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.
