42 Flashcards
time it takes for a substance to diffuse from one place to another
proportional to the square of the distance.
t = (x^2)/2D
(x = distance; D = diffusion coeff)
animals that lack a distinct circulatory
system
- In animals with a gastrovascular cavity, fluid bathes both
the inner and outer tissue layers, facilitating exchange of
gases and cellular waste. Only the cells lining the cavity have
direct access to nutrients released by
digestion. However, because the body
wall is a mere two cells thick, nutrients
need diffuse only a short distance to
reach the cells of the outer tissue layer - Planarians and most other flatworms also survive without a circulatory system. Their combination of a
gastrovascular cavity and a flat body is
well suited for exchange with the environment (Figure 42.2b). A flat body optimizes diffusional exchange by increasing surface area
and minimizing diffusion distances.
For animals with many cell layers,
diffusion
distances are too great for adequate exchange of nutrients
and wastes by a gastrovascular cavity. In these organisms, a
circulatory system minimizes the distances that substances
must diffuse to enter or leave a cell.
A circulatory system has three basic components:
a circulatory
fluid, a set of interconnecting vessels, and a muscular pump,
the heart
heart
powers circulation by using metabolic energy to elevate the hydrostatic pressure of the circulatory fluid,
which then flows through the vessels and back to the heart.
open circulatory system
Arthropods and most molluscs have an open circulatory
system, in which the circulatory fluid bathes the organs directly (Figure 42.3a).
- In these animals, the circulatory fluid,
called hemolymph, is also the interstitial fluid that bathes
body cells.
- Contraction of one or more hearts pumps the
hemolymph through the circulatory vessels into interconnected sinuses, spaces surrounding the organs. Within the sinuses, chemical exchange occurs between the hemolymph
and body cells. Relaxation of the heart draws hemolymph back in through pores, which are equipped with valves that
close when the heart contracts. Body movements help circulate the hemolymph by periodically squeezing the sinuses.
- The open circulatory system of larger crustaceans, such as
lobsters and crabs, includes a more extensive system of vessels as well as an accessory pump.
closed circulatory system
- Annelids (including earthworms),
cephalopods (including squids and octopuses), and all vertebrates have closed circulatory systems - a circulatory fluid
called blood is confined to vessels and is distinct from the
interstitial fluid (Figure 42.3b). One or more hearts pump
blood into large vessels that branch into smaller ones that infiltrate the organs. Chemical exchange occurs between the
blood and the interstitial fluid, as well as between the interstitial fluid and body cells
advantage of open circ
The lower hydrostatic pressures associated with open circulatory systems make them less costly
than closed systems in terms of energy expenditure.
- In some
invertebrates, open circulatory systems serve additional functions. For example, spiders use the hydrostatic pressure generated by their open circulatory system to extend their legs.
advantages of closed circ
The benefits of closed circulatory systems include relatively
high blood pressures, which enable the effective delivery of O2
and nutrients to the cells of larger and more active animals.
Among the molluscs, for instance, closed circulatory systems
are found in the largest and most active species, the squids and
octopuses.
- Closed systems are also particularly well suited to
regulating the distribution of blood to different organs
cardiovascular system
The closed circulatory system of humans and other vertebrates
is often called the cardiovascular system. Blood circulates
to and from the heart through an amazingly extensive network of vessels: The total length of blood vessels in an average
human adult is twice Earth’s circumference at the equator!
arteries
Arteries carry blood away from the heart to organs
throughout the body. Within organs, arteries branch into
arterioles, small vessels that convey blood to the capillaries.
capillaries
Capillaries are microscopic vessels with very thin, porous
walls. Networks of these vessels, called capillary beds, infiltrate every tissue, passing within a few cell diameters of every
cell in the body. Across the thin walls of capillaries, chemicals, including dissolved gases, are exchanged by diffusion
between the blood and the interstitial fluid around the tissue
cells. At their “downstream” end, capillaries converge into
venules.
veins
venules converge into veins, the vessels that
carry blood back to the heart.
Arteries and veins are distinguished by
the direction in
which they carry blood, not by the O2 content or other characteristics of the blood they contain. Arteries carry blood
from the heart toward capillaries, and veins return blood to
the heart from capillaries.
- The only exceptions are the portal
veins, which carry blood between pairs of capillary beds. The
hepatic portal vein, for example, carries blood from capillary
beds in the digestive system to capillary beds in the liver. From the liver, blood passes into the hepatic
veins, which conduct blood toward the heart.
The hearts of all vertebrates contain two or more muscular
chambers:
The chambers that receive blood entering the
heart are called atria (singular, atrium). The chambers responsible for pumping blood out of the heart are called
ventricles.
single circ
In bony fishes, rays, and sharks, the heart consists of two chambers: an atrium and a ventricle. The blood passes through the
heart once in each complete circuit, an arrangement called
single circulation (Figure 42.4a). Blood entering the heart
collects in the atrium before transfer to the ventricle. Contraction of the ventricle pumps blood to the gills, where there is a
net diffusion of O2 into the blood and of CO2 out of the blood.
