Download Circulatory System

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.
A jellyfish is a good example of an animal that can
do without a circulatory system. Circulation occurs
within the gastrovascular cavity.
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).
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.
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.
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 wouldn’t 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
fish
&
mammals
With a 4-chambered heart there are separate circuits
that:
a)oxygenate the blood by circulating the blood
through the lungs (the pulmonary circuit) that
occupies the right side of the heart, and
b)move the blood to all other body organs (brain,
digestive system, muscles, etc.) using the systemic
circuit.
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.
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.
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:
a) feeding and f___ing – don’t 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.
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 can’t have the whole heart contracting at once.
Instead, like squeezing toothpaste from a tube,
contraction has to squeeze the blood out by:
1)beginning contraction of the atria at the top,
moving the blood into the ventricles, then
2)beginning contraction of the ventricles at the
bottom, to move blood into the pulmonary artery
and the aorta.
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
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.
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.
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.
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”
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)
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
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…
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.
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.
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.
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 won’t be allowed
to donate.
Plasma contains water, a number of salts and ions,
proteins characteristic of blood, and substances
being transported.
2) erythrocytes – these cells are unique, in that they
have no nucleus or mitochondria.
Since they can’t 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. It’s a
protein in 4 subunits (2  and 2 β chains) with a
heme in the middle.
To maximize surface area for gas exchange, each
cell looks like a doughnut from which the hole
hasn’t been completely punched out.
3) leukocytes – there are at least five types of white
blood cells, with different functions established
for four of them.
Polymorphonuclear leukocyte = neutrophil
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.
Blood clotting
Assuming you’ve 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.
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).
Here’s what the process looks like graphically and as
a series of enzymatic steps: