AP
Biology
Notes: Circulation/Respiration
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).
Circulation
in Animals:
Transport systems functionally connect the organs of
exchange with the body cells. 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.
Three ways of increasing effesincey of diffusion:
Gastrovascular cavities
Open circulatory system
Closed Circulatory system
Gastrovascular
Most
invertebrates have a gastrovascular
cavity or a circulatory system for internal transport.
The body plan of a hydra and other cnidarians
makes a circulatory system unnecessary.
A body wall only
two cells thick encloses a central
gastrovascular cavity that serves for both digestion
and for diffusion of substances throughout the body.
Cnidarians:
The
mouth leads to an elaborate gastrovascular cavity that has
branches radiating to
and from the circular canal. The products
of
digestion in the gastrovascular cavity are directly available to the
cells of the inner layer, and it is
only a short distance to diffuse to
the cells of the outer layer.
Planarians:
Planarians and most other flatworms also have
gastrovascular cavities that exchange materials
with the environment through a single opening.
The flat shape of the body and the branching of
the gastrovascular cavity throughout the animal ensure that
cells are bathed by a suitable medium
and that diffusion distances are short.
For animals with many cell layers, gastrovascular cavities
are insufficient for internal distances because the diffusion transports are too
great.
Open Circulaaory system:
In insects, other arthropods, and most mollusks, blood bathes organs
directly in an open
circulatory system. There is
no distinction between blood and interstitial fluid, collectively
called
hemolymph. One or more hearts pump the hemolymph into
interconnected sinuses
surrounding the organs, allowing exchange
between hemolymph \and body cells.In insects
and other arthropods, the heart is an elongated dorsal tube.
When the heart contracts, it pumps
hemolymph through vessels out into sinuses. When the heart relaxes, it draws
hemolymph into
the circulatory system through pores called ostia.
Body movements that squeeze the sinuses
help
circulate the hemolymph.
Open Circulaaory system:
In
a closed circulatory system, as found in earthworms, squid, octopuses, and
vertebrates, blood is
confined
to vessels and is distinct from the interstitial fluid. One or more hearts pump
blood into
large vessels that branch into smaller ones moving through organs.
Materials are exchanged by
diffusion between the blood and the interstitial fluid bathing the cells.
Vertebrate phylogeny is reflected in adaptations of
the cardiovascular system. The
closed circulatory
system of humans and other vertebrates is often called the cardiovascular
system.
The heart consists of one atrium or
two atria, the chambers that receive blood returning to the heart,
and one or two ventricles, the chambers that pump blood out of the heart.
Arteries,
veins, and capillaries are the three main kinds of blood vessels.
Arteries carry blood away
from the heart to organs. Within organs, arteries branch into arterioles,
small vessels that convey
blood to capillaries. Capillaries with very thin, porous walls form networks,
called capillary beds, that
infiltrate each tissue.
At
their “downstream” end, capillaries converge into venules, and venules
converge into veins,
which return blood to the heart. Arteries and veins are distinguished by the
direction in which they
carry blood, not by the characteristics of the blood they carry. All arteries
carry blood from the heart
toward capillaries. Veins return blood to the heart from capillaries.
Fish:
A fish
heart has two main chambers, one atrium 
and one ventricle. Blood is pumped from the
ventricle to the gills (the gill circulation) where it
picks up oxygen and disposes of carbon dioxide
across the capillary walls. The
gill capillaries
converge into a vessel that carries oxygenated blood
to capillary beds in the other organs and back to the
heart.
Blood must pass through two capillary
beds, the gill capillaries and systemic capillaries.
When blood flows through a capillary bed, blood
pressure—the motive force for circulation—drop
Substantially.
Therefore, oxygen-rich blood leaving
the gills flows to the systemic circulation quite slowly
Amphibians:
Frogs and other amphibians have a three-chambered
heart with two atria and one ventricle.
The
ventricle pumps blood into a forked artery that splits the ventricle’s output.
into the
pulmocutaneous and systemic
circulations. The
pulmocutaneous circulation leads to capillaries
in the
gas-exchange organs (the lungs and skin of a frog),
where the blood picks up O2 and releases
CO2
before returning to the heart’s left atrium.
Most of the returning blood is pumped into the systemic
circulation, which supplies all body organs and then returns oxygen-poor blood
to the right atrium
via the
veins. This scheme, called double circulation, provides a vigorous flow of blood
to the brain,
muscles, and other organs because the blood is pumped a second time after it
loses pressure in the
capillary beds of the lung or skin
Reptiles:
Reptiles
also have double circulation with pulmonary (lung) and systemic circuits.
However,
there is even less mixing of oxygen-rich and oxygen-poor blood than in
amphibians. Although
the reptilian heart is three-chambered, the ventricle is partially divided.
In crocodilians, birds, and mammals, the ventricle is
completely divided into separate right and
left chambers. In this arrangement,
the left side of the heart receives and pumps only oxygen-rich
blood, while the right side handles only oxygen-poor blood.
Double circulation restores
pressure to the systemic circuit and prevents mixing of oxygen-rich
and oxygen-poor blood. 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.
Mammals:
In
the mammalian cardiovascular system, the pulmonary and system circuits operate
simultaneously.
The two ventricles pump almost in unison
While some blood is traveling in the pulmonary circuit,
the rest of the blood is flowing in the systemic circuit.
The pulmonary circuit carries blood from the heart to the lungs and back
again.
