Chapter 23 – Circulation
23.1 Circulatory Systems facilitate exchange with all body tissues
 A circulatory system is needed for any animal whose body is too large for vital chemicals to reach all parts by
diffusion alone. Diffusion is only adequate for transport over small distances, but for larger organisms it just is
not efficient. Nutrients must travel from the small intestine to the brain and well as the muscles in legs and
arms. The circulatory system accomplishes this internal transport
 In the human body, the heart pumps oxygenated blood from the lungs through blood vessels that gradually get
smaller to become capillaries. These capillaries form networks among the tissues in the body, meaning that no
substances have to go far to be picked up. In capillaries, the blood cells pass through single file. Each blood cell
comes close enough to the tissue that oxygen can diffuse out of the capillary and directly into the tissue
surrounding it.
 Materials are not exchanged directly between blood and body cells. Each body cell floats in interstitial fluid.
Molecules such as oxygen and nutrient diffuse out of the capillary, into the interstitial fluid, and then into the
tissue cells.
 The circulatory system has other functions besides transporting oxygen and nutrients. It takes wastes to waste
disposal organs, takes CO2 to the lungs, and takes metabolic wastes to the kidneys. It also helps maintain
homeostasis by exchanging the molecules with interstitial fluid. This exchange controls the makeup of the
environment in which the tissues live and moves the molecules through organs such as the kidneys and liver to
regulate blood contents. It is also involved in body defense, temperature regulation, and hormone distribution.
 Certain body plans make a circulatory system unnecessary. In some aquatic animals (like the hydra) the body
plan is only a few cells thick, so diffusion occurs directly from the water. In other aquatic animals, water can be
drawn into the gastrovascular cavity and nutrients will be collected this way. Jellyfish have a gastrovascular
cavity with branches radiating from the mouth to the circular canal. In this canal, ciliated cells circulate fluids.
 Most flatworms have a gastrovascular cavity that exchanges materials with the environment.
 Animals that have thick, multiple cell layers require more than simple diffusion to move fluids. Such animals
have true circulatory systems containing specialized transportation fluid, blood.
 Two circulatory systems exist:
 Open: Blood is pumped through open ended vessels and flows among the cells. No differentiation is made
between interstitial fluid and the blood.In insects, a tubular heart pumps blood into the head and the rest of the
body while body movements help circulate blood allowing chemical exchange
 Closed: Blood is confined to vessels and kept separate from interstitial fluid.
 Three types of vessels are present in this system:
o Arteries carry blood away from the heart
o Veins carry blood to the heart
o Capillaries connect the arteries and veins
 Generally, arteries are carrying oxygen rich blood to the body, while veins are carrying oxygen poor blood back
to the heart. However, two pulmonary arteries carry oxygen poor blood from the heart to the lungs and four
pulmonary veins carry oxygenated blood from the lungs to the heart
 Vertebrate circulatory systems are generally termed cardiovascular systems. Cardiovascular systems involve a
heart with two atria (atrium) that receive blood from veins while two ventricles pump blood out to the body.
Large arteries branch into arterioles that eventually branch into capillaries. These capillaries allow chemical
exchange between interstitial fluid and the blood. Capillaries converge into veins that return blood to the heart.
23.2 Vertebrate cardiovascular systems reflect evolution
 In single circulation, blood passes through the heart once in each circuit through the body. Blood pumps from
the ventricle travels first to the gill capillaries. Blood pressure drops as blood flows through the narrow
capillaries. An artery carries the oxygen-rich blood from gill capillaries to the tissues and organs, and then it
returns to the atrium of the heart
 In terrestrial vertebrates, blood is supplied to body organs by double circulation. Blood is pumped a second time
after it slows down in the capillary bed of the lungs. The pulmonary circuit carries blood between the heart and
the gas exchange tissues in the lungs while the systemic circuit carries blood between the heart and the rest of
the body
 In most amphibians, the heart has three chambers, two atria and one ventricle. Gas exchange occurs through the
skin (the pulmocutaneous circuit) as well as through the lungs. Oxygen rich blood returns to the left atrium and
is pumped by the ventricle into the systemic circuit. It mixes in the single ventricle, but most of the oxygen poor
blood is diverted to the pulmocutaneous circuit and most of the oxygen rich blood goes into the systemic circuit.
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Reptiles may also have a three-chambered heart, but the ventricle is partially divided, and less blood mixing
occurs.
In birds and mammals, the ventricle is completely divided, giving them two atria and two ventricles. The left
side of the heart receives and pumps the oxygen rich blood, while the right side receives only oxygen poor
blood.
23.3 The human cardiovascular system illustrates the double circulation of mammals
 The human heart is enclosed in a sac just under the breastbone. It is made of predominantly cardiac muscles.
The thin-walled atria collect blood returning to the heart, which then flows into the ventricles. Contraction of
the atria completes the filling of the ventricles. Thicker-walled ventricles pump blood to the lungs and other
body tissues.
 Pulmonary circuit: The right atrium fills and moves blood to the right ventricle. The right ventricle pumps
blood to the lungs through two pulmonary arteries. As blood flows through capillaries in the lungs, it picks up
oxygen and releases carbon dioxide. This oxygen rich blood flows back through the pulmonary veins to the left
atrium. The oxygen rich blood leaves the left atrium and enters the left ventricle. From the left ventricle, blood
is pumped to the body.
 Systemic circuit: The aorta is the largest blood vessel (2.5cm in diameter) and branches into coronary arteries,
which supply blood to the heart itself. Other arteries branch from the aorta leading to the head, chest, and arms
(top of body) or to the abdominal region and legs (bottom of body). Arteries lead to arterioles, which lead to
capillaries. These capillaries then connect to venules that take blood back into veins. This deoxygenated blood
is channeled into larger veins, the superior vena cava and inferior vena cava and they empty blood into the right
atrium.
23.4 The heart contracts and relaxes rhythmically
 A complete filling and pumping of the blood is called the cardiac cycle. It takes about 0.8 second to finish one
cardiac cycle, which equals to 75 beats per minute
 When the heart is relaxed, this is diastole and blood flows into all four chambers. It enters the right atrium from
the venae cavae and the left atrium from the pulmonary veins. The valves between the atria and ventricles
(atrioventricular, AV, valves) are open. This lasts about 0.4 second.
 Systole occurs when the atria contracting and filling the ventricles with blood (atrial systole) which takes 0.1
second. The ventricles contract for about 0.3 second (ventricular systole). The force of their contraction closes
the AV valves and open the semilunar valves located at the exit of each ventricle. The blood pumps into the
large arteries. Blood flows into the atria during the second part of systole
 Volume per minute is called cardiac output and is equal to the amount of blood pumped by the left ventricle
each time it contracts. Average person having 70 beats per minute, so the cardiac output would be 70 * 75mL
per beat, equaling 5,250mL per minute or 5.25L per minute, which is roughly equal to the total blood volume.
 Heart valves are made of connective tissue and prevent blood from backflow and keep it moving in the right
direction. The heart sounds heard by a stethoscope are caused by the closing of the valves. The “lub” is the coil
of blood against the closed AV valves, and the “dup” is from the semilunar valves shutting.
23.5 The SA Node sets the tempo of the heartbeat
 The pacemaker, SA (sinoatrial) node, maintains the heart’s pumping rhythm by setting the rate at which all the
muscle cells of the heart contract. It is found in the right atrium. The pacemaker generates electrical signals
 Cardiac muscles are connected by junctions between cells and signals spread through the atria, making them
contract at the same time. The signal passes through a relay point, the AV (atrioventricular) node, found in the
wall between the right atrium and ventricle. Cardiac fibers contract and relay the signals to the apex of the heart
and up through the walls of the ventricles triggering the contractions that force blood out of the heart.
 In certain diseases, the natural pacemaker fails and an artificial pacemaker can be installed. The artificial
pacemaker emits and electrical signal to trigger contractions.
23.6 What is a heart attack?
 Heart muscle cells require nutrients and oxygen to survive. The heart contracts over 100,000 times per day, so
those requirements are high. Blood exits the heart through the aorta, which immediately branches off into
coronary arteries to feed the heart muscle. If any of these vessels get clogged, then the heart does not get what it
needs and the cells die. When these cells die, then the heart fails to function properly causing a heart attack.
 One-third of those who have a heart attack die immediately. Those who survive will have damage to the heart.
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Cardiovascular disease is disease of the heart and blood vessels. Heart attack and stroke are among the leading
causes of death in the US (ranking first and third). Cardiovascular disease is responsible for over 1 million
deaths per year (or one every 30 seconds). Atherosclerosis is the gradual impairment of the arteries and occurs
gradually. Growths, plaques, develop in the inner wall of the arteries narrowing the passages.
The smooth muscle layer becomes thickened and filled with cholesterol and fibrous connective tissue. This can
lead to blood clots being trapped in the vessels, resulting in heart attack or stroke. Treatments involve
medication to dissolve clots, cholesterol and blood pressure lowering medication, angioplasty (inserting a
balloon into the artery to compress the clot), or bypass surgery. Prevention includes healthy diet, exercise,
lowering cholesterol, and not smoking.
23.7 The structure of blood vessels fits their function
 Capillaries have thin walls formed with a single layer of epithelial cells wrapped in a thin basement membrane.
The inner surface of the capillary is smooth to keep blood cells from being damaged as they go through.
 Other blood vessels have thicker walls with the same epithelium, but they are reinforced with other tissues. An
outer layer of connective tissue with elastic fibers that let the vessel expand and contract. The middle layer
consists of smooth muscle. These layers are thicker and sturdier in arteries to allow the high pressure and rapid
flow of blood leaving the heart.
 Arteries are able to regulate flow by use of the smooth muscle contractions.
 In veins, the pressure and flow are lower. One-way valves regulate direction of the flow.
23.8 Blood pressure and velocity reflect the structure and arrangement of blood vessels
 Blood pressure is the force that blood exerts on the walls of our vessels. This pressure is created by the pumping
of the heart to drive blood into the vessels. When the ventricles contract, the blood is driven into the arteries
faster than it can flow into the arterioles, so the walls of the arteries are stretched. This stretching is measured as
your pulse
 Blood pressure is partly cardiac output and partly the resistance of blood flow due to the narrowing into
arterioles. These openings are controlled by smooth muscle, so as the muscles relax, the vessels dilate and let
blood flow. Stress can raise blood pressure by triggering signals to constrict vessels.
 Blood pressure is expressed in mmHg (millimeters mercury) and blood velocity is expressed in cm/second.
 By the time blood reaches the veins, the pressure is nearly zero. In order to climb back to the heart, the veins are
sandwiched in skeletal muscles that are able to move. This movement pinches the veins and squeezes blood
back up to the heart. Breathing also helps move blood. As we inhale, the pressure change in our chest cavity
causes the large veins near the heart to expand and fill.
23.9 Measuring blood pressure can reveal cardiovascular problems
 Typical blood pressure in a healthy young adult is 110/70, and optimal for an adult is 120/80. The first number
is systolic pressure and the second is diastolic pressure. Blood pressure is a measure of cardiovascular health
and an abnormal reading can indicate a problem.
 Hypertension, high blood pressure, is usually over 140/90. It harms the system in several ways:
o Causes the heart to work harder to pump blood.
o May cause the left ventricle to enlarge. If the coronary blood supply does not keep up with the
demands of the new muscle mass, then the heart muscle weakens.
o Increased pressure on vessel walls causes them to weaken and may lead to small ruptures that
encourage plaque development.
o Can lead to heart attack, heart disease, stroke, and kidney failure
23.10 Smooth muscle controls the distribution of blood
Smooth muscle in the arteriole wall regulates the distribution of blood to the capillaries of the organs. Each
tissue has many capillaries, so the body is supplied with blood at all times. Capillaries in some organs (heart,
kidneys, liver, and brain) usually carry a full load of blood. Other organ capillaries carry a load fitting the
needs of the organ at that time. Precapillary sphincters are a secondary control mechanism. These are rings of
smooth muscle located at the entrance of the capillary bed. When they are relaxed, blood flows into the
capillary. When they are contracted, then blood bypasses that capillary bed. Nerves and hormones regulate
both mechanisms
23.11 Capillaries allow the transfer of substances through their walls
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Capillaries are the only blood vessels whose walls are thin enough to allow exchange between blood and
interstitial fluid. The capillary wall consists of adjoining epithelial cells that enclose a lumen, space, which is
just large enough for blood cells to travel through single file.
Substance exchange occurs in several ways:
o Diffusion through the wall – oxygen and carbon dioxide
o Carried by vesicles from endocytosis that are released in the fluid by exocytosis
o Water and small solutes (sugar and salts) move freely through narrow clefts in the wall
Most fluid that leaves the blood at the arterial end of the capillary reenters at the venous end. The remaining
fluid is returned by the lymphatic system.
23.12 Blood consists of red and white blood cells suspended in plasma
Plasma is the liquid portion of blood. Cellular pieces compose about 45% of blood’s volume, the rest is
plasma. It is a straw colored fluid made of 90% water.
Its solutes include:
 Salts in the form of dissolved ions that maintain osmotic balance, preserve body pH, and help with nerve
and muscle function
 Proteins that maintain osmotic balance between blood and interstitial fluid, or may play other important
functions such as in body defense or blood clotting
 substances in transit from one place to another, such as wastes, oxygen, carbon dioxide, or hormones
Two cell types are found in plasma:
 Red blood cells, erythrocytes, which carry oxygen. On average, a person has about 25 trillion in
circulation.
 White blood cells, leukocytes, which fall into five types. Their function is to fight infection. They include
monocytes, neutrophils, basophils, eosinophils, and leukocytes.
Also found in plasma are platelets. These are cell fragments responsible for clotting in blood.
23.13 Too few or too many red blood cells can be unhealthy
 Adequate numbers of red blood cells are critical for healthy body functioning. Red blood cells are circulated in
the body for 3-4 months, and then they are broken down and recycled. The iron of the hemoglobin is removed
and returned to the bone marrow at about 2 million per second.
 Low numbers of hemoglobin or low number of red blood cells is called anemia. Anemia causes a person to feel
tired and is susceptible to infection because cells are not getting enough oxygen. The most common cause of
anemia is iron deficiency, and it is more common in women than in men due to menstruation
 Production of red blood cells in the bone marrow is regulated by a negative feedback mechanism that is
sensitive to the amount of oxygen reaching the tissues through the blood. If the tissues are not receiving enough
oxygen, the kidneys produce the hormone, erythropoietin (EPO), which stimulates the bone marrow to produce
more red blood cells.
23.14 Blood clots plug leaks when blood vessels are injured
 Blood contains self-sealing materials that activate when blood vessels are injured. The sealants include platelets
and the protein, fibrinogen.
 Immediately after an injury, a damaged blood vessel constricts to reduce blood loss and allow time for repair to
begin. The epithelium lining a vessel is damaged; connective tissue is exposed to blood. Platelets adhere to the
tissue and release chemicals that make nearby platelets sticky. A cluster of sticky platelets form a plug that
provides fast protection against further blood loss. Clotting factors released from the platelets interact with
clotting factors in plasma.
 This set of reactions causes the patch to be reinforced and form a scab on the skin. This process involves over a
dozen clotting factors that result in the conversion of fibrinogen into fibrin. Threads of fibrin trap blood cells
and more platelets to make the fibrin clot. The platelets contract, pulling the torn edges closer together to reduce
the size of the damaged area. The clotting mechanism is important and any defect can be life threatening.
 In hemophilia, fatal bleeding can result from small injuries.
23.15 Stem cells offer a potential cure for blood cell diseases
 The red marrow of bones such as the ribs, vertebrae, breastbone and pelvis contain spongy tissue where
unspecialized cells, stem cells, differentiate into blood cells. One type of stem cell gives rise to lymphocytes
that function in the immune system. Another type produces erythrocytes, other white blood cells, and the cells
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that produce platelets. After forming in the early embryo, these cells continually produce all of the blood cells
that the body needs.
Leukemia is cancer of the white blood cells, or leukocytes. Leukocytes protect the body against infection and
cancer, but sometimes they become cancerous themselves. A person with leukemia has an unusually high
number of leukocytes; most of them do not function normally. This overabundance of white blood cells crowd
out red blood cells, causing severe anemia and impaired clotting. Leukemia is fatal if not treated, but usually
responds to chemotherapy and radiation. An alternative treating is transplanting healthy bone marrow from a
suitable donor after the recipient’s marrow has been purposefully destroyed. Another option is to remove the
patient’s own marrow, extract the healthy stem cells from it, and reinsert it into the patient.