Transcript Homeostasis - Cloudfront.net

Homeostasis
Homeostasis – (maintaining a “steady state”) maintenance of
the conditions of the tissue fluid bathing the cells at a
relatively constant level (pH, temperature, salt
concentration)
• usually achieved by negative feedback – a change in the
level of an internal factor causes effectors to restore the
internal environment to its original level
– an increase in body temperature causes the body to
lose more heat and vice versa
• occasionally positive feedback is used such as in childbirth
– oxytocin is released, contractions occur, stimulate more
release of oxytocin and therefore more contractions etc.
• ability of mammals to maintain a stable
internal environment makes them
independent of changing external
conditions and enables them to exploit a
wide range of habitats – ex. maintenance
of internal body temperature
Control of Blood Glucose Concentrations
Glucose is transported in solution in the blood
plasma
• maintenance of glucose at steady levels is
vital
• normal blood glucose concentration is about
90 - 100 mg of glucose per 100 cm3 of blood
• Two interacting mechanisms control blood
glucose concentrations
– insulin – compensates for glucose levels that are
too high
– glucagon – compensates for glucose levels that
are too low
Insulin
• small protein consisting of 51 amino acids
• secreted by special cells called beta cells in the islets of
Langerhans which are special endocrine tissue in the
pancreas
• when blood glucose concentrations rise above the set
point, more insulin is secreted by the pancreas which
results in:
– an increase in the uptake of glucose and amino acids
into cells
– an increase in the rate of cellular respiration and the use
of glucose as a respiratory substrate
– an increase in the rate of conversion of glucose to fat in
adipose cells
– an increase in the rate of conversion of glucose to
glycogen in liver and muscle cells (glycogenesis)
Glucagon
• secreted by the alpha cells in the islets of
Langerhans
• when blood glucose concentrations fall below set
point, the alpha cells secrete glucagons
• glucagon activates phosphorylase (enzyme in
liver) which catalyzes the breakdown of glycogen
to glucose (glycogenolysis)
• also increases the conversion of amino acids and
glycerol into glucose 6-phosphate (to enter cellular
respiration pathways)
• READ ABOUT DIABETES IN TEXTBOOK
The Liver
•
The liver is the largest organ in the human
abdomen – plays a central role in
metabolism, regulating the levels of a
wide range of chemicals in the blood and
preventing harmful substances from
reaching chemically sensitive organs such
as the brain
Liver has a double blood supply – circulation:
• hepatic artery – delivers oxygenated blood so that
liver cells can generate energy by aerobic
respiration to carry out all their energy demanding
functions
• hepatic portal vein – takes all the blood from the
intestines (carrying all digested food, sugars, and
amino acids) to the liver (blood vessel with the
highest sugar concentration in body)
• enables liver to process substances absorbed from
the digestive system before they enter general
circulation – ex. poisons can be made harmless by
detoxification before they damage brain
• hepatic vein – takes blood from the liver to the
heart by merging into the inferior vena cava (blood
vessel with highest concentration of urea)
•
lymphatic vessels from the digestive
system carry fatty substances to liver for
processing before they go to rest of body
(remember, fats are absorbed from the
small intestine into the lymphatic system
rather than into the blood)
Internal structure of the liver
Made of numerous structures called lobules
• each lobule is made of a central vein in the
center and tiny blood spaces called sinusoids
radiating from it
• hepatic (liver) cells line up on both sides of
the sinusoids
• hepatic cells have tiny canals between them
called bile canaliculi – bile made by the
hepatic cells trickle into these canaliculi and
drain out into the bile duct
Process of bile production:
• Bile is a yellow-green fluid containing water, bile
salts (derivatives of cholesterol that help to
emulsify fats), bile pigments (products of the
breakdown of hemoglobin, become yellow
pigments in intestine, have no function but add to
color of feces), inorganic salts, cholesterol and
bicarbonate ions (HCO3-) (to neutralize acid from
stomach as food enters small intestine)
• bile is made by hepatic cells, travels via the bile
canaliculi to the bile duct which empties into the
gall bladder
• bile is stored in gall bladder
•
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blood from the hepatic portal vein and
the hepatic artery pour blood into the
sinusoids past each hepatic cell
many chemical reactions are carried out
by the liver in these cells
sinusoids collect waste and CO2 from
hepatic cells and all of the blood is sent
to hepatic vein to leave the liver
The liver carries out many chemical processing
functions:
• storage of minerals (including iron) and vitamins A
(also called retinol – fat soluble), vitamin D (also
called calciferol – fat soluble), and B12