(1)
The right
ventricle pumps blood to the lungs via
(2)
the
pulmonary arteries. As blood
flows through
(3)
capillary
beds in the right and left lungs, it loads O2 and unloads CO2. Oxygen-rich
blood returns from the lungs via the
pulmonary veins to
(4)
the
left atrium of the heart. Next, the oxygen-rich blood flows to
(5)
the
left ventricle, as the ventricle opens and the atrium contracts.
The left ventricle pumps
oxygen-rich blood out to the body tissues through the systemic circulation.
Blood leaves the left ventricle via
(6)
the
aorta, which conveys blood to arteries leading throughout the
body.
The first branches from the aorta are the
coronary arteries, which supply blood
to the heart muscle. The next branches
lead to capillary beds
(7)
in
the head and arms. The aorta
continues in a posterior direction, supplying
oxygen-rich blood to arteries leading to
(8)
arterioles
and capillary beds in the abdominal organs and legs.
Within the capillaries,
blood gives up much of its O2 and
picks up CO2 produced by cellular respiration.
Venous return to the right side of the
heart begins as capillaries rejoin to form venules
and then veins. Oxygen-poor blood from the
head, neck, and forelimbs is channeled into
a large vein called
(9)
the
anterior (or superior) vena cava. Another large vein called the
(10)
posterior
(or inferior) vena cava drains blood from the trunk and hind limbs.
The two venae cavae empty their blood
into
(11)
the right
atrium, from which the oxygen-poor blood flows
into the right ventricle.
Between
each atrium and ventricle is an atrioventricular (AV) valve which keeps blood
from flowing
back into the atria when the ventricles contract. Two sets of semilunar
valves, one between the left
ventricle and the aorta and the other between the right ventricle and the
pulmonary artery, prevent
backflow from these vessels into the ventricles while
they are relaxing.
The
heart sounds we can hear with a stethoscope are caused by the closing of the
valves.
Cells
of the heart:
Certain cells of vertebrate cardiac muscle are
self-excitable, meaning they contract without any
signal from the nervous system. Each
cell has its own intrinsic contraction rhythm.
However,
these cells are synchronized
by the sinoatrial (SA) node, or pacemaker, which sets the rate
and timing at which all cardiac muscle cells
contract. The SA node is located in the wall of the
right atrium. The cardiac
cycle is regulated by electrical impulses that radiate throughout the heart.
Cardiac muscle cells are electrically coupled by intercalated disks
between adjacent cells.
Arteries,
veins, and capillaries:
All
blood vessels are built of similar tissues. The walls of both arteries and veins
have three
similar layers. On the outside, a layer of connective tissue with elastic fibers
allows the vessel to
stretch and recoil. A middle layer
has smooth muscle and more elastic fibers. Lining the lumen of
all blood vessels, including capillaries, is an endothelium, a single layer of
flattened cells that minimizes
resistance to blood flow.
Structural differences correlate
with the different functions of arteries, veins, and capillaries.
Capillaries lack the two outer layers and their very thin walls consist of
only endothelium thus
enhancing exchange. Arteries have thicker middle and outer layers than
veins. The thicker walls
of arteries provide strength to accommodate blood pumped rapidly and at high
pressure by the heart.
Their elasticity (elastic recoil) helps maintain blood pressure even when
the heart relaxes. The
thinner-walled veins convey blood back to the heart at low velocity and
pressure. Blood flows
mostly as a result of skeletal muscle contractions when we move that squeeze
blood in vein Within
larger veins, flaps of tissues act as one-way valves that allow blood to flow
only toward the heart.
Blood:
Blood is a connective tissue with cells suspended in
plasma 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.
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,
an 250,000 to 400,000
platelets. There are about 25
trillion red cells in the body’s 5 L of blood.
Red Blood cells (erythrocyte):
The main function of red blood cells, oxygen
transport, depends on rapid diffusion of oxygen
across the red cell’s 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 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.
White
Blood cells(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.
Platelets
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 bonemarrow. Platelets function in
blood clotting.
The
cellular elements of blood wear out and are replaced constantly throughout a
person’s life.
For example, erythrocytes usually circulate for only about 3 to 4 months and are
then destroyed by
phagocyte cells in the liver and spleen. Enzymes digest the old cell’s
macromolecules, and the
monomers are recycled. Many
of the iron atoms derived from hemoglobin in old red blood cells are
built into new hemoglobin molecules.
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.
Blood
contains a self-sealing material that plugs leaks from cuts and scrapes. A clot
forms when
the inactive form of the plasma protein fibrinogen is converted to fibrin, which
aggregates into threads that form the framework of
the clot. The clotting mechanism
begins with
the release of clotting factors from platelets. An inherited defect in any step of the clotting
process causes hemophilia, a disease characterized by excessive bleeding
from even minor cuts
and bruises.
(1) The clotting process begins when the endothelium
of a vessel is damaged and connective tissue
in the wall is exposed to blood. Platelets
adhere to collagen fibers and release a substan
ce that makes
nearby platelets sticky.
(2) The platelets form a plug.
(3) The seal is reinforced by a clot
of
fibrin when vessel damage is severe.
Anticlotting
factors in the blood normally prevent spontaneous clotting in the absence of
injury.
Sometimes,
however, platelets clump and fibrin coagulates within a blood vessel, forming a
clot
called a thrombus, and
blocking the flow of blood.