Lecture 20: Animal circulation and gas exchange
1) Introduction
Every organism must exchange materials and
energy with its environment, and this exchange
ultimately occurs at the cellular level.
Cells live in aqueous environments.
The resources that they need, such as nutrients
and oxygen, move across the plasma membrane to the cytoplasm.
Metabolic wastes, such as carbon dioxide, move
out of the cell.
Most animals have organ systems specialized for
exchanging materials with the environment, and many have an internal transport
system that conveys fluid (blood or interstitial fluid) throughout the body.
For aquatic organisms, structures like gills
present an expansive surface area to the outside environment.
Oxygen dissolved in the surrounding water
diffuses across the thin epithelium covering the gills and into a network of
tiny blood vessels (capillaries).
At the same time, carbon dioxide diffuses out
into the water.
Diffusion alone is not adequate for transporting
substances over long distances in animals - for example, for moving glucose
from the digestive tract and oxygen from the lungs to the brain of mammal
The bulk transport of fluids throughout the body
functionally connects the aqueous environment of the body cells to the organs
that exchange gases, absorb nutrients, and dispose of wastes.
For example, in the mammalian lung, oxygen from
inhaled air diffuses across a thin epithelium and into the blood, while carbon
dioxide diffuses out.
Bulk fluid movement in the circulatory system,
powered by the heart, quickly carries the oxygen-rich blood to all parts of the
body.
As the blood streams through the tissues within
microscopic vessels called capillaries, chemicals are transported between blood
and the interstitial fluid that bathes the cells.
Metabolic rate is an important factor in the
evolution of cardiovascular systems.
In general, animals with high metabolic rates
have more complex circulatory systems and more powerful hearts than animals
with low metabolic rates.
Similarly, the complexity and number of blood
vessels in a particular organ are correlated with that organs metabolic
requirements.
Perhaps the most fundamental differences in
cardiovascular adaptations are associated with gill breathing in aquatic
vertebrates compared with lung breathing in terrestrial vertebrates.
The evolution of a powerful four-chambered heart
was an essential adaptation in support of the endothermic way of life
characteristic of birds and mammals.
Endotherms use about ten times as much energy as
ectotherms of the same size.
Therefore, the endotherm circulatory system
needs to deliver about ten times as much fuel and O2 to their tissues and
remove ten times as much wastes and CO2.
Birds and mammals evolved from different reptilian ancestors, and their powerful four-chambered hearts evolved independently - an example of convergent evolution.
2) Blood pressure, exchange, and components of blood
Blood pressure is determined partly by cardiac
output and partly by peripheral resistance.
Contraction of smooth muscles in walls of
arterioles constricts these vessels, increasing peripheral resistance, and
increasing blood pressure upstream in the arteries.
When the smooth muscle relax,
the arterioles dilate, blood flow through arterioles increases, and pressure in
the arteries falls.
Nerve impulses, hormones, and other signals
control the arteriole wall muscles.
Stress, both physical and emotional, can raise
blood pressure by triggering nervous and hormonal responses that will constrict
blood vessels.
The exchange of substances between the blood and
the interstitial fluid that bathes the cells takes place across the thin
endothelial walls of the capillaries.
Some substances are carried across endothelial
cells in vesicles that form by endocytosis on one
side and then release their contents by exocytosis on
the other side.
Others simply diffuse between the blood and the
interstitial fluid across cells or through the clefts between adjoining cells.
As blood proceeds along the capillary, blood
pressure continues to drop and the capillary becomes hyperosmotic
compared to the interstitial fluids.
The resulting osmotic gradient pulls water into
the capillary by osmosis near the downstream end.
About 85% of the fluid that leaves the blood at
the arterial end of the capillary bed reenters from the interstitial fluid at
the venous end.
The remaining 15% is eventually returned to the
blood by the vessels of the lymphatic system.
Fluids and some blood proteins that leak from
the capillaries into the interstitial fluid are returned to the blood via the lymphatic
system.
Fluid enters this system by diffusing into tiny
lymph capillaries intermingled among capillaries of the cardiovascular system.
Once inside the lymphatic system, the fluid is
called lymph, with a composition similar to the interstitial fluid.
The lymphatic system drains into the circulatory
system near the junction of the venae cavae with the right atrium.
