Download Chapter 42 Part I Circulatory Systems and the Human

Biology 120
J. Greg Doheny
Chapter 42 Part I
Circulatory Systems and the Human Cardiovascular System
Notes:
As mentioned in the previous section, smaller life forms (ie-single-celled microbes) absorb
nutrients directly from their environment, and secrete waste products directly back into it. Large,
multicellular organisms can’t do this, and therefore have evolved networks of tubes that digest
food, bring the resulting nutrients around the body, and pick up and remove waste products. In
mammals (including humans) there is a separate tube system (the Digestive System) which
breaks down and absorbs nutrients. These nutrients are transferred from the small intestine,
across a membrane to a closed circuit of blood that leads to the liver via the Hepatic Portal
System and Hepatic Portal Vein. The liver then filters these nutrients to get rid of toxins, and
transfers them to the main circulatory system. The circulatory system then takes these nutrients
around to all the cells. The cells absorb these nutrients, but also dump their waste products back
into the bloodstream. These waste products must be cleaned from the bloodstream by the
kidneys (the Renal System, or the excretory system). Waste carbon dioxide (a waste product of
cellular respiration) is released into the lungs, and exchanged for fresh oxygen from the air (the
Respiratory System, also known as the Pulmonary System). There is usually some sort of a
muscular pumping system to pump these nutrients and gasses around this network of blood
vessels (a Heart). The network of blood vessels and the muscle-pump that moves the blood
around is collectively referred to as the Cardiovascular System. The Cardiovascular System is
closely associated with the Pulmonary System, which adds oxygen to the blood, and removes
carbon dioxide.
Topics
I.
II.
Variations on Circulatory Systems, and Variations on Hearts
The Human Heart
I.
Variations on Circulatory Systems and Hearts
Closed vs. Open Circulatory Systems (Figure 42.3): A circulatory system is a system of tubes
that carries oxygen and nutrients around to the various cells of the body. Most Arthropods
(insects, crustaceans etc.) have what’s called an open circulatory system, where a single fluid
called hemolymph is pumped from the heart to spaces surrounding the body’s organs through a
series of open-ended tubes. The hemolymph carries oxygen on special proteins similar to
hemoglobin (in mammals), but which is not contained in red blood cells (erythrocytes). Most
other types of organisms (including mammals, worms, birds, reptiles etc.) have a closed
circulatory system where blood carries oxygen and nutrients around to cells, and a parallel
system (the Lymphatic System) carries lymph (serum that does not contain blood cells) around to
the organs. The circulatory system containing the blood is ‘closed’ because it flows from the
heart, around a circuit of tubes, and then back to the heart again without ever leaving those tubes.
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In a closed circulatory system, blood flows from the heart via a pumping action, and has a
pulse. Blood flows away from the heart in a network of blood vessels called arteries. Arteries
are large, elastic, and have high blood pressure inside. These large arteries flow into networks of
smaller tubes called arterioles, and arterioles flow into extensive networks of very small blood
vessels called capillary beds. The capillarys (microscopic tubes) that make up these beds are
only one or two cells thick (squamous cell epithelium), allowing gasses, nutrients, and serum
(the liquid portion of blood) to diffuse across the membrane in both directions. From these
capillaries, fresh oxygen flows into cells, and waste carbon dioxide flows out. The same is true
for new nutrients flowing into cells and chemical waste products flowing out. Blood coming out
of these capillary beds then enters small veins called venules, which flow into larger vessels
called veins, which return the blood to the heart. Because the large capillary bed has acted as a
‘damper,’ blood flowing out from capillary beds no longer has a pulse. Hence, blood flows TOO
the capillary beds in high-pressure tubes that have a pulse (arteries), and AWAY from the
capillary beds in low-pressure tubes that don’t have a pulse (veins).
