BIOA02H3 Chapter Notes - Chapter 49: Circulatory System, Extracellular Fluid, Heart
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49.1 Why Do Animals Need a Circulatory System?
A circulatory system consists of a muscular pump (the heart), a fluid (blood), and a
series of conduits (blood vessels) through which the fluid can be pumped around the
body. Heart, blood, and vessels are also known collectively as a cardiovascular
system. The function of circulatory systems is to transport things around the body.
Circulatory systems transport: heat, hormones, respiratory gases, blood cells, platelets,
elements of the immune system, nutrients, and waste products. These all seem like
important tasks, so why do so many species not have circulatory systems?
Some animals do not have circulatory systems
Single-celled organisms serve all of their needs through direct exchanges with the
environment. Such organisms are mostly found in aquatic environments or very moist
terrestrial environments. Similarly, for multicellular organisms a circulatory system
is unnecessary if all of their cells are close enough to the external environment
that nutrients, respiratory gases, and wastes can diffuse between the cells and the
environment. Small aquatic invertebrates have structures and body shapes that permit
direct exchanges between cells and environment. Many of these animals have flattened,
thin body shapes that maximize the amount of surface area that is in contact with the
external environment and minimize the diffusion path length for exchanges between the
cells and the external environment. The cells of some other aquatic invertebrates are
served by highly branched central cavities called gastrovascular systems that
essentially bring the external environment into the animal. All the cells of a sponge, for
example, are in contact with, or very close to, the water that surrounds the animal and
circulates through its central cavity. Very small animals without circulatory systems can
maintain high levels of metabolism and activity, but bigger animals without circulatory
systems such as sponges, coelenterates, and flatworms tend to be inactive, slow, or even
sedentary. Larger, active animals need circulatory systems.
Larger and more active animals must support the metabolism of their cells by delivering
nutrients to them and taking wastes away from them with circulatory systems. The
critical environment for each cell is the extracellular fluid that surrounds it. All of the
cell’s nutrients—oxygen, fuel, essential molecules—comes from that fluid, and the waste
products of the cell go into that fluid. In many animals, the extracellular fluid is
continuous with the fluid in the circulatory system. The vessels of these animals empty
their fluid directly into the tissues. At other locations the extracellular fluid flows back
into the circulatory system to be pumped back out again. These circulatory systems are
called open. In contrast, closed circulatory systems completely contain the circulating
fluid, blood, in a continuous system of vessels. However, even in closed circulatory
systems, liquid and low-molecular-weight solutes are exchanged between the blood and
the extracellular fluids surrounding the cells of the body. In fact, the term extracellular
fluid refers to both the fluid in the circulatory system and the fluid between the cells of
the body. To distinguish the two, we refer to the fluid in the circulatory system as the
blood plasma and the fluid around the cells as interstitial fluid. A normal 70
kilogram person has a total extracellular fluid volume of about 14 liters. A little more
than a quarter of it, about 3 liters, is the blood plasma; the rest is interstitial fluid.
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Circulatory systems carry materials to and from all regions of the body to maintain the
optimum composition of the interstitial fluids, which in turn serve the needs of the cells.
Importantly, circulatory systems speed up the delivery of nutrients and deliver them
preferentially to the most active tissues.
Open circulatory systems move extracellular fluid
In open circulatory systems extracellular fluid squeezes through intercellular spaces
as the animal moves. A muscular pump usually assists the distribution of the fluid in
these systems. The contractions of this simple heart propel the extracellular fluid
through vessels leading to different regions of the body, but the fluid leaves those vessels
to trickle through the tissues and eventually return to the heart. Open circulatory
systems are found in arthropods, mollusks, and some other invertebrate groups. In the
generalized arthropod the fluid returns to the heart through valved openings called
ostia. In mollusks, open vessels aid in the return of extracellular fluid to the heart.
Closed circulatory systems circulate blood through a system of
In a closed circulatory system, a system of vessels keeps circulating blood separate
from the interstitial fluid. Blood is pumped through this vascular system by one or more
muscular hearts, and some components of the blood never leave the vessels. Closed
circulatory systems characterize vertebrates, annelids, and some other invertebrate
A simple example of a closed circulatory system is that of the earthworm. One large
ventral vessel carries blood from its anterior end to its posterior end. Smaller
vessels branch off and transport the blood to even smaller vessels serving the tissues in
each segment of the worm’s body. In the smallest vessels, respiratory gases, nutrients,
and metabolic wastes diffuse between the blood and the interstitial fluid. The blood then
flows from these vessels into larger vessels that lead into one large dorsal vessel,
which carries the blood from the posterior to the anterior end of the body. Five
pairs of muscular vessels connect the large dorsal and ventral vessels in the anterior
end, thus completing the circuit. The dorsal vessel and the five connecting vessels
serve as hearts for the earthworm; their contractions keep the blood circulating. The
direction of circulation is determined by one-way valves in the dorsal and connecting
Closed circulatory systems have several advantages over open systems:
Fluid can flow more rapidly through vessels than through intercellular spaces, and
can therefore transport nutrients and wastes to and from tissues more rapidly.
By changing resistance in the vessels, closed systems can be selective in directing
blood to specific tissues.
Specialized cells and large molecules that aid in the transport of hormones and
nutrients can be kept within the vessels, but can drop their cargo in the tissues where
it is needed.
It seems logical to accept the premise that in all but very small animals, closed
circulatory systems can support higher levels of metabolic activity than open systems
can. But we then have to ask: How do highly active insect species achieve high levels of
metabolic output with their open circulatory systems? The reason is that insects do not
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depend on their circulatory systems for respiratory gas exchange. Recall from the last
chapter that respiratory gas exchange in insects is through a system of air-filled tubes.
Circulatory systems consist of a pump and an open or closed set of vessels
through which is pumped a fluid that transports oxygen, nutrients, wastes,
and a variety of other substances.
49.2 How Have Vertebrate Circulatory Systems
Vertebrates have closed circulatory systems and hearts with two or more chambers.
When a heart chamber contracts, it squeezes the blood, putting it under pressure. Blood
then flows out of the heart and into vessels where pressure is lower. Valves prevent the
backflow of blood as the heart cycles between contraction and relaxation.
As circulatory systems become more complex, the blood that flows to the gas
exchange organs becomes more completely separated from the blood that
flows to the rest of the body. In fish, the phylogenetically oldest vertebrates, blood is
pumped from the heart to the gills and then to the tissues of the body and back to the
heart. In birds and mammals, the phylogenetically youngest vertebrates, blood is
pumped from the heart to the lungs and back to the heart in a pulmonary circuit, and
then from the heart to the rest of the body and back to the heart in a systemic circuit.
We will trace the evolution of the separation of the circulation into two circuits.
The closed vascular system of vertebrates begins with vessels called arteries that
carry blood away from the heart. Arteries give rise to smaller vessels called
arterioles, which feed blood into capillary beds. Capillaries are the tiny, thin-walled
vessels where materials are exchanged between the blood and the tissue fluid. Small
vessels called venules drain capillary beds. The venules join together to form larger
vessels called veins, which deliver blood back to the heart.
arteries arterioles capillary beds venules veins
Fish have two-chambered hearts
The fish heart has two chambers. An atrium (plural atria) receives blood from the body
and pumps it into a more muscular chamber, the ventricle. The ventricle pumps the
blood to the gills, where gases are exchanged. Blood leaving the gills collects in a large
dorsal artery, the aorta, which distributes blood to smaller arteries and arterioles
leading to all the organs and tissues of the body. In the tissues, blood flows through beds
of tiny capillaries, collects in venules and veins, and eventually returns to the atrium of
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