As blood leaves the gills, the capillaries converge into a vessel
that carries oxygen-rich blood to capillary beds throughout the
body. Blood then returns to the heart.
- In single circulation, blood that leaves the heart passes
through two capillary beds before returning to the heart.
When blood flows through a capillary bed, blood pressure
drops substantially. The
drop in blood pressure in the gills limits the rate of blood
flow in the rest of the animal’s body. As the animal swims,
however, the contraction and relaxation of its muscles help
accelerate the relatively sluggish pace of circulation.
- (blood flows under reduced pressure directly from the gas exchange organs to other organs)
double circ
The circulatory systems of amphibians, reptiles, and mammals
have two circuits, an arrangement called double circulation
(Figure 42.4b). The pumps for the two circuits are combined
into a single organ, the heart. Having both pumps within a
single heart simplifies coordination of the pumping cycles.
- One pump, the right side of the heart, delivers oxygen-poor
blood to the capillary beds of the gas exchange tissues, where
there is a net movement of O2 into the blood and of CO2 out of
the blood.
- This part of the circulation is called a pulmonary
circuit if the capillary beds involved are all in the lungs, as in
reptiles and mammals.
- It is called a pulmocutaneous circuit
if it includes capillaries in both the lungs and the skin, as in
many amphibians.
- After the oxygen-enriched blood leaves the gas exchange tissues, it enters the other pump, the left side of the heart. Contraction of the heart propels this blood to capillary beds in
organs and tissues throughout the body. Following the exchange of O2 and CO2, as well as nutrients and waste products, the now oxygen-poor blood returns to the heart, completing
the systemic circuit.
- Double circulation provides a vigorous flow of blood to
the brain, muscles, and other organs because the heart repressurizes the blood destined for these tissues after it passes
through the capillary beds of the lungs or skin. Indeed, blood
pressure is often much higher in the systemic circuit than in
the gas exchange circuit.
double circ in amphibians
- have a heart
with three chambers: two atria and one
ventricle. - A ridge within the ventricle diverts most (about 90%) of the oxygen-poor
blood from the right atrium into the pulmocutaneous circuit and most of the oxygen-rich blood from the left atrium into the
systemic circuit. - When underwater, a frog
adjusts its circulation, for the most part
shutting off blood flow to its temporarily
ineffective lungs. Blood flow continues to
the skin, which acts as the sole site of gas
exchange while the frog is submerged.
double circ in reptiles (except birds)
turtles, snakes, lizards:
- 3 chambered heart: an incomplete septum
partially divides the single ventricle into
separate right and left chambers.
- Two
major arteries, called aortas, lead to the
systemic circulation.
- In alligators, caimans, and other crocodilians, the ventricles are divided by a complete septum (not shown), but the pulmonary and systemic circuits connect where the arteries exit the heart. This connection enables arterial valves to shunt blood flow away from the lungs temporarily, such as when the animal is underwater.
double circ in mammals and birds
- two atria
and two completely divided ventricles. The
left side of the heart receives and pumps
only oxygen-rich blood, while the right
side receives and pumps only oxygen-poor
blood. - As
endotherms, mammals and birds use about
ten times as much energy as equal-sized
ectotherms. Their circulatory systems therefore need to deliver about ten times as
much fuel and O2 to their tissues (and remove ten times as much CO2 and other
wastes). - This large traffic of substances is
made possible by separate and independently powered systemic and pulmonary
circuits and by large hearts that pump the
necessary volume of blood. - A powerful
four-chambered heart arose independently
in the distinct ancestors of mammals and
birds and thus reflects convergent evolution
plasma
Vertebrate blood is a connective tissue consisting of cells suspended in a liquid matrix called plasma. Dissolved in the
plasma are ions and proteins that, together with the blood
cells, function in osmotic regulation, transport, and defense.
Separating the components of blood using a centrifuge reveals that cellular elements (cells and cell fragments) occupy
about 45% of the volume of blood (Figure 42.17). The remainder is plasma.
electrolytes
Among the many solutes in plasma are inorganic salts in the
form of dissolved ions, sometimes referred to as blood electrolytes (see Figure 42.17).
- Although plasma is about 90%
water, the dissolved salts are an essential component of the
blood.
- Some of these ions buffer the blood, which in humans
normally has a pH of 7.4.
- Salts are also important in maintaining the osmotic balance of the blood.
- In addition, the concentration of ions in plasma directly affects the composition of the interstitial fluid, where many of these ions have a vital role
in muscle and nerve activity.
- To serve all of these functions,
plasma electrolytes must be kept within narrow concentration
ranges
plasma proteins
Plasma proteins act as buffers against pH changes, help
maintain the osmotic balance between blood and interstitial
fluid, and contribute to the blood’s viscosity (thickness). Particular plasma proteins have additional functions.
- The immunoglobulins, or antibodies, help combat viruses and
other foreign agents that invade the body.
- Others are escorts for lipids, which are insoluble in water
and can travel in blood only when bound to proteins.