Definition of Arteries vs. Veins: Arteries are defined as blood vessels that carry blood AWAY
from the heart, and veins are defined as blood vessels that carry blood TO the heart. There is a
common misconception that arteries are blood vessels that carry oxygen rich blood (and are
coloured red, because oxygenated hemoglobin is red in colour), while veins are blood vessels
that carry oxygen-depleted blood (and are coloured blue because oxygen-depleted hemoglobin is
blue in colour). This definition of arteries vs. veins would be fine, until we get to the network of
blood vessels that carries blood from the heart to the lungs and vice versa. The vessels that
carry oxygen-depleted blood from the heart to the lungs are called ‘pulmonary arteries’, despite
the fact that the blood is oxygen poor; and the vessels that carry oxygen rich blood from the
lungs back to the heart are called ‘pulmonary veins,’ even though the blood is very oxygen-rich.
Thus, an artery is always defined as a blood vessel that carries blood AWAY from the heart, and a
vein is always defined as a blood vessel that carries blood TO the heart, regardless of the oxygen
content of the blood. There are also some physical differences between veins and arteries which
we’ll discuss later.
The Cardiovascular System’s Relationship to the Pulmonary System: If the heart is going to
carry oxygen-rich blood around to the cells, the blood must come into contact with the air. More
primitive organisms (like insects and worms) simply have holes in their exoskeleton or cuticle
that lets air inside, making contact with the vessels, and exchanging oxygen for carbon dioxide
directly across the blood vessel membranes. These organisms usually have a network of vessels
located near their skin (called a ‘cutaneous system’) to allow this to happen. (The word
‘cutaneous’ refers to the surface of the skin.)
More advanced organisms (like mammals) have air sacks on the inside that are lined with
capillary beds that allow oxygen exchange. These air sacks are called lungs, and the lung system
is referred to as the Pulmonary System. (the word ‘pulmonary’ refers to lungs.) These
organisms have a network of vessels that carry blood to the lungs, called the ‘Pulmonary
Circuit,’ to distinguish this network from the network of vessels that carries blood to the rest of
the body (called the ‘systemic circuit’).
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Some intermediate organisms, like amphibians (ie-frogs) have both a set of lungs, as well as the
ability to breath through their skin. The word ‘cutaneous’ means ‘on or just underneath the
skin.’ Frogs, therefore, have a network of vessels called the ‘Pulmocutaneous Circuit’ (a
combination of ‘pulmonary’ and ‘cutaneous’) which carries blood to capillary beds lining both
the lungs and the skin for purposes of gas exchange. When amphibians are underwater, they are
able to shut off blood flow to their lungs (because they can no longer breath air), and shunt all of
their blood to their skin.
Variations on Hearts I: One heart vs. many hearts. More primitive organisms often have
many hearts (ie-auxiliary hearts in worms, Figure 42.3b), or a dispersed heart (ie-tubular heart
in insects, Figure 42.3a). More advanced organisms have a single, large, centrally located heart
(ie-mammals, Figures 42.4 and 42.5).
Variations on Hearts II: One Chamber vs. Two Chambers. Some organisms (ie-mammals)
have a two chambered heart while others have a one chambered heart (Figure 42.4). In a one
chambered heart, one beat of the heart must be strong enough to send the blood all the way
around the entire body circuit, including the breathing apparatus (lungs or gills). This is what
fish do. (In the case of fish, they have gills instead of lungs.) They have a one chambered heart
that must send blood all the way around their body. This is actually quite difficult to do, and
blood pressure is quite low by the time it reaches the end of the circuit. By contrast, organisms
with a two chambered heart have two separate circuits for their blood. One chamber of the heart
sends blood to the lungs (the pulmonary circuit), and then back to the heart. The second
chamber then sends the oxygen-rich blood to the body (the systemic circuit). This is actually
more efficient because you can generate higher blood pressure when sending the blood around
the body, allowing the blood to return to the lungs and be oxygenated again more often. For this
reason, fish have a low metabolic rate when compared to mammals.
