Transcript Lecture 11 - Internal Environment

Chapter 32
Maintaining the Internal Environment
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
How the Animal Body Maintains
Homeostasis
• Homeostasis is defined as the dynamic
constancy of the internal environment
 conditions fluctuate continuously within
narrow limits
 most of the regulatory mechanisms of the
vertebrate body that are not devoted to
reproduction are concerned with maintaining
homeostasis
How the Animal Body Maintains
Homeostasis
• To maintain internal constancy, the
vertebrate body needs
 sensors that are able to measure each
condition of the internal environment
 an integrating center that contains the set
point, or proper value for a particular internal
condition
 effectors are generally muscles or glands
that can change the value of the condition
back toward the set point
How the Animal Body Maintains
Homeostasis
• The integrating center is often a particular
region of the brain or spinal cord, but could
also be cells of endocrine glands
 it receives messages from several sensors
and then determines if the condition is
deviating from the set point
 it sends a message to the certain effectors to
either decrease or increase their activity
• the activity of the effectors is influenced by the
effects they produce in a negative feedback loop
A generalized diagram of a
negative feedback loop
How the Animal Body Maintains
Homeostasis
• Examples of negative feedback loops in
homeostasis
 regulating body temperature
• humans, as well as mammals and birds, are endothermic
– this means that they can maintain relatively constant body
temperature
• other vertebrates are ectothermic, meaning their body
temperatures depend more or less on the environmental
temperature
– but they can modify their behavior to affect body temperature
 regulating blood glucose
Control of blood glucose levels
Regulating the Body’s Water
Content
• Animals use various mechanisms for
osmoregulation, the regulation of the
body’s osmotic composition
 this refers to how much water and salt the
body contains
 the proper operation of many vertebrate organ
systems of the body requires that the osmotic
concentration of the blood be kept within
narrow bounds
Regulating the Body’s Water
Content
• In many animals, the removal of water and
salts from the body is coupled with the
removal of metabolic wastes through the
excretory system
 for example, protists, like Paramecium,
employ contractile vacuoles
• water and metabolic wastes are collected by
endoplasmic reticula that connect to feeder canals,
which lead to the vacuole
• the water and wastes are then expelled through a
pore
The Vertebrate Kidney
• The kidney is a complex organ made up of
many, many units called nephrons
 blood pressure forces the fluid in the blood
through a capillary bed at the top of each
nephron, called a glomerulus
• the glomerulus excludes blood cells, proteins, and
other large molecules from the filtrate
 the remainder of the nephron tube reabsorbs
anything else useful from the filtrate
Basic organization of the vertebrate
nephron
Vertebrate Kidney Function
• Because the original glomerular filtrate is
isotonic blood, all vertebrates can produce a
urine that is
 isotonic to blood by reabsorbing ions
 hypotonic to blood by making the urine more dilute
• Only birds and mammals can reabsorb water
from the glomerular filtrate to produce a urine
that is hypertonic (more concentrated than)
blood
Evolution of the Vertebrate Kidney
• Kidneys are thought to have evolved first
among the freshwater fish
 Body fluids of a freshwater fish have a greater
osmotic concentration than the surround
water, these animals face two serious
problems
• water tends to enter the body from the
environment
• solutes tend to leave the body and enter the
environment
The Vertebrate Kidney - Fish
• Freshwater fish address these problems
by
 not drinking water
 excreting a large volume of dilute urine
 reabsorbing ions (mainly NaCl) across the
nephron tubule from the glomerular filtrate
 actively transporting NaCl across the gills
from the surrounding water into the blood
Fish Kidneys
• Marine fish probably evolved from freshwater
ancestors
 because their bodies are hypotonic to the surrounding
seawater, they faced problems in that
• water tends to leave their bodies through osmosis across the
gills
• they lose water in their urine
 to compensate, marine fish drink lots of seawater
• they excrete isotonic urine
Freshwater and marine teleosts (bony fish)
face different osmotic problems
Sharks’ (and their realtives’)Kidneys
• Elasmobranchs are the most common
subclass of cartilaginous fish
 they solve their osmotic problem posed by
their seawater environment by reabsorbing
urea from the nephron tubules
 this elevates the osmotic concentration in the
blood so that they do not have to continually
drink seawater
• the blood is approximately isotonic to the
surrounding sea
Figure 32.9 Osmoregulation in
elasmobranchs
32.3 Evolution of the Vertebrate
Kidney
• The amphibian kidney is identical to that of
freshwater fish
 amphibians produce a very dilute urine and actively
transport Na+ across their skin
• The kidneys of terrestrial reptiles absorb much
salt and water in the nephron tubules
 their urine is still hypotonic but they can absorb
additional water in the cloaca
Other Vertebrate Kidneys
• Because mammals and birds can produce
hypertonic urine, they can excrete their
waste products in a small volume of water
 the production of the hypertonic urine is
possible due to a looped portion of the
nephron, called the Loop of Henle
 marine birds additionally drink sea water and
excrete excess salt through salt glands
Osmoregulation by
some vertebrates
Marine birds drink seawater and then
excrete the salt through the salt glands
Functions of the Mammalian
Urinary System
1. Removal of metabolic wastes (especially
nitrogenous wastes e.g. urea & uric acid).