- A
third group of plasma proteins are clotting factors that help
plug leaks when blood vessels are injured.
plasma vs interstitial fluid
Plasma has a much higher protein concentration than interstitial fluid, although the two fluids are otherwise similar. (Capillary walls, remember, are not very permeable to proteins.)
Blood contains two classes of cells:
red blood cells, which transport O2, and white blood cells, which function in defense (see
Figure 42.17). Also suspended in blood plasma are platelets, fragments of cells that are involved in the clotting process.
erythrocytes
Red blood cells, or erythrocytes, are by far the
most numerous blood cells. Each microliter (μL, or mm3) of
human blood contains 5–6 million red cells, and there are
about 25 trillion of these cells in the body’s 5 L of blood.
- Their
main function is O2 transport, and their structure is closely related to this function. Human erythrocytes are small disks
(7–8 μm in diameter) that are biconcave—thinner in the center than at the edges. This shape increases surface area, enhancing the rate of diffusion of O2 across their plasma
membranes.
- Despite its small size, an erythrocyte contains about
250 million molecules of hemoglobin. Because each molecule of hemoglobin binds up to four molecules of O2, one
erythrocyte can transport about a billion O2 molecules.
Mature mammalian erythrocytes lack
nuclei.
This unusual characteristic leaves more space in these tiny
cells for hemoglobin, the iron-containing protein that transports O2 (see Figure 5.20). Erythrocytes also lack mitochondria
and generate their ATP exclusively by anaerobic metabolism.
Oxygen transport would be less efficient if erythrocytes were
aerobic and consumed some of the O2 they carry
erythrocytes and O2 diffusion
As
erythrocytes pass through the capillary beds of lungs, gills, or
other respiratory organs, O2 diffuses into the erythrocytes
and binds to hemoglobin. In the systemic capillaries, O2 dissociates from hemoglobin and diffuses into body cells.
sickle cell disease
- an abnormal form of hemoglobin
(HbS) polymerizes into aggregates. Because the concentration of
hemoglobin in erythrocytes is so high, these aggregates are large
enough to distort the erythrocyte into an elongated, curved
shape that resembles a sickle. - Sickle-cell disease significantly impairs the function of the
circulatory system. Sickled cells often lodge in arterioles and
capillaries, preventing delivery of O2 and nutrients and removal of CO2 and wastes. Blood vessel blockage and resulting organ swelling often result in severe pain. In addition,
sickled cells frequently rupture, reducing the number of red
blood cells available for transporting O2. The average life
span of a sickled erythrocyte is only 20 days—one-sixth that
of a normal erythrocyte. The rate of erythrocyte loss outstrips
the replacement capacity of the bone marrow.
siclke cell treatment
Short-term
therapy includes replacement of erythrocytes by blood transfusion; long-term treatments are generally aimed at inhibiting aggregation of HbS
leukocytes
The blood contains five major types of white
blood cells, or leukocytes. Their function is to fight infections.
- Unlike erythrocytes, leukocytes are also
found outside the circulatory system, patrolling both interstitial fluid and the lymphatic system.
platelets
Platelets are pinched-off cytoplasmic fragments of
specialized bone marrow cells. They are about 2–3 μm in diameter and have no nuclei. Platelets serve both structural and
molecular functions in blood clotting.
hemophiila
Any genetic mutation that blocks a step in the clotting process can cause
hemophilia, a disease characterized by excessive bleeding and
bruising from even minor cuts and bumps
blood clotting
- A break in a
blood vessel wall exposes proteins that attract platelets and
initiate coagulation, the conversion of liquid components of
blood to a solid clot. - The coagulant, or sealant, circulates in
an inactive form called fibrinogen. - In response to a broken
blood vessel, platelets release clotting factors that trigger reactions leading to the formation of thrombin, an enzyme that
converts fibrinogen to fibrin. - Newly formed fibrin aggregates
into threads that form the framework of the clot. - Thrombin
also activates a factor that catalyzes the formation of more
thrombin, driving clotting to completion through positive
feedback
anticlotting
Anticlotting factors in the blood normally prevent spontaneous clotting in the absence of injury. Sometimes, however,
clots form within a blood vessel, blocking the flow of blood.
Such a clot is called a thrombus.
stem cells
Erythrocytes, leukocytes, and platelets all develop from a common source: multipotent stem cells that are dedicated to replenishing the body’s blood cell populations (Figure 42.19).
The stem cells that produce blood cells are located in the red
marrow of bones, particularly the ribs, vertebrae, sternum, and
pelvis. Multipotent stem cells are so named because they have
the ability to form multiple types of cells—in this case, the
myeloid and lymphoid cell lineages. When a stem cell divides,
one daughter cell remains a stem cell while the other takes on
a specialized function.
differentiation of blood cells
stem cells –> lymphoid and myeloid stem cells
lymphoid –> lymphocytes (B and T cells)
myeloid –> erythrocytes, neutrophils, basophils, monocytes, platelets, eosinophils
Throughout a person’s life, erythrocytes, leukocytes, and
platelets arising from stem cell divisions replace the worn-out cellular elements of blood.
Erythrocytes, for example, circulate for only 120 days on average before being replaced; the
old cells are consumed by phagocytic cells in the liver and
spleen. The production of new erythrocytes involves recycling of materials, such as the use of iron scavenged from old
erythrocytes in new hemoglobin molecules
EPO
A negative-feedback mechanism, sensitive to the amount
of O2 reaching the body’s tissues via the blood, controls
erythrocyte production. If the tissues do not receive enough
O2, the kidneys synthesize and secrete a hormone called
erythropoietin (EPO) that stimulates erythrocyte production. If the blood is delivering more O2 than the tissues can use,
the level of EPO falls and erythrocyte production slows. Physicians use synthetic EPO to treat people with health problems
such as anemia, a condition of lower-than-normal erythrocyte
or hemoglobin levels that lowers the oxygen-carrying capacity
of the blood.
blood doping
Some athletes inject themselves with EPO to increase their erythrocyte levels, although this practice, a form of
blood doping, has been banned by the International Olympic
Committee and other sports organizations.
cardiovascular disease
- disorders of the heart and
blood vessels - Cholesterol metabolism plays a central role in cardiovascular disease. The presence of this
steroid in animal cell membranes helps maintain normal
membrane fluidity. - Another factor in cardiovascular disease is inflammation, the
body’s reaction to injury. Tissue damage leads to recruitment of two types of circulating
immune cells, macrophages and leukocytes. Signals released by
these cells trigger a flow of fluid out of blood vessels at the site
of injury, resulting in the tissue swelling characteristic of inflammation. Although inflammation is often a
normal and healthy response to injury, it can significantly disrupt circulatory function.
LDL/HDL
- Cholesterol travels in blood plasma mainly
in particles that consist of thousands of cholesterol molecules
and other lipids bound to a protein. - One type of particle—low-density lipoprotein (LDL)—delivers cholesterol to
cells for membrane production. - Another type—high-density
lipoprotein (HDL)—scavenges excess cholesterol for return
to the liver. - Individuals with a high ratio of LDL to HDL are at
substantially increased risk for heart disease.
atherosclerosis
- Circulating cholesterol and inflammation can act together to
produce a cardiovascular disease called atherosclerosis, the
hardening of the arteries by accumulation of fatty deposits - Healthy arteries have a smooth inner lining that reduces resistance to blood flow. Damage or infection can
roughen the lining and lead to inflammation. - Leukocytes are
attracted to the damaged lining and begin to take up lipids,
including cholesterol. - A fatty deposit, called a plaque, grows
steadily, incorporating fibrous connective tissue and additional cholesterol. - As the plaque grows, the walls of the artery
become thick and stiff, and the obstruction of the artery
increases.
heart attack
- one result of untreated atherosclerosis
- A heart attack, also called a myocardial infarction, is the damage or death of cardiac muscle tissue
resulting from blockage of one or more coronary arteries,
which supply oxygen-rich blood to the heart muscle. - Because
the coronary arteries are small in diameter, they are especially
vulnerable to obstruction. Such blockage can destroy cardiac
muscle quickly because the constantly beating heart muscle
cannot survive long without O2. - If the heart stops beating, the
victim may nevertheless survive if a heartbeat is restored by
cardiopulmonary resuscitation (CPR) or some other emergency procedure within a few minutes of the attack.
stroke
- one result of untreated atherosclerosis
- A stroke
is the death of nervous tissue in the brain due to a lack of O2. - Strokes usually result from rupture or blockage of arteries in
the head. - The effects of a stroke and the individual’s chance of
survival depend on the extent and location of the damaged
brain tissue. - Rapid administration of a clot-dissolving drug
may reduce the effects of a stroke or heart attack
atherosclerosis warning signs
- Although atherosclerosis often isn’t detected until critical
blood flow is disrupted, there can be warning signs. - Partial
blockage of the coronary arteries may cause occasional chest
pain, a condition known as angina pectoris. The pain is most
likely to be felt when the heart is laboring hard during physical
or emotional stress, and it signals that part of the heart is not
receiving enough O2. - An obstructed coronary artery may be
treated surgically, either by inserting a metal mesh tube called
a stent to expand the artery or by transplanting a healthy
blood vessel from the chest or a limb to bypass the blockage.
hypertension
- Hypertension (high blood pressure) is yet another contributor to heart attack and stroke as well as other health
problems. - According to one hypothesis, chronic high blood
pressure damages the endothelium that lines the arteries,
promoting plaque formation. - The usual definition of hypertension in adults is a systolic pressure above 140 mm Hg or a
diastolic pressure above 90 mm Hg. - Fortunately, hypertension is simple to diagnose and can usually be controlled by
dietary changes, exercise, medication, or a combination of
these approaches.