Along a lymph vessels
are organs called lymph nodes.
The lymph nodes filter the lymph and attack
viruses and bacteria.
Inside a lymph node is a honeycomb of connective
tissue with spaces filled with white blood cells specialized for defense.
When the body is fighting an infection, these
cells multiply, and the lymph nodes become swollen.
In addition to defending against infection and
maintaining the volume and protein concentration of the blood, the lymphatic
system transports fats from the digestive tract to the circulatory system.
In invertebrates with open circulation, blood (hemolymph) is not different from interstitial fluid.
However, blood in the closed circulatory systems
of vertebrates is a specialized connective tissue consisting of several kinds
of cells suspended in a liquid matrix called plasma.
The plasma includes the cellular elements (cells
and cell fragments), which occupy about 45% of the blood volume, and the
transparent, straw-colored plasma.
The plasma, about 55% of the blood volume,
consists of water, ions, various plasma proteins, nutrients, waste products,
respiratory gases, and hormones, while the cellular elements include red and
white blood cells and platelets.
Blood plasma is about 90% water.
Dissolved in the plasma are a variety of ions,
sometimes referred to as blood electrolytes,
These are important in maintaining osmotic
balance of the blood and help buffer the blood.
Also, proper functioning of muscles and nerves
depends on the concentrations of key ions in the interstitial fluid, which
reflects concentrations in the plasma.
Plasma carries a wide variety of substances in
transit from one part of the body to another, including nutrients, metabolic
wastes, respiratory gases, and hormones.
The plasma proteins have many functions.
Collectively, they acts
as buffers against pH changes, help maintain osmotic balance, and contribute to
the bloods viscosity.
Some specific proteins transport
otherwise-insoluble lipids in the blood.
Other proteins, the immunoglobins
or antibodies, help combat viruses and other foreign agents that invade the
body.
Fibrinogen proteins help plug leaks when blood
vessels are injured.
Blood plasma with clotting factors removed is
called serum.
Suspended in blood plasma are two classes of
cells: red blood cells which transport oxygen, and white blood cells,
which function in defense.
A third cellular element, platelets,
are pieces of cells that are involved in clotting.
Red blood cells, or erythrocytes, are by
far the most numerous blood cells.
Each cubic millimeter of blood contains 5 to 6
million red cells, 5,000 to 10,000 white blood cells, and 250,000 to 400,000
platelets.
There are about 25 trillion red cells in the
bodys 5 L of blood.
The main function of red blood cells, oxygen
transport, depends on rapid diffusion of oxygen across the red cells plasma
membranes.
Human erythrocytes are small biconcave disks,
presenting a great surface area.
Mammalian erythrocytes lack nuclei, an unusual
characteristic that leaves more space in the tiny cells for hemoglobin, the iron-containing protein that transports oxygen.
Red blood cells also lack mitochondria and
generate their ATP exclusively by anaerobic metabolism.
An erythrocyte contains about 250 million
molecules of hemoglobin.
Each hemoglobin molecule binds up to four
molecules of O2.
Recent research has also found that hemoglobin
also binds the gaseous molecule nitric oxide (NO).
As red blood cells pass through the capillary
beds of lungs, gills, or other respiratory organs, oxygen diffuses into the
erythrocytes and hemoglobin binds O2 and NO.
In the systemic capillaries, hemoglobin unloads
oxygen and it then diffuses into body cells.
The NO relaxes the capillary walls, allowing
them to expand, helping delivery of O2 to the cells.
There are five major types of white blood cells,
or leukocytes: monocytes, neutrophils,
basophils, eosinophils, and
lymphocytes.
Their collective function is to fight infection.
For example, monocytes
and neutrophils are phagocytes, which engulf and digest
bacteria and debris from our own cells.
Lymphocytes develop into specialized B cells and
T cells, which produce the immune response against foreign substances.
White blood cells spend most of their time
outside the circulatory system, patrolling through interstitial fluid and the
lymphatic system, fighting pathogens.
The third cellular element of
blood, platelets, are fragments of cells about 2 to 3 microns in
diameter.
They have no nuclei and originate as pinched-off
cytoplasmic fragments of large cells in the bone
marrow.
Platelets function in blood clotting.