2. Water balance (and therefore blood pressure).
3. Control of electrolyte balance.
4. Control of pH.
5. Removal of toxins.
The Mammalian Kidney
• Each kidney receives blood from a renal
artery, and it is from this blood that urine
is produced
 urine drains from each kidney through a
ureter
 the ureters carry urine to a urinary bladder
 urine passes out of the body through the
urethra
The mammalian
urinary system
contains two
kidneys, each of
which contain
about a million
nephrons within
the renal cortex
and renal
medulla
The Mammalian Kidney
• Within the kidney, the mouth of the ureter
flares open to form a funnel-like renal
pelvis
 the renal pelvis has cup-like extensions that
receive urine from the renal tissue
 the renal tissue is divided into
• an outer renal cortex
• an inner renal medulla
About 25% of
your cardiac
output flows
through your
kidneys each
minute!
The Nephron: functional unit of the kidney
Interlobular artery
Afferent Arteriole
Glomeruli
Urine formation
• The mammalian nephron is composed of three
regions. Each region has specific roles.
 Filtration
• the filtration device at the top of each nephron is called the
Bowman’s capsule which receives filtrate from the
glomerular capillaries
 Tubular reabsorption and secretion
• the Bowman’s capsule is connected to a long renal tubule,
which includes the Loop of Henle, that acts as a reabsorption
device
 Water and pH balance
• the renal tubule empties into a collecting duct that operates
as a water conservation device
• Formation of urine in the kidney
1. Filtration (glomerulus)
2. Tubular reabsorption (kidney tubules)
3. Tubular secretion (kidney tubules)
4. Water conservation (collecting system)
• Filtration
 occurs when, driven by blood pressure, small
molecules are pushed across the thin walls of
the glomerulus into the Bowman’s capsule
 large particles, such as blood cells and
proteins, are excluded from the filtrate
 the filtrate contains water, nitrogenous wastes
(mostly urea), nutrients (principally glucose
and amino acids), and a variety of ions
Filtration pressures: NFP must be positive for U2P
The filtration membrane
Tubular reabsorption
• Reabsorption of filtered solutes occurs in the
Proximal Convoluted Tubules.
• Reabsorbed substances:
• glucose
• amino acids
• water
• vitamins
• fatty acids
• urea
• electrolytes (Na, K, HCO3- etc.)
• Reabsorption of water
 The “thin” segment of the descending arm of
the Loop of Henle is permeable to the
passage of water, but impermeable to salts
and urea
 this leaves a more concentrated filtrate to go
to the ascending arm of the Loop of Henle
32.4 The Mammalian Kidney
• NaCl (salt) reabsorption
 as the filtrate passes up the ascending arm of
the Loop of Henle, the walls become thicker
and permeable to salts (but not to water)
which pass out into the surrounding tissues
and enter the blood
 in the upper regions of the ascending arm of
the Loop of Henle, active transport channels
pump out more salt.
32.4 The Mammalian Kidney
• Tubular secretion
 Mostly occurs in the “distal convoluted tubule”
and the collecting duct. This involves active
transport of other nitrogenous wastes, such
as uric acid and ammonia, as well as excess
hydrogen ions
32.4 The Mammalian Kidney
• Further reabsorption of water
 in the collecting duct, the lower reaches are
permeable to urea, which exits to the
surrounding tissues
 this makes water even more likely to leave
Eliminating Nitrogenous Wastes
• Amino acids and nucleic acids are
nitrogen-containing molecules
• When animals metabolize these
substances, they produce nitrogencontaining by-products, called
nitrogenous wastes, that must be
eliminated by the body
• The first step in the metabolism of amino
acids and nucleic acids is the removal of
the amino (--NH2 group)
 this group is then combined with H+ to form
ammonia (NH3)
 this takes place in the liver
Ammonia is toxic
• Ammonia is quite toxic and is safe only in very
dilute concentrations
 for fish and tadpoles, ammonia can be directly
eliminated across the gills or excreted in dilute urine
 in sharks, adult amphibians, and mammals, the
nitrogenous waste is eliminated as urea, which is less
toxic
 reptiles, birds, and insects excrete nitrogenous
wastes in the form of uric acid, which can be
excreted with very little water
Figure 32.14 Nitrogenous wastes
How mammals do it!
• Mammals also make some uric acid but it
is a waste product of the breakdown of
nucleotides
 most mammals have an enzyme, uricase,
that converts the uric acid into a more soluble
form called allantoin
 humans, apes, and dalmation dogs lack this
enzyme and must excrete the uric acid
• in humans, an excessive accumulation of uric acid
is called gout
Clinical
• Diseases that are harmful to the urinary
system:
 Hypertension
 Diabetes mellitus
 Liver dysfunction
 Congestive heart failure
 Hormone imbalances
These can all lead to renal failure! Some can
also result in …
Kidney stones
Renal Calculi
“Well Mr.
Osborne, I
don’t think
that it’s kidney
stone after all”