Chapter 21- Circulation and Respiration (22)
*Sections 21.1, 21.6, 21.9 will not be tested
This Chapter helps you to bring together ideas about the biochemistry of cellular
respiration and the diffusion of gases across membranes in the context of the familiar
organs and tissues of a multicellular organism.
Note the direction of blood flow through a mammalian heart (such as ours) - from
ventricles into arteries then into arterioles (small arteries) then into capillaries (where the
real exchange of gases, wastes, nutrients occurs from body tissues to the blood and vice
versa) then from capillaries into venules (small veins) then into veins and back to the
heart. Since the left side of the heart delivers oxygenated blood to the systemic
circulation (all the blood vessels in the body other than the lungs), this means that the left
ventricle has to be a lot stronger and larger and more muscular than the right ventricle,
which pumps blood through the much less extensive blood system that is in the lungs.
The lungs are also intimately involved with the functions of the blood, transferring
oxygen into the blood and removing carbon dioxide. The lungs can precisely match their
operation to the need for oxygen by a feedback mechanism that measures carbon dioxide
levels in the blood, in special receptors in blood vessels in the medulla region of the
All of this Chapter is relevant, Focus on the structure of blood as a tissue and the flow of
blood through the heart and body.
Figure 21.3 c and 21.4 are useful for your understanding of the way blood flows in the
body, you do not need to memorize them, but you should coordinate your reading of the
relevant material in the chapter on blood flow through the body with these figures so that
you can gain a better understanding.
Blood and Circulation.
When a test tube full of blood is centrifuged at the appropriate speed a number of layers
form, at bottom 40-50% of the tube is a packed layer of red blood cells, then a thin tissue
paper-like white layer of white cells which comprise much of the immune system, and an
upper layer of a pale yellowish fluid called plasma which consists of water packed with
protein and salt solutes, and many other nutrients as, well as wastes.
The bone marrow contains stem cells, that generate the cells of the blood. Red blood cells (aka erythrocytes), in which the nucleus degenerates and disappears.
Red blood cells are red because they contain large amounts of hemoglobin, a complex
protein that has heme groups which contain the iron that binds oxygen for transport
around the body.
Very little of the oxygen in the blood is in solution, nearly all of it is chemically bound to
hemoglobin ( a single hemoglobin protein molecule can bind 4 oxygen atoms), which
means that a steep gradient of oxygen concentration is maintained across the lungs
Megakaryocytes are cells that fragment to produce tiny membrane bound platelets that
are involved in blood clot formation.
Stem cells also produce two groups of white cells (leukocytes) – components of the
immune system, dealt with later.
Some animals, such as insects, have an open circulatory system, it is not a closed system
of heart veins, arteries etc, the heart pumps blood into a body cavity where it directly
bathes organs and then returns to blood vessels that collect the blood and return it to the
Fishes have a two chambered heart, a single atrium that collects blood from the body,
passes it to the single ventricle where it passes on through the blood vessels of the gills
and the body.
Amphibians and most reptiles have a three chambered heart, they have separate
capillary beds for the lungs and the body and these are collected by separate atria, but
these atria pump into a single ventricle. This means that oxygen poor blood from the
body and oxygen rich blood returning from the lungs is mixed in the single ventricle,
which is inefficient when compared to the four chambered heart, below.
Birds, mammals and a few reptiles (crocodiles for instance) have a four chambered
heart. In a four chambered heart one as essentially two separate pumps, each consisting of
an atrium and a ventricle, that are interconnected so that when one pump collects and
pumps out oxygen poor blood from the body, into the lungs, the blood from the lungs is
collected by the other pump and pumped out into the body.
The Human cardiovascular system
One can regard the fluid balance of the body as existing in three places, the
cardiovascular system - which transports the blood around the body, the lymphatic
system which drains fluids back from the tissues into the cardiovascular system, and the
fluids in and around the tissues and organs. The cardiovascular system transports oxygen to and carbon dioxide away from body
tissues, and transports nutrients from the digestive system to the tissues and wastes from
the tissues to the kidneys. Blood circulation is closed, it proceeds in from the heart in
arteries, into smaller arterioles, from these into single cell layered capillaries, on in to
venules and then into veins for transport back to the heart.
Arteries and veins are complex layered tubes. From the outermost layer inwards one
finds an outer coat, elastic tissue, smooth muscle, and a basement membrane that
surrounds and supports the innermost endothelium that is in contact with the blood.
Arteries, are thicker than veins, they generally have more substantial smooth muscle
layers that add strength, and the ability to actively resist blood pressure as the heart beats.
The elastic nature of arteries is a key feature, it maintains a small amount of blood
pressure between contractions of the heart; during a heartbeat (systole) the high pressure
in the artery causes the wall to bulge and thus retain some blood, when the heart is not
beating (diastole) the elastic artery wall which has bulged during systole now relaxes (the
so called elastic recoil effect) and in so doing exerts a pressure that serves to continue
pumping of the blood, arteries are thus described as "pressure reservoirs".
Arterioles are smaller arteries - with some differences in wall construction, and they
conduct blood from the arteries to the capillaries. Arterioles can respond to hormonal and
nervous signals that command them to reduce in diameter (vasoconstrict) which
increases blood pressure, or to increase their diameter (vasodilate) and this decreases
blood pressure. Arterioles thus play a key role in control of blood pressure and the rate of
blood flow to particular tissues. Arterioles conduct blood to the capillaries and onwards
to the veins
Capillaries are a single cell layer thick, they consist of an outermost basement
membrane and the single cell layer thick innermost endothelium. All cells in the body are
close to a capillary. Capillaries are “leaky”, there are tiny gaps in the walls, so that they
can release nutrients and gases to tissues and accept waste fluids and gases from tissues.
