Download Chapter 44: Controlling the Internal Environment

Chapter 44: Controlling the Internal Environment
Regulation of Body Temperature

Different species of animals are adapted to different temperature ranges, but each
has an optimal temperature range

Thermoregulation: maintenance of body temperature within a range that enables
cells to function efficiently

Organisms exchange heat with the external environment in four ways:
o Conduction: direct transfer of heat from molecules of the environment to
those of the body
o Convection: transfer of heat by the movement of air or liquid past a surface of
a body
o Radiation: emission of electromagnetic waves produced by objects warmer
than absolute zero, i.e. the sun and a human’s body
o Evaporation: loss of heat from the surface of a liquid that loses some
molecules as gas
o *Convection and evaporation most common

Humans are endotherms, or we derive most/all of the body heat from metabolism;
maintain relatively constant internal temperature
o Surprisingly, having a constant internal temperature does not distinguish
endotherms from ectotherms; some ectothermic fishes live in water with
stable temperatures to the extent that their internal body temperature varies
less than humans
o Ectotherm= body temperature a result of surroundings, Endotherm= body
temperature a result of organism’s metabolism

There are many benefits to being endothermic because it solved many problems
associated with living on land:
o Can maintain constant body temperature despite outside temperature
fluctuations
o Are generally warmer than surroundings but can cool the body in a hot
environment
o A higher internal body temperature leads to high levels of aerobic
metabolism (cellular respiration)
o There are bioenergetic connections between body temperature, active
aerobic metabolism, and motility; moving on land requires much more effort
than it does in the water

Mammals and birds are thought to have efficient circulatory and respiratory
systems because they are adaptations that accompanied the evolution of endothermy

Invertebrates have very little control over their body temperature; amphibians and
reptiles are generally ectothermic (low metabolic rates); fish are generally ectothermic,
with body temperature varying within 1-2 C of the surrounding water

In order to maintain a high internal temperature, mammals must balance heat
production with the rate of heat loss
o To increase: increase contraction of muscles (shivering) or by action of
hormones that increase metabolic rate by producing heat rather than ATP
o Nonshivering thermogenesis: hormonal triggering of heat production

Mammals, like humans in particular, can experience regional differences in internal
body temperature; the arms and legs can be several degrees colder than your trunk or
abdomen because most of the vital organs are located there

Humans rely on a layer of fat just beneath the skin to insulate and protect against
heat loss

Body temperature regulation in humans is an example of a complex homeostatic
system controlled by feedback mechanisms

Homeostasis is the property of a system, either open or closed, that regulates its
internal environment and tends to maintain a stable, constant condition

The nerve cells that control thermoregulation and others that control homeostatis
are concentrated in the hypothalamus; a portion of the brain that contains a number of
small nuclei with a variety of functions. One of the most important functions of the
hypothalamus is to link the nervous system to the endocrine system via the pituitary
gland (hypophysis)

The hypothalamus responds to change in temperature above and below a set point
and then active the respective mechanisms to regulate the temperature change

The actual nerve cells that sense body temperatures are located in the skin, the
hypothalamus and other nervous system organs

If your body temperature is too low, the heat mechanisms will be activated (erection
of fur/hair, shivering, nonshivering thermogenesis)

If your body is too hot, then the body will shut down heat-saving mechanisms and
promote sweating

Animals can adapt to environmental temperatures over a period of days or weeks,
called acclimatization

Torpor: physiological state when metabolism decreases and the heart and
respiratory system slow down
Water Balance and Waste Disposal

For those cells not in contact with the external environment the interstitial fluid is
serviced by blood
o Have to maintain balance between water loss and uptake as well as prevent
the by-products of metabolism from building up and poisoning body fluids
o Requires the movement of solutes between body fluids and environment

Maintaining water balance and getting rid of metabolic wastes requires the passage
of solutes through transport epithelium (layer or layers of epithelial cells that regulate
solute movement)
o Some transport epithelia face outside while others face a tubular channel
that leads to opening of body surface; joined by impermeable tight junctions

Any solute passing between extracellular fluid and environment must pass through
selective membrane of cells