– storage of iron – iron is a component of hemoglobin
– liver stores iron left from the breakdown of old red blood
cells – this iron will be used again to produce new
hemoglobin
• manufacture of plasma proteins (such as albumins)
and blood clotting agents (including fibrinogen)
• detoxification of poisons – liver has a group of
enzymes which break down chemicals into less
harmful products (ex. catalase breaks hydrogen
peroxide into water and oxygen)
The liver also has major roles in the metabolism of
carbohydrates, fat, and protein as well as
breaking down old red blood cells:
1. Carbohydrate metabolism – the liver helps to
regulate blood glucose levels
– glucose concentration of blood leaving liver
may be kept the same, reduced, or increased
as compared to the blood that entered through
the actions of glycogenolysis (converting
glycogen to glucose), gluconeogenesis
(converting amino acids or glycerol into
glucose), or glycogenesis (storing glucose as
glycogen) under influence of insulin and
glucagon
2. Protein metabolism – regulation of amino acids and
proteins
• the body cannot store excess proteins or amino acids
• non-nitrogenous parts of amino acids can be converted to
fats or molecules that can enter Krebs cycle
• nitrogen in amino group has to be eliminated from body
because it forms toxic substances
• amino acids are first deaminated – removal of amino group
from amino acid to form ammonia
• ammonia is very toxic and very soluble – terrestrial animals
convert it to a less toxic and less soluble substance to avoid
harmful effects and to conserve water - urea
• liver deaminates amino acid forming ammonia
• ammonia is immediately converted to urea in liver by
combining with carbon dioxide (process is actually a series
of enzyme-catalyzed reactions called the ornithine cycle or
urea cycle)
• urea is dumped into hepatic vein and leaves liver
Breakdown of old red blood cells in the liver
• the liver is responsible for destruction and removal of old red
blood cells (erythrocytes)
• old RBCs are eaten up by phagocytic cells of liver called
Kupffer cells – present in lining of sinusoids
• the heme part of hemoglobin is broken down to a green
pigment called biliverdin
• Biliverdin is broken into the yellow-brown pigment called
bilirubin
• bilirubin is mixed with the bile and it becomes one of the
component of bile salts
• bilirubin comes out with the feces and gives it a brown color
• globin part of hemoglobin is digested into amino acids by
liver – recycled and used again to make proteins
• iron from hemoglobin is stored in liver and is used for making
new hemoglobin
Temperature Control (Thermoregulation)
Humans are considered homeotherms
(maintain a constant body temperature and
are endotherms - regulate body
temperature from within by heat produced
by aerobic cellular respiration)
• body temperature is usually around 37o C –
optimum temperature for enzyme activity
• body needs to maintain temperature
regardless of changes in environmental
temperatures
heat is lost to the environment by:
• conduction (transfer of heat between two
objects in contact)
• radiation (release of heat in the form of waves
from hot or warm objects – you radiate heat to
surroundings)
• and convection (transfer of heat by convection
currents in liquids and gases – you heat the
air around you, it rises, and cold air sinks and
replaces it)
Heat regulation in humans involves coordination between
nervous and hormonal systems:
• thermoreceptors (nerve cells belonging to nervous system)
under skin sense the change in surrounding environment
• thermoreceptors send nerve messages to hypothalamus (link
between nervous and endocrine systems)
• hypothalamus releases TRH (thyroid releasing hormone) that
travels to pituitary gland which then releases TSH (thyroid
stimulating hormone) that travels to thyroid gland causing it to
release the hormone, thyroxine
• thyroxine increases the metabolic rate of body releasing more
heat – in hot weather, less thyroxine is released and less heat
is generated
• hypothalamus also sends nerve messages to sweat glands,
muscles, and blood capillaries in order to produce other
responses such as sweating, shivering, vasoconstriction, and
vasodilation
• heat regulation involves negative feedback – as temperature
goes up, hypothalamus produces less TRH, pituitary releases
less TSH, thyroid releases less thyroxine and metabolism
reduces, reduced heat production and vice versa
Body temperature responses in cold weather:
• thermoreceptors under skin sense decrease in
temp and send messages to hypothalamus in brain
• hypothalamus also has thermoreceptors to sense
temp of blood
• hypothalamus sends messages to different parts of
the body to cause the following changes:
– vasoconstriction of blood vessels under the skin – less
blood near surface of skin reducing heat lost to
atmosphere
– metabolic rate increases and more heat is generated
– involuntary shivering causes release of heat by muscles
– body hairs rise attempting to trap a layer of air between
them – trapped air is warmed by body and creates an
insulating layer
Body temperature responses to hot weather:
• thermoreceptors under skin send messages
