The Path of Blood through the Human Body
When a heart contracts and forces blood into the blood vessels, there is a certain path that
the blood follows through the body. The blood moves through pulmonary circulation and
then continues on through systemic circulation. Pulmonary and systemic are the two
circuits in the two-circuit system of higher animals with closed circulatory systems.
Humans and other mammals have two-circuit circulatory systems: one circuit is for
pulmonary circulation (circulation to the lungs; pulmo = lungs), and the other circuit is
for systemic circulation (the rest of the body). As each atrium and ventricle contract,
blood is pumped into certain major blood vessels, and from there, continues through the
circulatory system.
Pulmonary circulation
Blood that is lacking oxygen is said to be deoxygenated. This blood has just exchanged
oxygen for carbon dioxide across cell membranes, and now contains mostly carbon
dioxide. Deoxygenated blood enters the right atrium through the superior vena cava and
the inferior vena cava.
Superior means higher, and inferior means lower, so the superior vena cava is at the top
of the right atrium, and the inferior vena cava enters the bottom of the right atrium.
From the right atrium, the deoxygenated blood drains into the right ventricle through the
right atrioventricular (AV) valve, which is so named because it is between the atrium and
the ventricle. This valve is also referred to as the tricuspid valve because it has three flaps
in its structure. When the ventricles contract, the AV valve closes off the opening
between the ventricle and the atrium so that blood does not flow back up into the atrium.
As the right ventricle contracts, it forces the deoxygenated blood through the pulmonary
semilunar valve and into the pulmonary artery. Semilunar means half-moon and refers to
the shape of the valve. Note that this is the only artery in the body that contains
deoxygenated blood; all other arteries contain oxygenated blood. The semilunar valve
keeps blood from flowing back into the right ventricle once it is in the pulmonary artery.
The pulmonary artery carries the blood that is very low in oxygen to the lungs, where it
becomes oxygenated.
Systemic circulation
Freshly oxygenated blood returns to the heart via the pulmonary veins. Note that these are
the only veins in the body that contain oxygenated blood; all other veins contain
deoxygenated blood.
The pulmonary veins enter the left atrium. When the left atrium relaxes, the oxygenated
blood drains into the left ventricle through the left AV valve. This valve is also called the
bicuspid valve because it has only two flaps in its structure.
Now the heart really squeezes. As the left ventricle contracts, the oxygenated blood is
pumped into the main artery of the body — the aorta. To get to the aorta, blood passes
through the aortic semilunar valve, which serves to keep blood flowing from the aorta
back into the left ventricle.
The aorta branches into other arteries, which then branch into smaller arterioles. The
arterioles meet up with capillaries, which are the blood vessels where oxygen is
exchanged for carbon dioxide.
Capillary exchange
Capillaries bridge the smallest of the arteries and the smallest of the veins. Near the
arterial end, the capillaries allow materials essential for maintaining the health of cells to
diffuse out (water, glucose, oxygen, and amino acids).
To maintain the health of cells, it is also necessary for the capillaries to transport wastes
and carbon dioxide to places in the body that can dispose of them. The waste products
enter near the venous end of the capillary. Water diffuses in and out of capillaries to
maintain blood volume, which adjusts to achieve homeostasis.
Capillaries are only as thick as one cell, so the contents within the cells of the capillaries
can easily pass out of the capillary by diffusing through the capillary membrane. And,
because the capillary membrane abuts the membrane of other cells all over the body, the
capillary’s contents can easily continue through the abutting cell’s membrane and get
inside the adjoining cell.
The process of capillary exchange is how oxygen leaves red blood cells in the
bloodstream and gets into all the other cells of the body. Capillary exchange also allows
nutrients to diffuse out of the bloodstream and into other cells. At the same time, the
other cells expel waste products that then enter the capillaries, and carbon dioxide
diffuses out of the body’s cells and into the capillaries.
After the capillaries “pick up” the garbage from other cells, the capillaries carry the
wastes and carbon dioxide through the deoxygenated blood to the smallest of the veins,
which are called venules. The venules branch into bigger vessels called veins. The veins
then carry the deoxygenated blood toward the main vein, which is the vena cava. The two
branches of the vena cava enter the right atrium, which is where pulmonary circulation
begins.
Rule #1: All Arteries flow Away from the heart.