Transport epithelia are arranged in tubular networks

Most toxic by-products of metabolism are nitrogen-containing wastes (from
breakdown of protein and nucleic acids)- removed when macromolecules are broken
apart from energy or converted to carbohydrates or fats

Waste product is ammonia; efficient to excrete ammonia directly because virtually
no energy is expended
o Most animals must convert ammonia to compounds like urea or uric acid
(require ATP to be produced)
o Kind of nitrogenous waste an animal excretes depends on evolutionary
history and habitat

Ammonia: in aquatic animals, they excrete nitrogenous wastes as ammonia (very
soluble in water)

Urea: mammals excrete urea, which is 10,000 times less toxic than ammonia
o Ammonia too toxic for land animals
o Most animals can tolerate high concentrations of urea and they excrete a
very concentrated solution as a waste product to conserve water
o FYI: amphibians secrete ammonia as tadpoles and urea as land-dwelling
adults

Uric Acid: land snails, birds, and reptiles excrete uric acid

The cells of an animal must always balance water loss and uptake; burst if they have
too much water and shrivel up and die without enough

Osmosis: movement of water across a selectively permeable membrane

Humans are osmoregulators, or an animal that must adjust the internal solute
concentration because it is the same concentration as the outside environment
o This is energetically costly because it must drive the osmotic gradient that
allows water to move in and out

Humans will die if they lose 12% of their body water, so evolutionary adaptations
are in place to prevent that:
o For humans, it is the layer of skin that protects from water loss
 We lose a considerable amount of water despite this because of the
gas exchange that takes place on moist surfaces and through urine
o Replenish water supply by drinking and eating moist foods
Excretory Systems

Very important to homeostasis because they dispose of metabolic waste and have
the ability to respond to imbalances in body fluids by excreting more or less of a specific
ion (compensation)

Produce urine through two processes: filtration of body fluids and refinement of the
aqueous solution from filtration

Filtration: when blood is exposed to a filtering device made of a selectively
permeable membrane of transport epithelia

The membranes keep proteins and other large biological molecules, but blood
pressure forces water and small solutes (salts, sugars, amino acids, nitrogenous waste)
into excretory system

Urine is produced by two mechanisms using active transport
o Reabsorption: selective transport of water and valuable solutes (glucose,
salts, amino acids) from filtrate back into body fluids
o Filtration is nonselective so small molecules that are essential to the body
must be returned to the body fluids
o Secretion involves the adding of solutes (salt and toxins) from the body to
the filtrate

In vertebrates that osmoregulate (humans), the kidneys function in both excretion
an osmoregulation

Kidneys, the blood vessels that serve them, and the structures that carry urine
formed in the kidneys are part of the human excretory system
o The kidneys are bean-shaped (10 cm) organs
o Blood enters kidneys through renal artery and leaves through renal vein
o FYI: account for 1% of weight of body but receive 20% of blood pumped with
each heartbeat
o Urine exits kidneys through ureters which connect to the urinary bladder
o Urine leaves body through tube called the urethra (found near the vagina in
females and through penis in males)

There are two regions of the mammalian kidney: outer renal cortex and inner
renal medulla
o Both of these regions are filled with excretory tubules called nephrons and
collecting ducts
o The nephron is the functional unit of the kidney and contains a long tubule
and a ball of capillaries called the glomerulus, and the Bowman’s capsule
surrounds the glomerulus

Filtration occurs when the blood pressure forces water, urea, salts, and other
solutes from the blood in the glomerulus into the Bowman’s capsule
o The capillaries and specialized cells of the capsule called podocytes perform
the functions of a filter because they are permeable to water and small
solutes but not blood or large molecules
o Any molecule small enough to fit through the capillary wall with be
completely filtered because filtration is nonselective

After being filtered from the Bowman’s capsule, the filtrate travels through three
regions of the nephron: the proximal tubule, the loop of Henle (hairpin turn with a
descending limb and ascending limb), and the distal tubule
o The distal tube empties into the collecting duct that receives the filtrate
from the nephrons
 About 80% of nephrons are cortical nephrons that have reduced
loops of Henle in the renal cortex
 The other 20% of the nephrons are juxtamedulllary nephrons that
have well-develop loops that are located in the renal medulla

