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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 23. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. Why do large animals need an internal transport system such as the cardiovascular system?
  2. What is the difference between the pulmonary and systemic circuits of the cardiovascular system?
  3. What are the different parts of the human circulatory system?
  4. How do the different types of blood vessels differ depending on the role they play in the cardiovascular system?
  5. What is blood pressure and how is it related to the beating of the heart?
  6. What is the difference between an open and a closed circulatory system?
  7. How does high blood pressure influence our health?
  8. What is cholesterol and how does it interact with our circulatory system?

A Guide to the Reading

When exploring the content in Chapter 23 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.

Circuits of the Circulatory System

The human heart has four chambers: two large ventricles that pump blood to the lungs and body tissues, and two atria that collect blood returning from the lungs and the body and help fill the ventricles. The heart pumps blood through two circuits set up in series, the pulmonary circuit and the systemic circuit.  The pulmonary circuit moves blood from the heart to the lungs and back while the systemic circuit moves blood to the tissues of the body and back to the heart.  Both circuits are composed of arteries, capillaries, and veins.  Branching arteries carry blood away from the heart while veins carry blood back.  Connecting the arteries and veins are many tiny vessels known as capillaries.  The structure of the three different blood vessels reflects their function.  Arteries have large diameters and thick, muscular walls to provide rapid transport and withstand the high pressures generated by the heart. Capillaries have small diameters and thin porous walls to facilitate exchange of gases and nutrients with surrounding cells. Additionally, capillaries have a relatively large surface area compared with the volume of blood passing through them and this provides for a large surface area for exchange. Veins also have large diameters and thick, elastic walls with valves that prevent the backflow of blood to the heart. 

For more information on this concept, be sure to focus on:

  • In Section 23.1, The human heart pumps blood to the body through two circuits
  • In Section 23.1, The human heart has two pumps and four chambers
  • In Section 23.2, Blood vessels and factors affecting blood flow
  • Figure 23.1, The Human Cardiovascular System
  • Figure 23.10, Capillaries are Specialized for Exchanging Materials

Relationship Between a Heartbeat and Blood Pressure

During each heartbeat, the heart goes through the cardiac cycle with a relaxation phase known as diastole and a contraction phase known as systole.    During the contraction phase blood leaves the heart under high pressure and flows through the arteries.  During the relaxation phase of the cardiac cycle the heart refills with blood returning from the veins and prepares for the next contraction.  During the beating of the heart, blood pressure shows a regular rise and fall due to this contraction/relaxation cycle.  A typical blood pressure measured in the arteries for a healthy adult is 120/80.  These values represent a peak in blood pressure at the end of the contraction as blood is being pumped into the arteries and a minimum in blood pressure when the heart is relaxed and filling.  As blood flows away from the heart, the blood pressure decreases with increasing distance from the heart so that the pressure in the capillaries and veins is significantly lower than in the arteries.  The heartbeat originates in specialized cells of the heart known as the pacemaker or sinoatrial node (SA node) that stimulate the atria to contract.  The signal passes from the SA node and atria to the ventricles via the atrioventricular node (AV node).  The electrical signals during a heartbeat can be measured as an electrocardiogram.   Organisms have the ability to alter heart rate, blood pressure, and flow rate to meet their changing needs. The distribution of blood flow to the different tissues and organs can be adjusted depending on where the oxygen demand is highest.

For more information on this concept, be sure to focus on:

  • In Section 23.1, What’s in a heartbeat?
  • Figure 23.3, The Human Heartbeat
  • Figure 23.4, Pressure Changes in the Human Circulatory System
  • Figure 23.6, The Heart’s Signaling System

Internal Transport in Animals

Multicellular organisms require internal transport of gases, nutrients, metabolic wastes, and signaling molecules because diffusion is too slow to keep up with respiration and effectively move materials over distances larger than a few cell diameters.  Animals have evolved two distinct means to accomplish internal transport.  Humans, along with many other animals, have closed circulatory systems in which a muscular heart pumps blood through a continuous network of vessels.  Blood moves from the heart through arteries, capillaries, veins, and back to the heart.  Other animals, such as insects and some mollusks, have open circulatory systems that employ a muscular pump or simple heart to push blood into open body spaces between the organs.  The blood then percolates through the tissues and organs before being collected and returned to the heart.  In insects, the blood does not have to transport respiratory gases because of their respiratory system.  While these two solutions to internal transport differ, they both are efficient enough for the animals to survive and thrive.

For more information on this concept, be sure to focus on:

  • In Section 23.3, Comparing circulatory systems among animals
  • Figure 23.11, Open Circulatory Systems

Health and the Cardiovascular System

Numerous health conditions can have deleterious effects on the human cardiovascular system.  Abnormally high blood pressure is know as hypertension and can be due to improper water balance within the body.  An increase in blood volume will lead to an increase in blood pressure.  The kidneys of individuals who have hypertension improperly maintain a high blood volume, thus resulting in high blood pressure.  Hypertension causes the heart to work harder when pumping blood into the body and eventually weakens the walls of blood vessels.  An aneurysm is when the weakened blood vessel wall bulges out due to the high blood pressure.  The health concern of an aneurysm is that it has the potential to rupture. When an aneurysm bursts in the brain, it is called a stroke.  High cholesterol levels within the body can also have a detrimental influence on the cardiovascular system.  Cholesterol is produced in the body from saturated fats found in butter, lard, cheese, or meats.  The body needs some cholesterol because it is the precursor for steroid hormones and vitamin D.  In the blood, cholesterol is packaged with proteins into low-density lipoproteins and high-density lipoproteins.   High-density lipoproteins move waste cholesterol to the liver where it is removed from the body.  Atherosclerosis is a condition where there is a build-up of cholesterol plaques along the walls of blood vessels.  This occurs when the body has excess cholesterol circulating in the blood.   These plaques can lead to an increase in blood pressure and decrease flow within the vessel, thus decreasing the delivery of oxygen to the tissues and cells of the body. 

For more information on this concept, be sure to focus on:

  • In Section 23.4, Hypertension can have serious consequences
  • In Section 23.4, Fat in our diet affects cholesterol levels in our blood
  • Figure 23.13, Cerebral Aneurysm
  • Figure 23.14, Arteries and Cholesterol: A Fatal Attraction

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

Blood Vessels and Factors Affecting Blood Flow

  • Chapter 22 – Section 22.2, How Do Oxygen and Carbon Dioxide Enter and Leave Animals?

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