glucose or an amino acid from the
luminal fluid into the tubular cell, from which the nutrient eventually enters the plasma.
•Chloride is passively reabsorbed down the electrical gradient established by active Na +
reabsorption.
•Water is passively reabsorbed as a result of the osmotic gradient created by active Na+
reabsorption. Sixty-five percent of the filtered H2O is reabsorbed from the proximal tubule in this
unregulated fashion. This extensive reabsorption of H 2O increases the concentration of other
substances remaining in the tubular fluid, most of which are filtered waste products.
•About 50% of filtered urea (a waste product of protein metabolism) is reabsorbed. The other
waste products, failing to be reabsorbed, remain in the urine in highly concentrated form.
•The other electrolytes actively reabsorbed by the tubules, such as PO 43_ and Ca++, have their
own independently functioning carrier systems. Once the filtered load of an actively reabsorbed
substance exceeds the transport capacity ( Tm), reabsorption proceeds at a constant maximal
rate, with the additional filtered quantity of the substance being excreted in the urine.
•Na+ reabsorption is regulated in the proximal and distal tubules and collecting ducts, though
less than half the filtered Na+ is regulatable. The extent of this controlled Na+ reabsorption
depends on the renin-angiotensin-aldosterone system. Because Na + and its attendant anion, Cl-,
are the major osmotically active ions in the extracellular fluid (ECF), the ECF volume is
determined by the Na+ load in the body. In turn, the plasma volume, which reflects the total ECF
volume, is important in the long-term determination of arterial blood pressure. Whenever the
Na+ load/ECF volume/plasma volume/arterial blood pressure are below normal, the kidneys
secrete renin, an enzymatic hormone that triggers a series of events ultimately leading to
increased secretion of aldosterone from the adrenal cortex. Aldosterone increases Na
reabsorption from the proximal and distal tubules, correcting for the reduction in Na/ECF
volume/blood pressure.
Tubular Secretion
•By means of the process of tubular secretion, the kidney tubules are able to selectively add
some substances to the quantity already filtered. Secretion of substances hastens their excretion
in the urine.
•The most important secretory systems are for (1) H +, which is important in the regulation of
acid-base balance; (2) K+, which keeps the plasma K+ concentration at an appropriate level to
maintain normal membrane excitability in muscles and nerves; and (3) organic ions, which
accomplishes more efficient elimination of foreign organic compounds from the body.
•Only a small fraction of filtered fluid remains in the tubules to be excreted as urine. Only wastes
and excess electrolytes not wanted by the body are left behind, to be eliminated in the urine.
Because the excreted material is “cleared” from the plasma, the term plasma clearance refers to
the volume of plasma being cleared of a particular substance each minute by means of renal
activity.
Osmoconcentration
•The kidneys are able to excrete urine of varying volumes and concentrations to either conserve
or eliminate H2O, depending on whether the body has a H2O deficit or excess, respectively.
Human kidneys are able to produce urine ranging from 0.3 ml/min at 1,200 mOsm to 25 ml/min
at 100 mOsm by reabsorbing variable amounts of H2O from the distal portions of the nephron.
•This variable reabsorption is made possible by the establishment of a vertical osmotic gradient
ranging from 300 to 1,200 mOsm (in humans) in the medullary interstitial fluid by means of the
loop of Henle countercurrent system and urea recycling between the collecting duct and Henle’s
loops. This vertical osmotic gradient to which the hypotonic (100 mOsm) tubular fluid is exposed
as it passes through the distal portions of the nephron establishes a passive driving force for
progressive reabsorption of H2O from the tubular fluid, but the actual extent of H2O reabsorption
depends on the amount of vasopressin (antidiuretic hormone) secreted.
•Vasopressin increases the permeability of the distal tubules and collecting ducts to H 2O; they are
impermeable to H2O in its absence. Vasopressin secretion increases in response to a H 2O deficit,
and H2O reabsorption increases accordingly. Vasopressin secretion is inhibited in response to a
H2O excess, thereby reducing H2O reabsorption. In this way, adjustments in vasopressincontrolled H2O reabsorption help correct any fluid imbalances.