There are many, many organs in the body. Each has its own artery. For some, it is easy to
identify the organ for which they are intended - like the pancreatic artery feeds the
pancreas. Others are a little more challenging to remember:
pulmonary artery  lungs
gastric artery  stomach
hepatic artery  liver
renal artery  kidney
coronary artery  heart
carotid artery  brain
Aorta  Almost all arteries branch from this major blood highway (exception =
pulmonary artery)
Rule #2: Veins always lead to the heart.
Once the artery reaches its target destination, you will notice that it divides and
subdivides, like the branches of a tree. This is so that the oxygen rich blood is undiluted
before it reaches every nook and cranny of the receptive organ. Arteries divide into
arterioles which divide into capillaries. The fascinating transfer work happens here in the
capillaries. Imagine your right hand is the capillary end of an arteriole (your right arm).
Lace your 2 hands together. Your left hand represents the capillary end of a Vein. Here is
where the oxygen is discharged and the empty hemoglobin containers are recycled back
towards the heart on their path to re-filling. Once the oxygen portion is used, the blood
tends to lose its bright red colour. Venous blood (blood found in veins) tends to be darker
in appearance. It is also more sluggish in speed as it is now so far away from the heart,
and using your imagination and the diagram above, it also has to travel 'up-hill.' Veins are
lined with little valves, or shelves, that help this slow stuff from sliding 'backwards' along
the road back to the heart. Blood from an artery would literally spurt to a beat if it were
inadvertently opened. Most of these 'V' words (veins, venules, venous, vena...) all stem
from the Latin verb "venire," meaning 'to come.'
Rule #3: All Arteries carry Oxygenated Blood EXCEPT the Pulmonary Artery
2 Major Arteries: Pulmonary Artery and the Aorta
The Pulmonary Artery carries blood away from the heart and to the lungs. Once it
arrives, fresh, oxygenated blood plumps up the hemoglobin, like filling a gas tank at a gas
station pump. Now it continues back to the heart's clover-leaf for distribution first to the
coronary arteries and then on to the rest of the body.
Oxygenated blood flows out of the heart via the Aorta - the Mother of all arteries and also
the largest. When you see a diagram of the circulatory system you'll note that on exiting
the Left Ventricle, this blood vessel is called the Ascending Aorta. The next portion,
giving rise to multiple branches, is called the Arch (not unlike the golden McDonald's
arches, only upside down). These tributaries are: a) the Carotid artery, which supplies
blood to the head and neck. b) the Subclavian arteries, which supply blood to the upper
chest and arms, c) the Coronary arteries, which supply the heart itself with oxygen rich
blood. Once past these exits, the Aorta dives down in direction and is aptly named the
Descending Aorta.
Rule #4 All Veins carry de-oxygenated blood EXCEPT the Pulmonary Vein
Circulatory System
All animals must exchange materials with their environment, including nutrients and
wastes, O2, CO2, etc., thus need a system that will do this. The more complex the
organism, the more complex this system must be. Arthropods, like insects and spiders,
have an open circulatory system, in which the blood is pumped forward by the heart,
but then flows through the body cavity, directly bathing the internal organs. Vertebrates,
like humans, have a closed circulatory system in which the blood stays in the circulatory
system as it circulates, and chemicals are exchanged by diffusion. Our system is also
called our cardiovascular system, and is composed of our heart plus our arteries and
veins. In a person’s heart, the atria (plural of atrium) receive blood from the veins and
the ventricles send blood to the arteries. As the arteries become more finely divided, they
are called arterioles. The finest divisions of our vascular system are called capillaries.
As the vessels get larger again, the smallest are called venules which join and enlarge to
form veins. Note that the distinction between arteries and veins is by direction of blood
flow, not oxygen content. Veins carry blood toward the heart and arteries carry it away
from the heart. Because of this, not all arteries carry oxygenated blood. The two major
exceptions, in which arteries are carrying deoxygenated blood are the pulmonary artery
which carries deoxygenated blood from the heart to the lungs (to pick up oxygen there)
and the umbilical arteries which carry deoxygenated blood away from the baby’s body to
the placenta (to pick up oxygen there). We have double circulation: we have a separate
pulmonary circuit to the lungs and a systemic circuit to the body.
1. The superior (a) and inferior (b) vena cava are the main veins that receive blood
from the body. The superior vena cava drains the head and arms, and the inferior
vena cava drains the lower body.
2. The right atrium receives blood from the body via the vena cavae. The atria are
on the top in the heart.
3. The blood then passes through the right atrioventricular valve, which is forced
shut when the ventricles contract, preventing blood from reentering the atrium.