Capillaries in the brain are an exception, they are much less leaky, they bar many
molecules from crossing into the brain fluids - this is the so-called “blood-brain” barrier.
The blood is under high hydrostatic pressure when it has just entered a capillary from an
arteriole and this forces fluids into the tissue space from the capillary so that nutrients and
oxygen are delivered, further along the capillary towards its venule end, osmotic forces
and hydrostatic forces begin to dominate in a way which forces fluids into the capillary,
which is how wastes and carbon dioxide are taken away from the tissues.
Venules are similar to veins but smaller in diameter, they pass the blood on from the
capillaries to the veins. Veins are smaller and thinner than arteries in cross section and
also react to blood pressure by expanding, this is because veins are thinner walled than
arteries and “stretch” more in response to blood pressure but without the strong recoil of
arteries. Veins are blood reservoirs, about 60% of the blood is in the veins. During times of
exercise when blood pressure needs to be increased the smooth muscle layer in the walls
of veins contracts somewhat to exert pressure on the blood, but in fact there is very little
involvement of veins in actually moving blood back to the heart. Veins return blood to
the heart, and to prevent backflow they contain one way valves.
Structure and function of the human heart
The human heart is composed mostly of myocardium (cardiac muscle cells) that
contract is response to electrical stimulation, the myocardium cells are embedded within
elastin and collagen fibers. Myocardium cells are joined by adherent junctions, and
possess gap junctions that allow the electrical excitation that causes the heartbeat to
spread rapidly. The inner part of the heart that is contacted by blood is lined with
The human heart is a double pump which connects two separate but interlinked
circulatory loops, the systemic circuit in which the left side of the heart pumps oxygen
rich blood around the body, and the pulmonary circuit in which the right side of the
heart collects oxygen depleted blood from the body which is high in carbon dioxide, and
pumps it into lungs where it picks up oxygen and offloads carbon dioxide before “re-
connecting” with the heart once again at the left side for transport to the body once more.
The cardiac cycle
Each half of the heart (each “pump”) consists of two chambers, the atrium and the
ventricle. The atrium is a collecting chamber, it receives blood and then passes it to the
ventricle which is a pumping chamber.
The heart receives oxygen depleted and low pressure blood from the body through
essentially, two large veins, the superior and inferior vena cava. These veins connect
with the right atrium. When the right atrium is full it passes the blood to the right
ventricle, which then contracts forcefully and pumps the blood through the pulmonary
arteries to both lung, where it picks up oxygen and offloads carbon dioxide.
From the lungs, the blood, now oxygen rich and at lowered pressure (because it lost
pressure making its way through the lungs) passes to the left atrium through the
pulmonary veins. The left ventricle is much thicker and more muscular than the right
ventricle because it has to develop sufficient pressure to force the blood through the
extensive systemic circulation which has a much larger capillary bed than in the lung
The left ventricle passes the blood into the aortic arch from which issue a number of
arteries to the body, including smaller coronary arteries that supply the heart itself with
blood. There are mitral valves between the atria and the ventricles that prevent back-flow of
blood from the ventricles into the atria, and also semilunar valves in the arteries leaving
the heart, which also prevent backflow between heart beats.
When the ventricles are contracting (and the atria are relaxed), the blood is at its highest
pressure in the body and this is called systole, when the heart is not pumping, in that very
short interval between beats, the blood pressure is lowest and this is diastole. This
process of systole and diastole is what creates the pulse that can be felt at the wrist. When
a blood pressure is recorded as 80/120 the first figure is diastole, the second is systole.
The heart is under electrical control (the cardiac conduction system). Nerves in the
spinal cord and the brain can adjust the rate of the heartbeat, but the intrinsic mechanism
of control of the cardiac system itself, the cycle of atrial and ventricular contraction and
relaxation, is precisely controlled by nerve fibres and bundles in the heart, the heart can
beat entirely on its own, controlled by electrical signals generated and delivered in the
A small clump of cells in the wall of the right atrium called the sinoatrial node (SA
node, aka the cardiac pacemaker) generates waves of electrical signals, each wave
triggers both atria to contract simultaneously and empty blood into the ventricles.
The signal next spreads to the atrioventricular (AV) node, which is located in the
septum dividing the two atria. The AV node then conducts the signal along long fibres to
each ventricle and causes them to contract. A slight but significant pause at the AV node
allows the atria to finish contracting before the ventricles receive the signal to contract.
The SA node is the pacemaker of the heart.
The lymphatic system is a widespread network of lymph capillaries and lymph vessels
that drain excess fluids from the tissues and place them back into the blood circulation.
The lymphatic system also contains numerous lymph nodes where infection fighting
white cells are in residence, and these react against infectious agents, which is why lymph
nodes swell during infections. The spleen is considered to be a part of the lymphatic
Hypertension (high blood pressure) forces the heart to work harder and may eventually
cause heart attacks and heart failure, but can be treated with drugs, dietary measures and
exercise. Atherosclerosis is the accumulation of cholesterol and other lipids in an artery
that both restricts blood flow and causes the artery wall to harden. A fat-laden diet is a
factor, as is smoking. Irregular electrical activity of the heart can also cause various
problems including an inefficient pumping action.
Human Lungs; Respiration
The human respiratory system begins with the oral cavity, and the nasal cavity, where
the nose warms, filters, and humidifies the air before it enters the pharynx