•Different osmoconcentrating abilities among species depend on nephron anatomy and metabolic
rates. Desert rodents, for example, have relatively long nephron loops and high metabolic rates,
and can concentrate urine up to 6,000 mOsm. Peristaltic pumping by the renal pelvis may
contribute to the concentrating process.
Bladder Storage and Micturition
•Once formed, urine is propelled by peristaltic contractions through the ureters from the kidneys
to the urinary bladder for temporary storage. The bladder can accommodate a relatively large
volume of urine before stretch receptors within its wall initiate the micturition reflex. This reflex
causes involuntary emptying of the bladder by simultaneous bladder contraction and opening of
both the internal and external urethral sphincters. Micturition can transiently be voluntarily
prevented until a more opportune time for bladder evacuation by deliberate tightening of the
external sphincter and surrounding pelvic diaphragm.
Other Vertebrate Urinary Systems and Extrarenal Organs
•Freshwater bony fishes must maintain an internal osmotic pressure far above that of the
environment, and are faced with constant influx of water through the gills and mouth. Their
kidneys excrete a highly dilute voluminous urine. Dietary intake and active transport by the gills
brings in NaCl and other ions to replace those lost in the urine. Thus the gills serve as extrarenal
osmoregulatory organs.
•Marine bony fishes use their gills for most excretory functions. Most are highly hypoosmotic
compared to seawater, so they face constant influx of excess salt through the gills and diet, and
constant water loss through the gills. Gills compensate for this, using specialized epithelial
chloride cells, which actively transport NaCl outwards.
•Cartilaginous fishes are isosmotic or slightly hyperosmotic relative to seawater, because they
retain urea and TMAO as major osmolytes (Chap. 13). The gills are relatively impermeable to
urea and TMAO, and the nephrons reabsorb most of these osmolytes from the filtrate, excreting
only what is necessary for nitrogen balance. Extrerenal organs perform most other excretion and
osmoregulatory roles. Gills are thought to remove some of the excess NaCl, but in particular,
these fish have a specialized hindgut organ called the rectal gland, which excrete a hypertonic
fluid high in NaCl.
•Amphibians have kidney nephrons similar to those of freshwater fish, with similar functions (to
excrete water and reabsorb ions and nutrients). They also excrete urea by filtration and tubular
secretion. In terrestrial amphibians, the urinary bladder has a secondary role, that of a temporary
water reservoir. With dehydration stress, the release of the hypothalamic hormone arginine
vasotocin (AVT) triggers water uptake into the main body from the wall of the bladder. Uptake
through the skin may also occur.
•Reptiles have kidney nephrons similar to aquatic vertebrates. The ureters carry urine in a liquid
or semisolid form into the cloaca, the final chamber of the hindgut. They can conserve water in
three ways (1) uric acid as the primary nitrogenous waste conserves water; (2) the cloaca or
lower intestine can reabsorb some water by first transporting salt. This precipitates uric acid even
further; (3), marine and some desert reptiles have an extrarenal organ, the nasal salt gland that
is dedicated to excreting a highly salty fluid.
•Birds have kidneys that send a liquid or semisolid urine into a cloaca, with a function similar to
that of reptiles. The kidneys are usually dominated by the typical non-mammalian nephron, called
“reptilian type.” However they also have some nephrons, called “mammalian type,” with
countercurrent loops that can form a concentrated urine. Their primary nitrogenous waste is uric
acid. precipitated as crystals which exert no osmotic pressure. Marine birds also have a nasal salt
gland located near the eyes, with ducts leading to the nasal passages. These glands transport
NaCl out of the blood without concomitant osmotic water movement.
Insect Malpighian Tubules
•These blind-end ducts in terrestrial arthropods (e.g., insects) project into the hemolymph from
the hindgut. They filter the hemolymph by secreting ions into the lumen. This makes the tubule
fluid more concentrated, so water from the hemolymph moves in by osmosis.
•Wastes such as uric acid are also transported into the lumen. The tubule empties into the
hindgut, where epithelial cells of the rectum can further modify the urine, often
osmoconcentrating it (i.e., making it hyperosmotic to conserve water). Uric acid precipitates as
crystals as water is removed.