4. The blood goes into the right ventricle (note that it has a thinner wall; it only
pumps to lungs). The ventricles are on the bottom of the heart.
5. The right semilunar valve marks the beginning of the artery. Again, it is
supposed to close to prevent blood from flowing back into the ventricle.
6. The pulmonary artery or pulmonary trunk is the main artery taking
deoxygenated blood to the lungs.
7. Blood goes to the right and left lungs, where capillaries are in close contact with
the thin-walled alveoli so the blood can release CO2 and pick up O2.
8. From the lungs, the pulmonary vein carries oxygenated blood back into the heart.
9. The left atrium receives oxygenated blood from the lungs.
10. The blood passes through the left atrioventricular valve.
11. The blood enters the left ventricle. Note the thickened wall; the left ventricle
must pump blood throughout the whole body.
12. The blood passes through the left semilunar valve at the beginning of the aorta.
13. The aorta is the main artery to the body. One of the first arteries to branch off is
the coronary artery, which supplies blood to the heart muscle itself so it can
pump. The coronary artery goes around the heart like a crown. A blockage of the
coronary artery or one of its branches is very serious because this can cause
portions of the heart to die if they don’t get nutrients and oxygen. This is a
coronary heart attack. From the capillaries in the heart muscle, the blood flows
back through the coronary vein, which lies on top of the artery.
14. The aorta divides into arteries to distribute blood to the body.
15. Small arteries are called arterioles.
16. The smallest vessels are the capillaries.
17. These join again to form venules, the smallest of the veins.
18. These, in turn, join to form the larger veins, which carry the blood back to the
superior and inferior vena cava.
The atrioventricular and semilunar valves prevent backflow as the heart contracts.
Defects in any of these that allow some blood to leak backwards cause distinctive sounds
through a stethoscope, thus are called heart murmurs.
The sinoatrial node controls the heart beat. This natural pacemaker is located in the
upper wall of the right atrium, and is composed of muscle tissue that sends electrical
impulses to the rest of both atria to contract. The impulse then spreads to the ventricles,
causing them to contract. The heart cycle involves three phases:
1. The atria contract and force blood into the ventricles. If the atria don’t contract,
this is called atrial fibrillation and pooled blood in the atria can begin to clot.
When the atria start beating normally again, these clots may be sent throughout
the person’s system. If one of these clots lodges in an arteriole somewhere, it
could cause a stroke, heart attack, or similar problem. As blood is pushed into the
ventricles, when the A-V valves close, the ventricular walls vibrate a little casuing
the first sound of the heart beat, the “lubb” sound.
2. The ventricles contract and force blood into the arteries. This is called systole and
the systolic blood pressure (BP) is the higher of the two numbers, when the heart
is actively contracting and putting pressure on the blood. When the semilunar
valves snap shut, this causes the second sound of the heart beat, the “dup.”
3. The heart relaxes and blood flows into the atria and ventricles. This is called
diastole. The diastolic BP is the lower of the two numbers, when the heart is
relaxed, and so, is a measure of how much pressure the arteries, themselves, are
putting on the blood. Clogged arteries are less elastic, so the blood is under more
pressure, thus more likely to cause the arteries to burst.
The rate of contraction is the heart rate. A baby’s heart starts beating when it is about
four weeks old (the mother’s period is two weeks late, and she’s just beginning to suspect
she might be pregnant). A newborn’s heart rate is around 135 to 140 beats per minute
(bpm). By age 15 to 30, the rate decreases to about 65-75 bpm, then speeds up slightly as
the person ages. The pulse is a wave of contraction of the artery walls (which roughly
corresponds to the heart rate) as blood is forced into the arteries. Pulse is usually
measured using the radial artery (the one along the radius). To find your pulse, rest your
right arm in the palm of your left hand. Curl the fingers of your left hand up around the
thumb side of your right wrist. Place several fingers of your left hand along and just to
the outside (thumb side) of the tendon that runs along your wrist. With gentle pressure,
you should be able to feel your pulse.