Variations on Hearts III: Two Separate Chambers vs. Two Joined Chambers (Figure 42.5):
The mammalian heart is very efficient because it has two separate chambers, and is able to
generate high pressure when sending blood out to the body. In humans, the two chambers are
separated by a wall of tissue called a ‘septum.’ Occasionally an unfortunate person is born with
a hole in their septum (called a ‘septal defect’) which allows the blood in the two chambers to
mix. Such people tire easily because their circulatory system can’t generate as much pressure,
and because the oxygen-poor blood returning from the body is able to mix with the oxygen rich
blood returning from the lungs. (ie-What’s the point of oxygenating the blood in the lungs if it’s
just going to mix with oxygen-poor blood again?)
Other life forms have the same problem. Fish have only one chamber to begin with.
Amphibians theoretically have two chambers, but they are only separated by a ridge rather than a
complete septum, allowing the oxygen rich blood to mix extensively with the oxygen poor blood.
Reptiles have separate chambers that are joined by a hole (like a human with a septal defect),
thus also allowing the oxygen rich and oxygen poor blood to mix. Mammals, by contrast, have
two chambers that are completely separated from each other. This is the most efficient system,
because it can generate higher blood pressure when sending newly oxygenated blood out to the
body, and the blood that is sent out to the body is very rich in oxygen (because it wasn’t remixed with the oxygen-poor blood).
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II The Human Heart
Like all mammas, humans have a two chambered heart with a complete septum (separating the
two chambers), as well as a pulmonary circuit that sends blood from the heart to the lungs, and
a separate systemic circuit that sends the oxygen-rich blood out to the body. In addition,
humans have one set of arteries and veins sending blood to the head (because the brain is very
important) and another set sending blood to the lower body (Figure 42.6). Oxygen poor blood
returning from the body enters the RIGHT side of the heart, and is sent from there to the lungs
(the pulmonary circuit). Oxygen rich blood then flows back from the lungs to the LEFT side of
the heart where it is pumped out into the body (the systemic circuit).
The human heart actually has FOUR chambers. Two of the chambers are called ventricles, and
the other two are called atria (singular: atrium). The ventricles are surrounded by a thick layer
of cardiac muscle, and provide the major pumping force. The atria merely store blood before it
enters the ventricles (the word atrium is Latin for an entry room in a house). When the heart
contracts (beats) it ejects blood away from itself, and creates a spike in blood pressure called
systole. Then the heart relaxes in order to fill with blood again, causing a drop in blood pressure
called diastole. These peaks and troughs in blood pressure are what create the pulse, which can
be felt in the arteries. The pulse cannot be felt in the veins, because the variations in pressure
have been absorbed in the capillary beds, which act like shock absorbers in a car.
The atria do not contract as strongly as the ventricles, and are simply compartments where the
blood waits before entering the ventricles. Atrial contraction only needs to be strong enough to
send blood from the atrium to the ventricle. In order to avoid backflow of the blood (also called
‘reflux’), the various chambers of the heart are separated by valves (which are really just flaps of
connective tissue) that ensure that the blood only flows in the forward direction. There are four
valves, each of which has its own name (see below). Two of them are called atrioventricular
(AV) valves because they prevent backflow from ventricles back into atria, and two are called
Semilunar valves. The semilunar valves prevent backflow of blood as it leaves the heart, en
route to either the lungs or the body.
Parts of the Human Heart (Figures 42.6 and 42.7): If you were a red blood cell returning from
the body to the heart, this is the order in which you would see these structures. (Learn this,
because it’s one of the practice questions!)
1. Superior Vena Cava and Inferior Vena Cava: Large veins on the right side of the body
where blood returning from the head (superior) and body (inferior) empties into the right
atrium.
2. Right Atrium: chamber where oxygen-poor blood waits before entering the right
ventricle (where it will be sent to the lungs).
3. Tricuspid Valve: Valve separating the right atrium from the right ventricle, ensuring
blood only flows from the atrium to the ventricle, rather than the reverse.
4. Right Ventricle: Cardiac muscle-lined chamber where blood is sent out of the heart, and
into the pulmonary circuit.