cardiovascular dsease risk factors
- Although the tendency to develop particular cardiovascular
diseases is inherited, it is also strongly influenced by lifestyle. - Smoking and consumption of certain processed vegetable
oils called trans fats increase the ratio of LDL
to HDL, raising the risk of cardiovascular disease. - In contrast,
exercise decreases the LDL/HDL ratio.
cardiovascular disease treatment: cholesterol
Many individuals at high risk
are now treated with drugs called statins, which lower LDL levels and thereby reduce the risk of heart attacks.
cardiovascular disease treatment: inflammation
- The recognition that inflammation plays a central role in
atherosclerosis and thrombus formation is also changing the treatment of cardiovascular disease. - For example, aspirin,
which inhibits the inflammatory response, has been found to
help prevent the recurrence of heart attacks and stroke. - Researchers have also focused on C-reactive protein (CRP),
which is produced by the liver and found in the blood during
episodes of acute inflammation. Like a high level of LDL cholesterol, the presence of significant amounts of CRP in blood
is a useful risk indicator for cardiovascular disease
endothelium
Blood vessels contain a central lumen (cavity) lined with an
endothelium, a single layer of flattened epithelial cells.
- The
smooth surface of the endothelium minimizes resistance to
the flow of blood.
- Surrounding the endothelium are layers of
tissue that differ in capillaries, arteries, and veins, reflecting
the specialized functions of these vessels
capillary structure
Capillaries are the smallest blood vessels, having a diameter only slightly greater than that of a red blood cell.
- Capillaries also have very thin walls,
which consist of just the endothelium and its
basal lamina. This structural organization facilitates the exchange of substances between the blood in capillaries and the
interstitial fluid.
artery and vein structure
- Both arteries and
veins have two layers of tissue surrounding the endothelium: an
outer layer of connective tissue
containing elastic fibers, which
allow the vessel to stretch and recoil, and a middle layer containing smooth muscle and more
elastic fibers. - The walls of arteries are thick
and strong, accommodating blood
pumped at high pressure by the heart. Arterial walls also have an elastic recoil that
helps maintain blood pressure and flow to
capillaries when the heart relaxes between contractions. Signals from the nervous system and hormones circulating in the blood act on the smooth muscle in arteries and arterioles, dilating or constricting these vessels
and thus controlling blood flow to different parts of the body. - Because veins convey blood back to the heart at a lower
pressure, they do not require thick walls. For a given blood vessel diameter, a vein has a wall only about a third as thick as that
of an artery. Valves inside the veins maintain a unidirectional
flow of blood despite the low blood pressure.
blood slows as it moves from arteries to arterioles to capillaries. Why?
- The reason is that the number of capillaries is
enormous. Each artery conveys blood to so many capillaries
that the total cross-sectional area is much greater in capillary
beds than in the arteries or any other part of the circulatory
system. - The result is a dramatic decrease in
velocity from the arteries to the capillaries: Blood travels
500 times slower in the capillaries (about 0.1 cm/sec) than in
the aorta (about 48 cm/sec).
reduced velocity of blood flow in capillaries
The reduced velocity of blood flow in capillaries is essential to the function of the circulatory system.
- The exchange of substances between the blood and interstitial
fluid occurs only in capillaries because only capillaries
have walls thin enough to permit this transfer. Diffusion,
however, is not instantaneous. The slower flow of blood
through capillaries is thus necessary to provide time for
exchange to occur.
- After passing through the capillaries,
the blood speeds up as it enters the venules and veins,
which have smaller total cross-sectional areas than the
capillaries.
blood pressure
- Blood, like all fluids, flows from areas of higher pressure to
areas of lower pressure. - Contraction of a heart ventricle generates blood pressure, which exerts a force in all directions.
- The force directed lengthwise in an artery causes the blood to
flow away from the heart, the site of highest pressure. - The
force exerted against the elastic wall of an artery stretches the
wall, and the recoil of arterial walls plays a critical role in
maintaining blood pressure, and hence blood flow, throughout the cardiac cycle. - Once the blood enters the millions of
tiny arterioles and capillaries, the narrow diameter of these
vessels generates substantial resistance to flow. This resistance
dissipates much of the pressure generated by the pumping
heart by the time the blood enters the veins.
systolic pressure
Arterial blood pressure is highest when the heart contracts
during ventricular systole. The pressure at this time is called
systolic pressure (see Figure 42.11).
- The spikes in blood
pressure caused by the powerful contractions of the ventricles stretch the arteries.
pulse
By placing your fingers on the inside
of your wrist, you can feel a pulse—the rhythmic bulging of
the artery walls with each heartbeat.
- The surge of pressure is
partly due to the narrow openings of arterioles impeding the
exit of blood from the arteries.
- Thus, when the heart contracts, blood enters the arteries faster than it can leave, and
the vessels stretch from the rise in pressure.
diastolic pressure
During diastole, the elastic walls of the arteries snap back. As
a consequence, there is a lower but still substantial blood pressure when the ventricles are relaxed (diastolic pressure).
- Before enough blood has flowed into the arterioles to completely
relieve pressure in the arteries, the heart contracts again.