Blood pressure is maximum during systole, when the heart is pushing, and minimum
during diastole, when the heart is relaxed. In a living person, the blood pressure doesn’t
go to zero because the thick, elastic artery walls exert pressure on the blood. A
sphygmomanometer is the instrument used to determine BP. The artery used to
determine BP is the brachial artery, which runs down the upper arm, splitting into the
radial and ulnar arteries near the elbow. The cuff of the sphygmomanometer is wrapped
around the arm just above the elbow and pumped up to block off blood flow (the pressure
exerted by the cuff is higher than the systolic pressure). The pressure in the cuff is
gradually decreased, and when it equals the person’s systolic pressure, the heart can force
blood under the cuff, and a sound is heard as the pulses of blood surge under the cuff. As
the pressure in the cuff is lowered, when it equals the diastolic pressure, blood can flow
freely, so the sound disappears (not enough pressure is exerted by cuff to restrict blood
flow). Thus, by listening for the first sound, and when the sound becomes faint, while
watching the pressure indicator on the sphygomomanometer, it is possible to determine
someone’s blood pressure. Typically, when you go to the doctor’s office, one of the first
things that is done to you is that someone (a nurse?) takes your blood pressure. I have
frequently had the experience that when I ask what the results were, I initially get the
answer “It’s OK.” Here’s a tip: you, not they, are in charge of your health. The only way
you can educate yourself to how your body works is to keep re-asking the question until
you get a real answer. You need to know the actual numbers to be able to evaluate if
things have changed or are good or bad. Be persistent and eventually they’ll tell you what
your BP is.
(clipart edited from Corel Presentations 8)
A neonate’s BP is around 80/45 mm Hg meaning that the systolic pressure is equivalent
to air pressure that will support a column of mercury 80 millimeters high in a barometer,
and the diastolic is equivalent to the air pressure that will support a column of mercury 45
millimeters high. For adults in their 20s, 120/80 mm Hg is considered average for a male
and 115/75 mm Hg for a female, thus the accepted average is said to be 120/80 mm Hg.
With age, the arteries become less elastic (due in part to undesirable lipid deposits in their
walls), so the BP rises. Hypertension is when the BP is too high. There are two ways this
could happen: either the systolic pressure is greater than 145 to 160 mm Hg and/or the
diastolic is greater than 90 to 100. Major contributing factors include the amounts of salt,
cholesterol (and other lipids), and sugar in one’s diet and the amount of exercise the
person gets. Frequently, diuretics are prescribed to try to remove water from the person’s
blood, thus lowering the blood volume and hopefully thereby, the BP. However, many
diuretics also remove potassium (and other beneficial minerals?) from the person’s
system, and if serum potassium levels are not carefully monitored and go to low, this
could cause a heart attack!
A thrombus is a blood clot (platelets and fibrin) which forms within a vessel and blocks
the blood flow. These can result from surgery or from conditions like atrial fibrillation.
An embolus is a moving thrombus which may “get stuck” somewhere. If thrombi or
emboli lodge in an artery supplying blood to the heart, this can cause a coronary
embolism or heart attack or myocardial infarction. If one of these becomes lodged in an
artery in the lungs, it is also a life-threatening pulmonary embolism, and if in the brain,
a cerebral (or cerebellar) embolism or stroke or cerebrovascular accident (CVA).
A hemorrhage is bleeding, especially profuse, and can be severe if internal.
A hematoma is a local swelling or tumor filled with blood; a bruise, especially a large
one. Sometimes, if the injury is extensive, it can calcify as it heals, leading to a hard lump
(which may need to be surgically removed).
Hemorrhoids are dilated or varicose veins in the anal area. Typically, these are caused
not enough fiber in diet causing the feces to be very hard so the person has to strain to
pass them. Increasing the amount of fiber in one’s diet can help prevent hemorrhoids and
possibly aid in healing mild cases. Because vitamin C is necessary for collagen synthesis,
it is necessary for strong capillary walls (one of the first signs of a vitamin C deficiency is
easy bruising), so that and the bioflavonoid rutin (found in buckwheat) have proven
useful for strengthening blood vessels and preventing/treating hemorrhoids and other
varicose veins.
Edema is an accumulation of fluid (plasma) within tissues and/or the lymph system.
There are many possible causes of edema from injury, to too much salt, to improperly
functioning kidneys, to lack of exercise, to female hormonal changes, to a number of
other possible causes. If in doubt, see a doctor.
Much like a heat pump for your house or your refrigerator coils, your cardiovascular
system is also involved in countercurrent heating/cooling of your body. Arteries and
veins lying near each other in your extremities, but flowing in opposite directions can
absorb heat from each other as needed. When your core temperature is too high, the
arteries carry heat to the extremities to be dissipated. As the blood returns via the veins,
any excess heat still in the blood is transferred to the arterial blood and sent to the
extremities, again. When your core temperature is too low, as the blood flows out in the
arteries to nourish the extremities, its heat is transferred to the venous blood and sent
back into the body to keep it warm.