5. Pulmonary Semilunar Valve: valve that ensures blood goes to lungs, rather than back
into the right ventricle.
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6. Left and Right Pulmonary Arteries: Arteries that carry blood away from the heart and
into the left and right lungs (into lung capillary beds).
7. Left and Right Pulmonary Veins: Veins that carry oxygenated blood from the lungs
back to the heart.
8. Left Atrium: compartment for holding newly oxygenated blood before it enters the left
ventricle, and is then sent out to the body.
9. Mitral Valve: Valve that prevents backflow between the left ventricle and atrium.
10. Left Ventricle: Cardiac muscle-lined compartment that fills with oxygenated blood and
then contracts to send it out to the body.
11. Aortic Semilunar Valve: Valve that prevents backflow of oxygenated blood leaving the
heart and entering the aorta (going to the systemic circuit).
12. Aorta: Artery leaving the heart. This is the most important artery, because it supplies
oxygenated blood to the whole body. The aorta forms a bend called the Aortic Arch,
with three smaller arteries coming off of it (called the Carotid Arteries) that send blood
to the head. The rest of the aorta then proceeds downwards to the body (the Descending
Aorta).
Heart Rhythm and the Electrocardiogram (ECG; Figure 42.9): As mentioned in Chapter 40,
cardiac muscle fibers are able to transmit signals to each other, meaning that a signal to contract
(beat) can originate in one part of the heart, and then be rapidly relayed to the other parts of the
heart. The signal to beat originates in a part of the heart called the sinoatrial node (SA) or
‘pacemaker,’ located in the right atrium. This signal spreads to both atria, causing them to
contract, and fill the ventricles with blood. The electrical signal then spreads to another spot
called the atrioventricular (AV) node, and from there to the apex (pointed part) of the heart,
causing the ventricles to contract. When the electrical signals spread normally in this pattern it is
referred to as a normal sinus rhythm. The transmission of electrical signals from one part of
the heart to another can be picked up by electrodes placed on the skin, resulting in an
electrocardiogram (Figure 42.9). A physician can read an ECG, and tell if there are any
problems with the various parts of the heart based on these electrical impulses.
The parts of an ECG are traditionally labelled as P, Q, R,S, and T, with P representing activation
of the atria, QR and S representing activation of the ventricles (a ‘pump’ of the heart), and T
representing recovery of the heart in preparation for the next pump. An abnormally fast heart
rate (more than about 100 beats per minute) is known as tachycardia, and an abnormally slow
heart rate (less than about 50 beats per minute) is known as bradycardia. A heart beat that
deviates from the standard PQRST pattern is called a cardiac dysrhythmia. Two main types of
dysrhythmia are atrial fibrillation (where the atria contract in an uncoordinated pattern), and
ventricular fibrillation (where the ventricles contract in an uncoordinated pattern). Both
conditions can be corrected by a pacemaker (an implanted device that sends electrical signals to
the heart).
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PATHOLOGY OF THE HEART: Congenital Heart Defects.
You have learned that the human heart has four chambers (two atria and two ventricles)
separated by walls called septa (singular: septum). A “septal defect” is a hole in one of the
septa which allows oxygen poor blood from the left side of the heart to mix with oxygen rich
blood in the right side of the heart. There are two general types of septal defects. An Atrial
Septal Defect (ASD) is a hole in the septum that separates the left and right atria. A
Ventricular Septal Defect (VSD) is a hole in the septum that separates the left and right
ventricles.
ASDs are more common than VSDs, and are often caused by a failure of the fetal heart to
develop properly. Inside the womb, a developing fetus does not have access to air to oxygenate
its own blood. Instead, it gets oxygen from its mother via the placenta and umbilical cord, and
the developing fetal lungs and heart are bypassed. Oxygenated blood from the umbilical cord
flows into the right atrium from a spot where the umbilical cord is attached to the inferior vena
cava. A hole in the fetal atrial septum, called the foramen ovale, allows the blood to flow
directly from the right atrium to the left atrium, bypassing the fetal lungs. The foramen ovale is
supposed to close when the baby is born, but sometimes it remains partly or completely open.
