Circulation

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Circulation Except for the smallest, simplest
multicellular animals, all organisms need a
circulatory system of some sort. In those animals
without a circulatory system, all cells must be
close to the surface, so that simple diffusion
can bring oxygen and food molecules to cells and
carry wastes out.
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A jellyfish is a good example of an animal that
can do without a circulatory system. Circulation
occurs within the gastrovascular cavity.
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Flatworms similarly have an extensive gastric
system that moves both food and wastes. In most
species, no cell is more than a millimeter or so
from the body surface to exchange gases (O2 in,
CO2 out).
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For those that have circulatory systems, the
simpler systems are open circulatory systems.
Blood is pumped by a heart into vessels that are
open to the body cavity at their ends. Blood is
returned to the heart through pores. This is
the way circulation works in insects.
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In a closed circulatory system (like ours) blood
is pumped by a heart out to regions of the body
in vessels called arteries, which divide into
finer vessels called arterioles, then, in the
immediate neighborhood of the cells served, cells
in the blood move single-file through
capillaries. The return of blood to the heart
occurs by joining capillaries together into
venioles, then combining them into larger
vessels, the veins.
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In animals with low metabolic rates, the heart
may be only two or three chambers (which mixes
blood recently oxygenated with blood having
little oxygen. That wouldnt work for us.
Warm-blooded animals (homeotherms) need to move
far more oxygen and food into tissues, and have a
4-chambered heart to achieve that.
frogs
birds mammals
fish
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• With a 4-chambered heart there are separate
  circuits that
• oxygenate the blood by circulating the blood
  through the lungs (the pulmonary circuit) that
  occupies the right side of the heart, and
• move the blood to all other body organs (brain,
  digestive system, muscles, etc.) using the
  systemic circuit.

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Follow the numbers in the diagram to follow the
movement of the blood.
In this sequence circulation to the head and to
the lower body occurs in parallel, so that 7
and 8 are occurring at the same time, as are 9
and 10.
9
The heart beats (the muscle contracts) to move
blood through the system. There is significant
pressure in the system as the heart muscle
contracts (systole). Arteries have smooth muscle
in their walls to tolerate this pressure, then to
recover during the phase of the heartbeat when
the heart muscle is relaxed (diastole). Veins
also have smooth muscle in their walls, but the
thickness of the muscle layer is much smaller,
since the pressure in veins is lower.
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• Heart muscle contraction rate (your pulse rate)
  is regulated by both the nervous system and by
  hormones.
• The sympathetic nervous system can affect heart
  rate by accelerating pulse rate. The sympathetic
  system has two parts, commonly named by their
  actions
• feeding and f___ing dont accelerate heart
  rate
• b) fighting or fleeing these do produce an
• acceleration of heart rate epinephrine is
  released
• c) There is also a CO2 sensor in the brain that
• accelerates heart rate when exercise
  increases
• blood CO2 level.

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• The heartbeat is initiated by a natural
  pacemaker, a batch of nerve cells located in the
  sino-atrial node that fire automatically at a
  basal rate.
• You cant have the whole heart contracting at
  once. Instead, like squeezing toothpaste from a
  tube, contraction has to squeeze the blood out
  by
• beginning contraction of the atria at the top,
  moving the blood into the ventricles, then
• beginning contraction of the ventricles at the
  bottom, to move blood into the pulmonary artery
  and the aorta.