In Raynaud’s Phenomenon, when the person (more common in women than men) gets
cold, spasms in the tiny arteriole muscles cause the circulation in portions of the fingers
or toes to completely “turn off,” and that portion of the finger/toe turns completely white.
As the person warms up and circulation is restored, initially these areas of the fingers/toes
will be cyanotic (blue), then will be flushed and red, before returning to normal. The
Merck Manual suggests that there may be a relationship between migraine headaches and
Raynaud’s. Diagnosis is confirmed by testing the blood pressure in not only the brachial
artery, but also the radial and ulnar arteries, and using tiny cuffs made of Velcro® and
aquarium tubing, each finger, both when the person is comfortably warm and when the
person’s hands have been soaking in ice water. People with Raynaud’s need to make sure
to wear warm mittens and heavy socks in winter weather, and since much heat is lost
from our heads, wearing a scarf or hat can actually help to keep the person’s whole body
warm and lessen the chances of a Raynaud’s episode in the fingers/toes!
Hardening of the arteries is also called arteriosclerosis, a generic term for a number of
diseases in which the artery walls become thickened and lose elasticity. One special form
of this is atherosclerosis which is a build-up of lipids on the inside of blood vessels.
Major risk factors for atherosclerosis include hypertension, elevated serum lipids,
elevated LDL (low-density lipoproteins, the bad guys) and lowered HDL (high-density
lipoproteins, the good guys), smoking, diabetes, obesity, male sex, and family history.
Female hormones offer protection against accumulation of arterial plaque, so usually,
premenopausal women do not have as many problems with this as men do. However,
after menopause, lipids will start to accumulate. I once hear a statistic that the average
55-year-old woman has a build-up equivalent to the average 18-year-old man.
Blood |
Plasma is the liquid component of the blood. Mammalian blood consists of a liquid
(plasma) and a number of cellular and cell fragment components as shown in Figure 21.
Plasma is about 60 % of a volume of blood; cells and fragments are 40%. Plasma has
90% water and 10% dissolved materials including proteins, glucose, ions, hormones, and
gases. It acts as a buffer, maintaining pH near 7.4. Plasma contains nutrients, wastes,
salts, proteins, etc. Proteins in the blood aid in transport of large molecules such as
cholesterol.
Red blood cells, also known as erythrocytes, are flattened, doubly concave cells about 7
µm in diameter that carry oxygen associated in the cell's hemoglobin. Mature
erythrocytes lack a nucleus. They are small, 4 to 6 million cells per cubic millimeter of
blood, and have 200 million hemoglobin molecules per cell. Humans have a total of 25
trillion red blood cells (about 1/3 of all the cells in the body). Red blood cells are
continuously manufactured in red marrow of long bones, ribs, skull, and vertebrae. Lifespan of an erythrocyte is only 120 days, after which they are destroyed in liver and
spleen. Iron from hemoglobin is recovered and reused by red marrow. The liver degrades
the heme units and secretes them as pigment in the bile, responsible for the color of feces.
Each second two million red blood cells are produced to replace those thus taken out of
circulation.
White blood cells, also known as leukocytes, are larger than erythrocytes, have a nucleus,
and lack hemoglobin. They function in the cellular immune response. White blood cells
(leukocytes) are less than 1% of the blood's volume. They are made from stem cells in
bone marrow. There are five types of leukocytes, important components of the immune
system. Neutrophils enter the tissue fluid by squeezing through capillary walls and
phagocytozing foreign substances. Macrophages release white blood cell growth factors,
causing a population increase for white blood cells. Lymphocytes fight infection. T-cells
attack cells containing viruses. B-cells produce antibodies. Antigen-antibody complexes
are phagocytized by a macrophage. White blood cells can squeeze through pores in the
capillaries and fight infectious diseases in interstitial areas
Platelets result from cell fragmentation and are involved with clotting, as is shown by
Figures 17 and 18. Platelets are cell fragments that bud off megakaryocytes in bone
marrow. They carry chemicals essential to blood clotting. Platelets survive for 10 days
before being removed by the liver and spleen. There are 150,000 to 300,000 platelets in
each milliliter of blood. Platelets stick and adhere to tears in blood vessels; they also
release clotting factors. A hemophiliac's blood cannot clot. Providing correct proteins
(clotting factors) has been a common method of treating hemophiliacs. It has also led to
HIV transmission due to the use of transfusions and use of contaminated blood products.