Erythrocytes, leukocytes, and platelets all
develop from a single population of cells, pluripotent
stem cells, in the red marrow of bones, particularly the ribs, vertebrae,
breastbone, and pelvis.
Pluripotent means
that these cells have the potential to differentiate into any type of blood
cells or cells that produce platelets.
This population renews itself while replenishing
the blood with cellular elements.
A negative-feedback mechanism, sensitive to the
amount of oxygen reaching the tissues via the blood, controls erythrocyte
production.
If the tissues do not produce enough oxygen, the
kidney converts a plasma protein to a hormone called erythropoietin,
which stimulates production of erythrocytes.
If blood is delivering more oxygen than the
tissues can use, the level of erythropoietin is reduced, and erythrocyte
production slows.
3) Gas exchange
Gas exchange is the uptake of molecular
oxygen (O2) from the environment and the discharge of carbon dioxide (CO2) to
the environment.
While often called respiration, this process is
distinct from, but linked to, the production of ATP in cellular respiration.
The part of an animal where gases are exchanged
with the environment is the respiratory surface.
Movements of CO2 and O2 across
the respiratory surface occurs entirely by diffusion.
The rate of diffusion is proportional to the
surface area across which diffusion occurs, and inversely proportional to the square
of the distance through which molecules must move.
Therefore, respiratory surfaces tend to be thin
and have large areas, maximizing the rate of gas exchange.
In addition, the respiratory surface
of terrestrial and aquatic animals are moist to maintain the cell
membranes and thus gases must first dissolve in water.
3a) Gas exchange in fish
The flow pattern is countercurrent exchange.
As blood moves anteriorly
in a gill capillary, it becomes more and more loaded with oxygen, but it
simultaneously encounters water with even higher oxygen concentrations because
it is just beginning its passage over the gills.
All along the gill capillary, there is a diffusion gradient favoring the transfer of oxygen from water to blood.
3b) Gas exchange on land (introduction)
In contrast to fish, most reptiles and all birds
and mammals rely entirely on lungs for gas exchange.
Turtles may supplement lung breathing with gas
exchange across moist epithelial surfaces in their mouth and anus.
Lungs and air-breathing have evolved in a few fish species as adaptations to living on oxygen-poor water or to spending time exposed to air.
The process of breathing, the alternate
inhalation and exhalation of air, ventilates lungs.
A frog ventilates its lungs by positive
pressure breathing.
During a breathing cycle, muscles lower the
floor of the oral cavity, enlarging it and drawing in air through the nostrils.
With the nostrils and mouth closed, the floor of
the oral cavity rises and air is forced down the trachea.
Elastic recoil of the lungs, together with
compression of the muscular body wall, forces air back out of the lungs during
exhalation.
In contrast, mammals ventilate their lungs by negative
pressure breathing.
This works like a suction pump, pulling air
instead of pushing it into the lungs.
Muscle action changes the volume of the rib cage
and the chest cavity, and the lungsfollow suit.
Since the lungs do not completely empty and
refill with each breath cycle, newly inhaled air is mixed with oxygen-depleted
residual air.
Therefore, the maximum oxygen concentration in
the alveoli is considerably less than in the atmosphere.
This limits the effectiveness of gas exchange.
Ventilation is much more complex in birds than
in mammals.
Besides lungs, birds have eight or nine air sacs
that do not function directly in gas exchange, but act as bellows that keep air
flowing through the lungs.
The system in birds completely exchanges the air
in the lungs with every breath.
Therefore, the maximum lung oxygen
concentrations are higher in birds than in mammals.
Partly because of this efficiency advantage,
birds perform much better than mammals at high altitude.
For example, while human mountaineers experience tremendous difficulty obtaining oxygen when climbing the Earths highest peaks, several species of birds easily fly over the same mountains during migration.
3c) Control of gas exchange
Mammal breathing control centers are
located in two brain regions, the medulla oblongata and the pons.
Aided by the control center in the pons, the medullas center sets basic breathing rhythm,
triggering contraction of the diaphragm and rib muscles.
A negative-feedback mechanism via stretch
receptors prevents our lungs from overexpanding by
inhibiting the breathing center in the medulla.
The medullas control center monitors the CO2
level of the blood and regulated breathing activity appropriately.
Its main cues about CO2 concentration come from
slight changes in the pH of the blood and cerebrospinal fluid bathing the
brain.