When the foramen ovale fails to close properly it creates a condition called Patent Foramen
Ovale (PFO). (The word ‘patent’ means open, or unclosed.)
Types of ASDs: If the atrial defect is located up near the area where the superior vena cava
empties into the right atrium it is called a “Sinus Venous ASD.” If it is located lower down, but
still closer to the back of the right atrium than middle, it is called an “Ostium Secundum ASD,”
and if it is located directly between the two atria it is called an “Ostium Primum ASD.” (See
Figure 1.)
FIGURE 1: Types of Atrial Septal Defects (ASDs).
1. Sinus Venous ASD
2. Ostium Secundum ASD
3. Ostium Primum ASD
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Types of VSDs: VSDs are less common than ASDs, but like the ASDs they are given different
names depending on where the defect is located. Refer to Figure 2 for how the VSDs are named.
FIGURE 2: Types of Ventricular Septal Defects (VSDs)
1. Perimembranous VSD
2. Muscular VSD
3. Canal Type VSD
4. Subpulmonary VSD
Other Types of Congenital Defects: In addition to the foramen ovale, there is another hole
that is normally present in the developing fetal heart that connects the pulmonary artery directly
to the aorta. This connection between the pulmonary artery and the aorta is called the ductus
arteriosus, and it normally closes before the baby is born. If it does not, it leads to a condition
called patent ductus arteriosus (PDA). Finally, when VSD is combined with a narrowing of
the pulmonary artery, it leads to a condition called tetralogy of Fallot (ToF). ToF is a very
serious condition, and is sometimes called “blue baby syndrome” because babies that are born
with it appear blue, due to lack of oxygen in the blood (blue colouring due to lack of oxygen in
the blood is called cyanosis.)
PRACTICE QUESTIONS
Short Answer Questions:
1. Which side of the human heart receives oxygen-poor blood from the body?
2. Which side of the heart sends oxygen-rich blood to the body?
3. Name the two large veins that return oxygen poor blood from the head and body to the
heart (2 points).
4. Name the large artery that carries oxygen-rich blood from the heart to the body.
5. General term for blood vessels that carry blood away from the heart.
6. General term for blood vessels that carry blood to the heart.
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7. Name for network of microscopic blood vessels that bring oxygen and nutrients to
tissues, as well as removing waste products from tissues.
8. Name for the heart valve that prevents backflow from the aorta back into the heart.
9. Name for the heart valve that prevents backflow from the lungs back into the heart.
10. Name for the heart valve that prevents backflow from the right ventricle to the right
atrium.
11. Name for the heart valve that prevents backflow from the left ventricle to the left atrium.
12. Name for the circuit that carries blood to the lungs.
13. Name for the spike in blood pressure that occurs when the heart contracts, and pumps
blood out.
14. Name for the drop in blood pressure when the heart relaxes in order to fill with blood
again.