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That pattern is logical for the atria, but to get
the ventricles to begin at the bottom there are
special muscle fibers, called the bundle of His,
that carry the excitation from the atria down to
the base of the heart, then spread over the
ventricles as Purkinje fibers.
bundle of His
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The electrical activity of heart muscle cells is
evident at the skin, and is recorded in an EKG
(an electrocardiogram by doctors (the K comes
from the original German). Changes in the shape
of the wave are indicators of change in heart
function, e.g. a heart attack. Heart muscle cells
die. Why do heart attacks occur? Because blockage
occurs in an artery supplying the heart muscle.
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The blockage may develop in the cardiac artery
itself, due to buildup of atherosclerotic plaque,
or may be a clot formed within the vessel. The
picture shows both in the same artery
We all have some plaque in our arteries. The
object is to limit the amount of buildup, e.g.
through a low cholesterol diet and exercise.
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The same sort of blockage, preventing blood flow
to a part of the brain, is what we call a
stroke. A different problem can result if blood
pressure rises to high enough pressures that
arteries fail, and the rupture leaks blood into
surrounding tissues. The set of problems that
result from circulatory problems are collectively
called cardiovascular disease, and accounts for
40 of deaths in North America.
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Normal blood pressure for a healthy young adult
averages about 120/80 (a systolic pressure of 120
mm Hg, and a diastolic pressure of 80 mm
Hg). Systolic pressures from 130 to 140 are now
called high normal. Blood pressures above 140
systolic or above 90 diastolic are high.
Essential hypertension can be treated by 1)
Increasing kidney output, draining fluid from
the system 2) Adjusting the strength of the
heartbeat with drugs called blockers
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Blood pressure varies quite a bit over the course
of a day, by as much as 20-30 mm Hg without any
obvious cause, and by more under stress. The
occurrence of hypertension also varies. Factors
such as 1) sex (men at younger age and women at
higher age more frequently show
hypertension) 2) race (blacks have a higher
frequency of hypertension than other
races) 3) genetics (the propensity for
hypertension seems to run in families) 4)
lifestyle (sedentary jobs/lifestyle are more
prone to hypertension)
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Blood moves through the circulatory system in
blood vessels (arteries, arterioles, capillaries,
venioles, veins) in what we call a closed system.
How, then, do gases, food, and waste get
exchanged between tissue cells and the
circulatory system? The answer comes from a)
designed-in leakiness and b) the balance between
blood pressure and osmotic pressure
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Leakiness water, salts and sugar simply leak
out into the tissue fluid through the clefts
between cells.
The gases (O2 and CO2) simply diffuse through the
capillary walls. There are, however, two other
components to substance movement
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Some molecules too large to pass through the gaps
between capillary wall cells are moved actively
from the lumen of the capillary into cells
forming the epithelial wall by endocytosis
(engulfed into a vesicle of the cell), then moved
out into tissue fluid by exocytosis (emptied out
of the vesicle when it joins the outside cell
membrane). The balance between blood and osmotic
pressure also moves materials. At the arterial
end of a capillary, blood pressure is higher than
osmotic pressure, so materials are pushed out.
At the venous end, osmotic pressure is greater
than blood pressure, and the net movement is
inwards.
21
Not all fluid leaving capillaries can be
recollected by this pressure difference. A second
system of vessels, called the lymphatic system,
also returns fluid to circulation by joining the
large veins (particularly the superior and
inferior vena cava) near the heart.
22
There are four major components of blood 1)
plasma 2) red blood cells (erythrocytes) 3) white
blood cells (leukocytes) 4) platelets Both plasma
and leukocytes are more complex than a single
term suggests.
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1) plasma constitutes slightly more than ½ of
normal blood volume. A measure called the
hematocrit tells you what fraction of blood
is (mostly) red blood cells. If you donate
blood, you will remember having your finger
poked and a droplet of blood tested. If your
hematocrit is too low, you wont be allowed to
donate. Plasma contains water, a number of
salts and ions, proteins characteristic of
blood, and substances being transported.
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2) erythrocytes these cells are unique, in that
they have no nucleus or mitochondria. Since they
cant repair themselves, they have a limited
lifespan (100 120 days). Defective cells are
broken down in the liver and spleen. The heme
(iron) is mostly recycled back into bone marrow
to build new erythrocytes. The oxygen-carrying
molecule is hemoglobin. Its a protein in 4
subunits (2 ? and 2 ß chains) with a heme in the
middle.
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To maximize surface area for gas exchange, each
cell looks like a doughnut from which the hole
hasnt been completely punched out.
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3) leukocytes there are at least five types of
white blood cells, with different functions
established for four of them.
Polymorphonuclear leukocyte neutrophil
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a. basophils fight infection by releasing
chemicals (e.g. histamine) that are part of
the inflammation process b.neutrophils and
monocytes both types move out of the
capillaries and act as phagocytes, engulfing
(eating) foreign bacteria and proteins that
have entered through wounds. They die in the
process. c.lymphocytes are the key cells in
the immune response (next lecture).
d.eosinophil gets its name from staining with
eosin, but its functions are not well
understood. They seem to be released when
parasitic infection occurs, and release
chemicals that kill the invaders.
29
Blood clotting Assuming youve been injured and
have a bleeding wound, there are two basic parts
to the initial healing 1) initially stopping the
flow of blood by plugging the hole 2)
constructing a longer lasting clot (a
scab). Phase 1 is a response of the platelets
when connective tissue (collagen) is exposed. The
platelets at the site of injury adhere to the
collagen and release clotting factors that cause
more platelets to become sticky and together form
a plug.
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In phase 2 the clotting factors initiate a chain
of reactions that result in a clot. The key
clotting factor (which is in circulation in the
tissue fluid, as well released by platelets) is
thromboplastin. Thromboplastin (with Ca) acts
as a catalyst for the conversion of prothrombin
to thrombin. Thrombin, in turn, acts as a
catalyst in the conversion of fibrinogen to
fibrin (whose name suggests what it is like
fibers that trap erythrocytes, other cells, and
debris to form the clot (scab).
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Heres what the process looks like graphically
and as a series of enzymatic steps