The nephron and the collecting duct are lined with transport epithelium that
processes the filtrate to form urine
o 1100-2000 L of blood pass through the human kidneys per day; nephrons
and collecting duct process about 180 L of filtrate and only create 1.5 L of
urine
 The rest of the filtrate is reabsorbed into the blood (99% of which is
water)

Each nephron is supplied with blood by an afferent arteriole, a branch of the renal
artery (supplies nephrons and kidneys with blood from the heart) which divides into
capillaries of the glomerulus
o The capillaries converge after the leave the glomerulus to form the efferent
arteriole
 This vessel divides further to form network of capillaries called
peritubular capillaries
These capillaries work with the proximal and distal tubules of the
nephron (two of steps after leaving the Bowman’s capsule
 More capillaries are found at the loop of the Henle, which are called
the vasa recta

Tubules and capillaries are submerged in a interstitial fluid and various substances
pass back and forth between the plasma within capillaries
Secretion

Proximal and distal tubules involved in secretion- process that involves both active
and passive transport
o Controlled secretion of hydrogen ions from interstitial fluid into nephron
tubule is important for maintaining pH
Reabsorption

Proximal and distal tubules and the loop of Henle all contribute to reabsorption
o Nearly all sugars, vitamins, and other organic nutrients are reabsorbed along
with a majority of the water from the filtrate

Together, selective reabsorption and secretion control the concentrations of salts in
body fluids
Flow from Blood Filtrate to Urine

1: proximal tubule
o Help maintain pH of body by controlling secretion of hydrogen ions
o Cells secrete ammonia to neutralize the acid and prevent the filtrate from
becoming too acidic
o Proximal tubule absorbs 90% of bicarbonate buffer (HCO3-)
o Any drug or poison processed in the liver are secreted into the filtrate in the
proximal tubule
o Glucose and amino acids actively transported into the interstitial fluid and
then into the blood
o Potassium (K+) is also reabsorbed with NaCl (salt) and water
o Na+ is transported actively out of the cells and in response, Cl- is passively
transported out of tubule
o As salt moves from filtrate to interstitial fluid, water follows by osmosis

2: descending limb of loop of Henle
o Transport epithelium freely permeable to water but not to salt and small
solutes
o Water leaves loop of Henle because it is more hypertonic than the interstitial
fluid around it
o NaCl concentration continues to increase



3: ascending limb of loop of Henle
o Permeable to salt, not water
o Two regions: thin segment (loop tip) and thick segment (leads to distal tube)
o NaCl diffuses out in thin segment into interstitial fluid
o NaCl actively transported out of thick segment into interstitial fluid; filtrate
becomes more dilute
4: distal tube
o Secretion and reabsorption happen here
o Monitors amount of K+ secreted into filtrate and amount of NaCl that is
reabsorbed from the filtrate
o Controls pH regulation

5: collecting duct
o Carries filtrate to renal pelvis and medulla; determines how much salt is
actually in urine by actively absorbing NaCl
o Permeable to water, not salt and therefore loses more and more water by
osmosis
o At the end of the duct, the high concentration of urea diffuses out of duct
Conserving Water by Kidney Action

NaCl and urea responsible for osmotic gradient

1: At the proximal tubule, a large amount of water and salt are reabsorbed so
volume of filtrate decreases (molarity still equal to blood)

2: In the descending limb of the loop of the Henle, water leaves the tubule by
osmosis and NaCl concentration increases
o Salt concentration peaks at elbow of loop of Henle

3: salt concentration decreases in ascending limb of loop of Henle because
permeable to salt, not water
o Two limbs combined maintain osmotic gradient in the interstitial fluid of
kidneys
o Two limbs are close enough to one another to affect each other’s chemical
changes

4: filtrate is hypotonic when it reaches the distal tubule because lost NaCl
concentration in ascending limb

5: as filtrate flows from cortex to medulla, it loses water by osmosis but the
concentration of urea increases
o Urine is isotonic in relation to interstitial fluid in inner medulla but
hypertonic to blood and interstitial fluid anywhere else in the body