to hypothalamus in brain causes the
following responses:
– vasodilation increases flow of blood under skin
so more heat is lost to atmosphere
– sweating increases - when sweat evaporates it
takes heat from body surface resulting in
cooling
– metabolic rate decreases and less heat is
generated
– body hairs lay flat so they do not trap a layer of
air between them
Gas Exchange
• Gas exchange takes place at the alveoli
• Composition of alveolar air:
– gas in alveoli does not have same
composition as atmospheric air
– each breath brings in fresh air that mixes
with residual air
Composition of inspired (atmospheric air), alveolar,
and expired air (percentage composition by
volume)
Gas Inspired air
alveolar air
expired air
Oxygen 20.95
13.8
16.4
CO2
0.04
5.5
4.0
Nitrogen 79.01
80.7
79.6
• Blood arriving in lungs has a relatively high
concentration of carbon dioxide and
relatively low concentration of oxygen
– both gases diffuse down their
concentration gradients to equalize
between blood and air
Partial gas pressures
• partial pressure is usually used to compare
the proportion of gases in a mixture
• the partial pressure of a gas in a mixture of
gases is the pressure exerted by that gas
(measured in kilopascals, kPa)
• ex.at sea level, total atmospheric pressure
is 101.3 kPa
• atmosphere contains 21% oxygen, therefore
oxygen has a partial pressure of
.21 x 101.3 kPa or 21.3 kPa
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Hemoglobin and the transport of oxygen
oxygen enters blood from alveoli and diffuses
into red blood cells
oxygen then combines with hemoglobin to
form oxyhemoglobin (HbO2)
as hemoglobin picks up the first molecule of
oxygen, it increases its affinity for oxygen and
picks up the next molecule even faster, the
third and fourth are picked up even faster
the degree of oxygenation of hemoglobin is
determined by the partial pressure of oxygen
(p(O2)) in the immediate surroundings
• If p(O2) is low (as in the capillaries at the
tissues needing oxygen) hemoglobin
releases oxygen and carries relatively
small amounts of oxygen
• If p(O2) is high (such as at the alveoli)
hemoglobin becomes almost saturated
with oxygen
• an oxygen dissociation curve shows the degree
of hemoglobin saturation with oxygen plotted
against different values of p(O2) – the curve is Sshaped
• at p(O2) close to zero there is no oxygen bound to
the hemoglobin
• at low p(O2), the polypeptide chains are tightly
bound together, making it difficult for an oxygen
molecule to attach to iron in heme group
• as one molecule of oxygen attaches, the
polypeptide chain opens up exposing the other
heme groups to oxygen and allowing oxygen to
attach – the curves rises sharply
• at very high p(O2), the hemoglobin becomes
saturated and the curve levels off
•
oxygen at the muscles is taken over and
stored by myoglobin
– myoglobin has a much higher affinity for
oxygen than hemoglobin
– it binds with oxygen at a high rate and does
not dissociate its oxygen unless the p(O2)
drops to very low levels
– myoglobin stores oxygen in muscles until the
demand becomes very great – during heavy
exercise, muscles will get oxygen from
hemoglobin first, then when supply oxygen
from hemoglobin is exhausted, myoglobin will
begin to release its oxygen
•
Fetal (Foetal) hemoglobin
– mother and child have separate circulatory
systems
– fetus must be able to take oxygen from
mother’s hemoglobin in placenta
– fetal hemoglobin is structurally different from
maternal hemoglobin (slightly different) – has a
slightly higher affinity for oxygen than adult
hemoglobin – oxygen released by maternal
hemoglobin is bonded to fetal hemoglobin
Transport of carbon dioxide
Carried from tissues to lungs in different ways:
• most of the CO2 enters the red blood cells
and reacts with water to form carbonic acid
(H2CO3) (catalyzed by the enzyme, carbonic
anhydrase) – H2CO3 splits into H+ and
bicarbonate ions (HCO3-)
• the bicarbonate ions leave the red blood
cells and are transported in the plasma
• Chloride ions (Cl-) diffuse inwards from the
plasma to maintain electrical neutrality – this
process is called the chloride shift
• Protons (H+) left inside the cell are taken up
by the hemoglobin to form hemoglobinic acid
(HHb) – this process causes the hemoglobin
to release its oxygen
• therefore, higher CO2 causes hemoglobin to
lose its affinity for oxygen – causes the Bohr
shift
• higher CO2 levels cause the oxygen
dissociation curve to shift to the right
• excess protons would cause acidity of blood
to increase (lower pH) – hemoglobin acts as
a buffer by taking up excess protons and
preventing blood from becoming too acidic
Gas exchange at high altitudes
• pressure is less at high altitudes therefore partial
pressure of oxygen is also less
• mountain sickness may occur – fatigue, nausea,
breathlessness, headaches
• body begins to acclimatize by:
– producing more RBCs – results in more
hemoglobin to bind with oxygen
– breathing rate and depth increases by
autonomic control from breathing center in
brain stem
• People who live at high altitude have bigger lungs,
larger vital capacity, and hemoglobin with an
increased affinity for oxygen