- Because the arteries remain pressurized throughout the cardiac
cycle, blood continuously flows into arterioles and capillaries.
BP changes
Changes in arterial blood pressure are not limited to the oscillation during each cardiac cycle. Blood pressure also fluctuates on
a longer time scale in response to signals that change the state
of smooth muscles in arteriole walls. For example, physical or emotional stress can trigger nervous and hormonal responses
that cause smooth muscles in arteriole walls to contract or relax.
vasoconstriction/dilation
- when smooth muscles in arteriole walls contract, the arterioles narrow, a process called
vasoconstriction. Narrowing of the arterioles increases blood
pressure upstream in the arteries. - When the smooth muscles
relax, the arterioles undergo vasodilation, an increase in diameter that causes blood pressure in the arteries to fall. - Vasoconstriction and vasodilation are often coupled to
changes in cardiac output that also affect blood pressure. This
coordination of regulatory mechanisms maintains adequate
blood flow as the body’s demands on the circulatory system
change. - During heavy exercise, for example, the arterioles in
working muscles dilate, causing a greater flow of oxygen-rich
blood to the muscles. By itself, this increased flow to the
muscles would cause a drop in blood pressure (and therefore
blood flow) in the body as a whole. However, cardiac output
increases at the same time, maintaining
blood pressure and supporting the necessary increase in blood flow
signaling molecules
- Researchers have identified a gas, nitric oxide (NO), as a
major inducer of vasodilation and a peptide, endothelin, as
the most potent inducer of vasoconstriction. Both produced in blood vessels
in response to cues from the nervous and endocrine systems. - Each kind of molecule binds to a specific receptor, activating
a signal transduction pathway that alters smooth muscle
contraction and thus changes blood vessel diameter.
normal BP
For a healthy
20-year-old human at rest, arterial blood
pressure in the systemic circuit is typically about 120 millimeters of mercury
(mm Hg) at systole and 70 mm Hg at
diastole, expressed as 120/70. (Arterial
blood pressure in the pulmonary circuit
is six to ten times lower.)
how is BP measured?
Blood pressure is generally measured for
an artery in the arm at the same height
as the heart.
BP and gravity
- Gravity has a significant effect on blood pressure. When you are standing, for example, your head is roughly 0.35 m higher than your chest, and the arterial blood pressure in your brain is about 27 mm Hg less than that near your heart. - If the blood pressure in your brain is too low to provide adequate blood flow, you will likely faint. By causing your body to collapse to the ground, fainting effectively places your head at the level of your heart, quickly increasing blood flow to your brain.
gravity and veins
Gravity is also a consideration for blood flow in veins, especially those in the legs. Although blood pressure in veins is relatively low, several mechanisms assist the return of venous
blood to the heart.
- First, rhythmic contractions of smooth
muscles in the walls of venules and veins aid in the movement
of the blood.
- Second, and more important, the contraction of
skeletal muscles during exercise squeezes blood through the veins toward the heart.
- Third, the change in
pressure within the thoracic (chest) cavity during inhalation
causes the venae cavae and other large veins near the heart to
expand and fill with blood.
why is it that exercising heavily immediately after
eating a big meal may cause indigestion?
- blood flow to the
skin is regulated to help control body temperature, and blood
supply to the digestive tract increases after a meal. - During
strenuous exercise, blood is diverted from the digestive tract
and supplied more generously to skeletal muscles and skin.
empty capillaries
- At any given time, only about 5–10% of the body’s capillaries
have blood flowing through them. However, each tissue has
many capillaries, so every part of the body is supplied with
blood at all times. - Capillaries in the brain, heart, kidneys,
and liver are usually filled to capacity, but at many other sites
the blood supply varies over time as blood is diverted from
one destination to another
capillaries lack ___
smooth muscle
Given that capillaries lack smooth muscle, how is blood
flow in capillary beds altered?
- One mechanism involves contraction of the smooth muscle in
the wall of an arteriole, which reduces the vessel’s diameter and
decreases blood flow to the adjoining capillary beds. When the
smooth muscle relaxes, the arterioles dilate, allowing blood to
enter the capillaries. - The other mechanism involves the action of precapillary
sphincters, rings of smooth muscle located at the entrance to
capillary beds. The signals that regulate blood flow include
nerve impulses, hormones traveling throughout the bloodstream, and chemicals produced locally.
histamine
the chemical histamine released by cells at a wound site causes
smooth muscle relaxation, dilating blood vessels and increasing blood flow. The dilated vessels also give disease-fighting
white blood cells greater access to invading microorganisms.
where does exchange of substances take place?
the critical exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries.
how does exchange of substances take place?
- Some substances are
carried across the endothelium in vesicles that form on one
side by endocytosis and release their contents on the opposite side by exocytosis. - Small molecules, such as O2 and CO2,
simply diffuse across the endothelial cells or, in some tissues,
through microscopic pores in the capillary wall. - These openings also provide the route for transport of small solutes such
as sugars, salts, and urea, as well as for bulk flow of fluid into
tissues driven by blood pressure within the capillary.