Carbon dioxide reacts with water to form
carbonic acid, which lowers the pH.
When the control center registers a slight drop
in pH, it increases the depth and rate of breathing, and the excess CO2 is
eliminated in exhaled air.
Oxygen concentrations in the blood usually have
little effect of the breathing control centers.
However, when the O2 level is severely depressed
- at high altitudes, for example, O2 sensors in the aorta and carotid arteries
in the neck send alarm signals to the breathing control centers, which respond
by increasing breathing rate.
Normally, a rise in CO2 concentration is a good
indicator of a fall in O2 concentrations, because these are linked by the same process - cellular
respiration.
However, deep, rapid breathing purges the blood
of so much CO2 that the breathing center temporarily ceases to send impulses to
the rib muscles and diaphragm.
The breathing center responds to a variety of
nervous and chemical signals and adjusts the rate and depth of breathing to
meet the changing demands of the body.
However, breathing control is only effective if
it is coordinated with control of the circulatory system, so that there is a
good match between lung ventilation and the amount of blood flowing through alveolar
capillaries.
For example, during exercise, cardiac output is
matched to the increased breathing rate, which enhances O2 uptake and CO2
removal as blood flows through the lungs.
For a gas, whether present in air or dissolved
in water, diffusion depends on differences in a quantity called partial
pressure, the contribution of a particular gas to the overall total.
At sea level, the atmosphere exerts a total
pressure of 760 mm Hg.
Since the atmosphere is 21% oxygen (by volume),
the partial pressure of oxygen (abbreviated PO2) is 0.21 x 760, or about 160 mm
Hg.
The partial pressure of CO2 is only 0.23 mm Hg.
Blood arriving at the lungs via the pulmonary
arteries has a lower PO2 and a higher PCO2 than the air in the alveoli.
As blood enters the alveolar capillaries, CO2
diffuses from blood to the air within the alveoli, and oxygen in the alveolar
air dissolves in the fluid that coats the epithelium and diffuses across the
surface into the blood.
By the time blood leaves the lungs in the
pulmonary veins, its PO2 have been raised and its PCO2 has been lowered.
In the tissue capillaries, gradients of partial
pressure favor the diffusion of oxygen out of the blood and carbon dioxide into
the blood.
Cellular respiration removes oxygen from and
adds carbon dioxide to the interstitial fluid by diffusion, and from the
mitochondria in nearby cells.
After the blood unloads oxygen and loads carbon
dioxide, it is returned to the heart and pumped to the lungs again, where it
exchanges gases with air in the alveoli.
The low solubility of oxygen in water is a
fundamental problem for animals that rely on the circulatory systems for oxygen
delivery.
Most animals transport most of the O2 bound to
special proteins called respiratory pigments instead of dissolved in
solution.
Respiratory pigments, often contained within
specialized cells, circulate with the blood.
The presence of respiratory pigments increases
the amount of oxygen in the blood to about 200 mL of
O2 per liter of blood.
Like all respiratory pigments, hemoglobin must
bind oxygen reversibly, loading oxygen at the lungs or gills and unloading it
in other parts of the body.
Loading and unloading depends on cooperation
among the subunits of the hemoglobin molecule.
The binding of O2 to one subunit induces the
remaining subunits to change their shape slightly such that their affinity for
oxygen increases.
When one subunit releases O2, the other three
quickly follow suit as a conformational change lowers their affinity for oxygen.
Cooperative oxygen binding and release is
evident in the dissociation curve for hemoglobin.
Where the dissociation curve has a steep slope,
even a slight change in PO2 causes hemoglobin to load or unload a substantial
amount of O2.
As with all proteins, hemoglobins conformation
is sensitive to a variety of factors.
For example, a drop in pH lowers the affinity of
hemo-globin for O2, an effect called the Bohr
shift.
Because CO2 reacts with water to form carbonic
acid, an active tissue will lower the pH of its surroundings and induce
hemoglobin to release more oxygen.
In addition to oxygen transport, hemoglobin also
helps transport carbon dioxide and assists in buffering blood pH.
Carbon dioxide from respiring cells diffuses
into the blood plasma and then into red blood cells, where some is converted to
bicarbonate, assisted by the enzyme carbonic anhydrase.