Chapter 43 Salt-Water Balance and Temperature Control

I. Maintaining the Extracellular Fluid
A. Water Gains and Losses
1 Water is gained by two processes:
a. Absorption of water from liquids and solid foods occurs in the gastrointestinal tract.
b. Metabolism of nutrients yields water as a byproduct.
2 Water is lost by at least five processes:
a. Excretion of water is accomplished by the urinary system.
b. Evaporation occurs from respiratory surfaces.
c. Evaporation occurs through the skin.
d. Sweating occurs on the skin surface.
e. Elimination of water in feces is a normal occurrence.
3 The nervous system regulates these processes by regulation of thirst and temperature control
(sweating).
B. Solute Gains and Losses
1 Solutes are added to the internal environment by three processes:
a. Nutrients, mineral ions, drugs, and food additives are absorbed by the gastrointestinal tract.
b. Secretion from endocrine glands adds hormones.
c. Metabolism reactions contribute waste products.
2 Carbon dioxide waste is disposed of by the respiratory system, but the following must be
excreted in urine:
a. Ammonia is formed when amino groups are removed from amino acids.
b. Urea is formed by reactions in the liver that unite two ammonia molecules with carbon
dioxide.
c. Uric acid is formed in reactions that degrade nucleic acids.
d. Phosphoric acid and sulfuric acid are also produced during protein breakdown.
C. Urinary System of Mammals
1 Kidneys (2) filter a variety of substances from the blood.
a. Most of the filtrate is returned to the blood; about 1 percent ends up in the kidney’s central
cavity (renal pelvis) as urine.
b. The kidneys regulate the volume and solute concentrations of extracellular fluid.
c. Each kidney is composed of two zones—cortex and medulla—containing numerous blood vessels
and working units called nephrons.
2 Urine flows from each kidney through a ureter to a urinary bladder (for storage) and then out of
the body through the urethra.
3 Kidney stones are deposits of uric acid that collect in the renal pelvis or lodge in the urethra. D. Nephron Structure
1 Filtration occurs in the glomerulus—a ball of capillaries nestled in the Bowman’s capsule.
2 The Bowman’s capsule collects the filtrate and directs it through the continuous nephron tubules:
proximal Æ loop of Henle Æ distal Æ collecting duct.
3 The capillaries exit the glomerulus, converge, then branch again into the peritubular capillaries
around the nephron tubules where they participate in reclaiming water and essential solutes.

II. Urine Formation
A. Urine-Forming Processes
1 In filtration, blood pressure forces filtrate out of the glomerular capillaries into Bowman’s
capsule, then into the proximal tubule.
a. Blood cells, proteins, and other large solutes cannot pass the capillary wall into the capsule.
b. Water, glucose, sodium, and urea are forced out.
2 Reabsorption takes place in the tubular parts of the nephron where water and solutes move across
the tubular wall and out of the nephron and into the surrounding capillaries.
3 Secretion moves substances from the capillaries into the nephron walls.
a. Capillaries surrounding the nephrons secrete excess amounts of hydrogen ions and potassium
ions into the nephron tubules.
b. This process also rids the body of drugs, uric acid, hemoglobin breakdown products, and other
wastes.
B. Factors Influencing Filtration
1 The kidneys can process about 1.5 quarts of blood each minute because of two factors:
a. Blood enters the glomerulus under high pressure—these arterioles have wider diameters than
most arterioles do.
b. Glomerular capillaries are highly permeable to water and small solutes.
2 The rate at which the kidneys filter a given volume of blood depends on the flow of blood through
them and the rate of reabsorption in the tubules; neural and hormonal controls operate.
C. Reabsorption of Water and Sodium
1 Mechanisms within the kidney carefully regulate the excretion and retention of substances based
on intake and bodily need.
a. Proximal Tubule
1) Sodium ions are pumped out of the tubule (filtrate) and into the interstitial fluid
surrounding the peritubular capillaries.
2) Significant amounts of water follow passively down the gradient that has been created.
b. Urine Concentration and Dilution
1) In the descending limb of the loop of Henle, water moves out by osmosis, but in the
ascending portion sodium is pumped.
2) This interaction of the limbs of the loop produces a very high solute concentration in the
deeper parts of the kidney medulla and delivers a rather dilute urine to the distal
tubule.
c. Hormone-Induced Adjustments
1) When sodium levels fall so does the volume of extracellular fluid; this triggers the
juxtaglomerular apparatus to secrete renin, which calls forth angiotensin II, which
acts on the adrenal cortex to release aldosterone, which promotes sodium
reabsorption.
2) Antidiuretic hormone (ADH) from the posterior pituitary is secreted in response to a
decrease in extracellular fluid; ADH causes the distal tubules and collecting ducts to
become permeable to water, which moves back into the blood capillaries.
2 Sodium retention is accompanied by water retention, which can lead to increased blood
pressure—hypertension, which can affect kidney function.
D. Thirst Behavior
1 When solute concentration in the extracellular fluid rises, the thirst center of the hypothalamus
responds by decreasing saliva production.
2 The dry sensation in the mouth causes a liquid-seeking behavior.
E. Acid–Base Balance
1 Overall acid–base balance is maintained by controlling hydrogen ions through: buffer systems,
respiration, and excretion by the kidneys.
2 Buffers can neutralize hydrogen ions, the lungs can eliminate carbon dioxide.
3 Only the urinary system can eliminate excess hydrogen ions.
a. The HCOo(3,–) that forms in the nephron cells is moved to the capillaries where it neutralizes
excess acid.
b. The H+ that forms in the cells is secreted into the tubular fluid where it combines with
bicarbonate ions to form carbon dioxide (which is returned to the blood and excreted by
the lungs) and water (which is excreted in the urine).
c. Thus, hydrogen ions are permanently removed from extracellular fluid.
F. On Fish, Frogs, and Kangaroo Rats
1 Marine bony fish lose water by osmosis to the hypertonic sea; they drink sea water for
replacement and excrete the excess salt by means of special cells in the gills.
2 In fresh water, bony fish tend to gain water through gills (or in the case of amphibians, their skin)
and lose solutes in the dilute urine.
3 The kangaroo rat survives on so little water because of the very long loops of Henle, which
superconcentrate the urine.