Two opposing forces control the movement of fluid between the capillaries and the surrounding tissues:
- Blood pressure tends to drive fluid out of the capillaries, and the presence
of blood proteins tends to pull fluid back. - Many
blood proteins (and all blood cells) are too large to pass readily
through the endothelium, and they remain in the capillaries.
These dissolved proteins are responsible for much of the
blood’s osmotic pressure (the pressure produced by the difference in solute concentration across a membrane). - The difference in osmotic pressure between the blood and the interstitial
fluid opposes fluid movement out of the capillaries. - On average, blood pressure is greater than the opposing forces, leading
to a net loss of fluid from capillaries. The net loss is generally
greatest at the arterial end of these vessels, where blood pressure is highest.
lymphatic system
Each day, the adult human body loses approximately 4–8 L
of fluid from capillaries to the surrounding tissues. There is
also some leakage of blood proteins, even though the capillary wall is not very permeable to large molecules.
- The lost
fluid and proteins return to the blood via the lymphatic
system, which includes a network of tiny vessels intermingled among capillaries of the cardiovascular system.
lymph
- After entering the lymphatic system by diffusion, the fluid
lost by capillaries is called lymph; its composition is about
the same as that of interstitial fluid. - The lymphatic system
drains into large veins of the circulatory system at the base of
the neck. - The joining of the lymphatic and circulatory systems functions in
the transfer of lipids from the small intestine to the blood.
movement of lymph
- The movement of lymph from peripheral tissues to the
heart relies on much the same mechanisms that assist blood
flow in veins. - Lymph vessels, like veins, have valves that prevent the backflow of fluid. Rhythmic contractions of the vessel
walls help draw fluid into the small lymphatic vessels. - In addition, skeletal muscle contractions play a role in moving lymph.
lymphatic disorders
- Disorders that interfere with the lymphatic system highlight its role in maintaining proper fluid distribution in the
body. Disruptions in the movement of lymph often cause
edema, swelling resulting from the excessive accumulation of
fluid in tissues. - Severe blockage of lymph flow, as occurs
when certain parasitic worms lodge in lymph vessels, results
in extremely swollen limbs or other body parts, a condition
known as elephantiasis.
lymph nodes
- Along a lymph vessel are organs called lymph nodes.
- By filtering the lymph and by housing cells
that attack viruses and bacteria, lymph nodes play an important role in the body’s defense. - Inside each lymph node is a
honeycomb of connective tissue with spaces filled by white
blood cells. When the body is fighting an infection, these
cells multiply rapidly, and the lymph nodes become swollen
and tender (which is why your doctor may check for swollen
lymph nodes in your neck, armpits, or groin when you feel
sick). - Because lymph nodes have filtering and surveillance
functions, doctors may examine the lymph nodes of cancer
patients to detect the spread of diseased cells.
In recent years, evidence has surfaced demonstrating that
the lymphatic system also plays a role in
harmful immune
responses, such as those responsible for asthma. Because of these and other
findings, the lymphatic system, largely ignored until
the 1990s, has become a
very active and promising
area of biomedical research.
human heart: location, size, composition
Located behind the sternum (breastbone), the human heart is
about the size of a clenched fist and consists mostly of cardiac muscle
atria
The two atria have relatively
thin walls and serve as collection chambers for blood returning to the heart from the lungs or other body tissues.
- Much
of the blood that enters the atria flows into the ventricles
while all heart chambers are relaxed.
- The remainder is transferred by contraction of the atria before the ventricles begin
to contract.
ventricles
- The ventricles have thicker walls and contract
much more forcefully than the atria—especially the left ventricle, which pumps blood to all body organs through the
systemic circuit. - Although the left ventricle contracts with greater force than the right ventricle, it pumps the same volume of blood as the right ventricle during each contraction.
cardiac cycle
- The heart contracts and relaxes in a rhythmic cycle. When
it contracts, it pumps blood; when it relaxes, its chambers fill
with blood. - One complete sequence of pumping and filling is
referred to as the cardiac cycle. - The contraction phase of
the cycle is called systole, and the relaxation phase is called
diastole
cardiac output
The volume of blood each ventricle pumps per minute is
the cardiac output.
- Two factors determine cardiac output:
- the rate of contraction, or heart rate (number of beats per
minute), and the
- stroke volume, the amount of blood
pumped by a ventricle in a single contraction.
average stroke volume
- The average
stroke volume in humans is about 70 mL. - Multiplying this
stroke volume by a resting heart rate of 72 beats per minute
yields a cardiac output of 5 L/min—about equal to the total
volume of blood in the human body. - During heavy exercise,
cardiac output increases as much as fivefold.
valves
Four valves in the heart prevent backflow and keep blood
moving in the correct direction (see Figures 42.7 and 42.8).
Made of flaps of connective tissue, the valves open when
pushed from one side and close when pushed from the other.