15. What is the technical name for oxygen-carrying red blood cells.
16. What does P represent in a classic PQRST wave seen in an ECG?
17. What does QRS represent in a classic PQRST wave seen in an ECG?
18. What does T represent in a classic PQRST wave seen in an ECG?
19. What do you call an abnormally fast heart rate (more than about 100 BPM)?
20. What do you call an abnormally slow heart rate (less than about 40 BPM)?
21. What do you call uncoordinated contractions of the atria?
22. What do you call uncoordinated contractions of the ventricles?
23. What is an ASD?
24. What is a VSD?
25. What is the foramen ovale?
Essay Questions:
1. Explain the difference between a pulmonary circuit and a pulmocutaneous circuit. (Hint:
mammals have one type of circuit, and frogs have the other. 20 points)
2. In cardiac pathology (diseases of the heart), what is a ‘septal defect?’ (5 points)
3. Explain why it is advantageous to have a two chambered heart rather than a one
chambered one. (Hint: Fish have a one chambered heart, while mammals have a two
chambered heart. 10 points)
4. Explain why having a two chambered heart (ie-mammals), comprising separate
pulmonary and systemic circuits is more efficient than having A) a one chambered heart
(ie-fish), or B) a two chambered heart with a hole connecting the two chambers (iereptiles and amphibians). (20 points)
5. What is the difference between the pulmonary circuit and the systemic circuit of an
animal with a two chambered heart? (10 points)
6. Both humans and frogs have a systemic circuit for their circulatory system which is
basically the same for both organisms. Explain how their pulmonary circuits differ. (20
points)
7. Both the esophagus and the heart have methods to avoid reflux. Contrast these two
methods. How are they different, and which is more efficient? (20 points)
8. What is PFO? (20 points)
9. What is ToF? (20 points)
10. What is a PDA? (20 points)
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Heart Valves:
A. Tricuspid Valve
B. Pulmonary Semilunar Valve
C. Mitral Valve
D. Aortic Semilunar Valve
1. If you were a red blood cell returning from the body to the heart, in which order would
you encounter these valves?
2. Which are classified as atrioventricular (AV) valves?
3. Which are classified as semilunar valves?
4. Which prevents backflow from the right ventricle and the right atrium?
5. Which prevents backflow from the left ventricle to the left atrium?
6. Which prevents backflow from the lungs to the heart?
7. Which prevents newly oxygenated blood that is leaving the heart and entering the body
from flowing back into the heart.
If you were a newly oxygenated red blood cell traveling away from the heart, which of
these things would you encounter first, second, third etc. Put these things in order on their
journey away from, and then back to the heart (start with the moment the oxygenated red
blood cell leaves the heart):
A. Descending Aorta
I. Aortic Semilunar Valve
B. Capillary bed
J. Arteriole
C. Lungs
K. Artery
D. Aortic Arch
L. Vein
E. Left and right pulmonary veins
M. Right Ventricle
F. Left and right pulmonary arteries
N. Left Ventricle
G. Venule
O. Left Atrium
H. Inferior Vena Cava and Superior
P. Right Atrium
Vena Cava
Extended Matching Inventory: Match the term to the definition.
A. Arteries
J. Pulmonary Semilunar
B. Arterioles
K. Subcutaneous
C. Atrium
L. Systolic
D. Capillary beds
M. Tricuspid
E. Cardio
N. Vasoconstriction
F. Diastolic
O. Vasodilation
G. Erythrocyte
P. Ventricle
H. Hemolymph
Q. Venules
I. Pulmonary
R. Veins
1. General term for blood vessels that carry blood BACK TO the heart.
2. Name for the liquid that insects and other animals with an open circulatory system use
instead of blood (which contains special proteins that carry oxygen around instead of red
blood cells).
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3. Technical name for a red blood cell.
4. A heart chamber where blood goes before entering a heart ventricle.
5. Revers to a temporary and deliberate narrowing of the arteries, resulting in an increase in
blood pressure.
6. Valve that prevents backflow from the lungs to the heart.
7. Word meaning ‘just underneath the skin.’
8. Valve that prevents backflow between the left ventricle and left atrium.
9. Valve that prevents backflow between the heart and the systemic circuit.
10. Valve that prevents backflow between the heart and the pulmonary circuit.
11. Valve that prevents backflow between the right ventricle and right atrium.
12. A heart chamber where blood goes after leaving an atrium.
13. Smaller version of blood vessels that carry blood back to the heart.
14. Spike in blood pressure that arises when the Heart contracts.
15. Refers to opening up of blood vessels, resulting in a drop in blood pressure.
16. Term for blood vessels that carry blood AWAY from the heart.
17. Lower blood pressure associated with relaxation of the heart in between heartbeats.
18. Smaller version of blood vessels that carry blood away from the heart.
19. Microscopic networks of blood vessels that exchange gasses, nutrients and waste
products with various tissues and organs throughout the body.
20. Term referring to the lungs.
21. Term referring to the heart.
© J. Greg Doheny 2014
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