III. Maintaining Body Temperature
A. Many different physiological and behavioral responses help to maintain the body’s internal core
temperature.
B. Temperatures Suitable for Life
1 Enzymes remain functional within the 0–40°C range.
2 Above 41°C denaturation occurs rendering the enzyme ineffective.
3 Cooler temperatures may not disrupt activity, but will slow it by half for every ten-degree drop.
C. Heat Gains and Losses
1 Radiation is the gain of heat from some source, or the loss of heat from the body to the
surroundings depending on the temperatures of the environment.
2 Conduction is the transfer of heat from one object to another when they are in direct contact, as
when a human sits on cold (or hot!) concrete.
3 Convection is the transfer of heat by way of a moving fluid such as air or water.
4 Evaporation is a process whereby a heated substance changes from a liquid to a gaseous state
with a loss of heat to the surroundings.
D. Classification of Animals Based on Temperature
1 Ectotherms
a. Animals with low metabolic rates gain their heat from the environment.
b. Ectotherms, such as lizards, make adjustments to changing external temperatures in what we
call behavioral temperature regulation.
2 Endotherms and Heterotherms
a. Endotherms generate heat from metabolic activity and also exercise controls over heat
conservation and dissipation.
b. Endotherms have adaptations such as feathers, fur, or fat to reduce heat loss; they also
adjust their behavior by moving underground during the heat of the day, for instance.
c. Heterotherms, such as the hummingbird, generate body heat during their active periods but
resemble ectotherms during inactive times.
3 Thermal Strategies Compared
a. Ectotherms are at an advantage in the tropics where they do not have to expend much energy
to maintain body temperature.
b. Endotherms have an advantage in moderate to cold settings.
E. Temperature Regulation in Birds and Mammals
1 Responses to Cold Stress
a. Mammals respond to cold by constricting the smooth muscles in the blood vessels of the skin
(peripheral vasoconstriction), which retards heat loss.
b. In the pilomotor response, the hairs or feathers become more erect to create a layer of still
air that reduces convective and radiative heat losses.
c. Rhythmic tremors (shivering) is a common response to cold but is not effective for very long
and comes at high metabolic cost.
d. Hibernating mammals can produce nonshivering heat by a hormonal stimulation of a special
brown adipose tissue.
e. Hypothermia is a condition in which the core temperature drops below normal; it may lead to
brain damage and death; frostbite is localized cell death due to freezing.
2 Responses to Heat Stress
a. Peripheral vasodilation is the enlargement of the diameters of blood vessels to allow greater
volumes of blood to reach the skin and dissipate the heat.
b. Evaporative heat loss by sweating is a common and obvious cooling mechanism.
c. Panting is used by animals with very little ability to sweat.
d. Hyperthermia is a rise in core temperature, with devastating effects.
3 Fever
a. During a fever, the hypothalamus resets the body’s “thermostat” to a new temporary core
temperature.
1) At the onset of fever, heat loss decreases and heat production increases; the person feels
chilled.
2) When the fever breaks, peripheral vasodilation and sweating increase as the body
attempts to reduce the core temperature to normal.
b. The controlled increase in body temperature (within limits) during a fever seems to enhance
the body’s immune response.