AV valve
- An atrioventricular (AV) valve lies between each atrium
and ventricle. - The AV valves are anchored by strong fibers
that prevent them from turning inside out. - Pressure generated by the powerful contraction of the ventricles closes the
AV valves, keeping blood from flowing back into the atria.
semilunar valves
- Semilunar valves are located at the two exits of the heart:
where the aorta leaves the left ventricle and where the pulmonary artery leaves the right ventricle. - These valves are
pushed open by the pressure generated during contraction of the ventricles. - When the ventricles relax, blood pressure built
up in the aorta closes the semilunar valves and prevents significant backflow.
valve sounds
You can follow the closing of the two sets of heart valves
either with a stethoscope or by pressing your ear tightly
against the chest of a friend.
- The sound
pattern is “lub-dup, lub-dup, lub-dup.”
- The first heart sound
(“lub”) is created by the recoil of blood against the closed AV
valves.
- The second sound (“dup”) is produced by the recoil of
blood against the closed semilunar valves
heart murmur
If blood squirts backward through a defective valve, it may
produce an abnormal sound called a heart murmur.
- Some
people are born with heart murmurs; in others, the valves
may be damaged by infection (from rheumatic fever, for instance).
- When a valve defect is severe enough to endanger
health, surgeons may implant a mechanical replacement
valve. However, not all heart murmurs are caused by a defect,
and most valve defects do not reduce the efficiency of blood
flow enough to warrant surgery
SA node
- In vertebrates, the heartbeat originates in the heart itself.
- Some
cardiac muscle cells are autorhythmic, meaning they contract
and relax repeatedly without any signal from the nervous system. You can even see these rhythmic contractions in tissue
that has been removed from the heart and placed in a dish in
the laboratory! - Because each of these cells has its own intrinsic
contraction rhythm, their contractions are coordinated
in the intact heart by a group of autorhythmic
cells located in the wall of the right atrium, near where the superior vena cava enters the heart. This cluster of cells is called
the sinoatrial (SA) node, or pacemaker, and it sets the rate
and timing at which all cardiac muscle cells contract.
EKG
- The SA node generates electrical impulses much like those
produced by nerve cells. - Because cardiac muscle cells are electrically coupled through gap junctions, impulses from the SA node spread rapidly within heart tissue.
- In
addition, these impulses generate currents that are conducted
to the skin via body fluids. In an electrocardiogram (ECG
or, often, EKG, from the German spelling), these currents are
recorded by electrodes placed on the skin. - The resulting graph
of current against time has a characteristic shape that represents the stages in the cardiac cycle
sympathetic/parasympathetic nervous system
- Physiological cues alter heart tempo by regulating the SA
node. Two portions of the nervous system, the sympathetic
and parasympathetic divisions, are largely responsible for
this regulation. - The sympathetic division speeds up the pacemaker, and the parasympathetic division slows it down.
- when you stand up and start walking, the sympathetic division increases your heart rate, an adaptation that
enables your circulatory system to provide the additional O2
needed by the muscles that are powering your activity. - If you
then sit down and relax, the parasympathetic division decreases your heart rate, an adaptation that conserves energy
node impulses
- Impulses from the SA node first spread rapidly through
the walls of the atria, causing both atria to contract in unison. - During atrial contraction, the impulses originating at
the SA node reach other autorhythmic cells located in the
wall between the left and right atria. These cells form a relay
point called the atrioventricular (AV) node. - Here the impulses are delayed for about 0.1 second before spreading to
the heart apex. This delay allows the atria to empty completely before the ventricles contract. - Then the signals from
the AV node are conducted to the heart apex and throughout
the ventricular walls by specialized
muscle fibers called bundle branches and Purkinje fibers.
circulation: getting fresh O2
Contraction of the right ventricle pumps blood to the lungs via the pulmonary arteries.
- As the blood flows through capillary beds in the left and
right lungs, it loads O2 and unloads CO2.
- Oxygen-rich blood
returns from the lungs via the pulmonary veins to the
left atrium of the heart.
circulation: pumping O2-rich blood out
- Next, the oxygen-rich blood flows
into the heart’s left ventricle, which pumps the oxygenrich blood out to body tissues through the systemic circuit. - Blood leaves the left ventricle via the aorta, which conveys blood to arteries leading throughout the body.
- The first
branches leading from the aorta are the coronary arteries, which supply blood to the heart muscle itself. - Then
branches lead to capillary beds in the head and arms (forelimbs). - The aorta then descends into the abdomen, supplying
oxygen-rich blood to arteries leading to capillary beds in the
abdominal organs and legs (hind limbs). - Within the capillaries,
there is a net diffusion of O2 from the blood to the tissues and of CO2 (produced by cellular respiration) into the blood.
circulation: getting O2-poor blood back to heart
- Capillaries rejoin, forming venules, which convey blood to
veins. - Oxygen-poor blood from the head, neck, and forelimbs is channeled into a large vein, the superior vena cava.
- Another large vein, the inferior vena cava, drains
blood from the trunk and hind limbs. - The two venae cavae
empty their blood into the right atrium, from which the
oxygen-poor blood flows into the right ventricle
