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Homeostasis and the Circulatory System
Slide 1 – Welcome
Welcome to unit 3. Many students find the subjects covered in this unit challenging. We will be
studying the process of homeostasis, and exploring the circulatory, immune, and reproductive
Slide 2 – Homeostasis
Your body is like a car in that both you and your car have normal limits you must operate in to
run smoothly. Think about a car; if the oil pressure becomes too high or too low, the car will not
function properly, and may even break down. Your body is the same way. If your blood
pressure gets too high or too low, there can be serious health consequences. The process by
which your body maintains itself within specific limits is called homeostasis. Your body was
meant to have a constant internal environment. Small changes in the internal environment could
escalate into larger problems.
Slide 3 – Feedback Mechanisms
So how does the body maintain a constant internal environment? The body uses feedback
mechanisms. There are two type of feedback mechanism in the body, positive and negative.
Think of an annual work evaluation. If your boss gives you positive feedback, he wants you to
continue what you are doing, but if he gives you negative feedback, he wants you to stop what
you are doing. The body works the same way. In negative feedback, a changed is detected by
the body and actions are taken to reverse the change. Negative feedback is much more common
in the body. Generally you do not want something like rising blood pressure to continue out of
control, because this could cause major problems; therefore the body takes measures to stop the
rising blood pressure.
Slide 4 – Negative Feedback
Negative feedback is a way of reversing a change in the body. When negative feedback occurs
in the body, a change is sensed and the body uses different mechanisms to stop the change and
return things to normal. We will examine two types of negative feedback in more detail; the
maintenance of a body temperature around 98.6 degrees F and the regulation of blood glucose
Slide 5 – Temperature regulation
Before we examine the body, let’s start with an example of negative feedback that we are
familiar with, a thermostat in our homes. A thermostat has a sensor that detects when the
temperature deviates from the set point. For this example let’s say the set point is 70 degrees F.
If the temperature drops below 70 degrees F the sensor tells the thermostat to turn on the heater
until the temperature returns to the set point. If the temperature rises above 70 degrees F, the
thermostat turn on the air conditioner until the temperature returns to the set point. Regulation of
body temperature works in a very similar manner.
Slide 6 – Regulation of body temperature
Let’s examine the regulation of body temperature. Two things can go wrong with body
temperature; you can get too hot or too cold. The bottom portion of the diagram on this slide
shows what happens in the body when the body temperature drops below the set point and the
top indicates what happens when you get too hot. In the middle we see the hypothalamus, a part
of the brain. The hypothalamus works like a sensor and detects changes in body temperature.
When the body temperature drops below 98.6 degrees F, the body has several mechanisms to
raise the temperature. These mechanisms include: shivering, constricting blood vessels in the
skin, and producing goose bumps on the skin. All of these mechanisms help return the body to a
normal temperature. On the other hand if the body goes above the set point, the body triggers
mechanisms to lower the temperature. These mechanisms include: sweating and the dilation of
blood vessels near the skin.
Slide 7 – Blood glucose regulation
Another example of negative feedback in the body is regulation of blood glucose levels. After
eating a meal, the amount of glucose in the blood increases. Remember from our discussion of
digestion that food is converted from polymers to monomers. Glucose is a monomer of
carbohydrates. The body does not need all of this glucose circulating, so a hormone called
insulin is released from the pancreas to convert the excess glucose into glycogen. Recall that
glycogen was one of the polysaccharides discussed in the organic molecules section of this
course. Glycogen is stored in the muscles and liver. On the other hand, if you haven’t had a
meal for a while, the amount of glucose in the blood drops. When this occurs, the body releases
a hormone called glucagon from the pancreas that converts glycogen into glucose for use by the
cells in your body.
Slide 8 – Types of Diabetes
A condition known as diabetes is when insulin is either not produced by the body, type 1 or
juvenile diabetes; or the body does not recognize the presence of insulin, type 2 diabetes. This
causes the blood glucose levels of these individuals to rise. High levels of glucose in the blood
can cause damage to cells. Individuals that have type 1 diabetes have to taken insulin to
maintain their blood glucose levels within normal limits. People that have type 2 diabetes can
often maintain healthy levels of blood glucose levels through a healthy diet and exercise.
Individuals with diabetes have a higher risk of heart disease, stroke, kidney disease, blindness,
and nerve damage.
Slide 9 – YouTube Video – Big Bang Theory Homeostasis
Slide 10 – Check Your Understanding
Now that we have learned about negative feedback control, let’s check your knowledge of the
subject. The following slides will have a series of questions on the topic. Be sure to click
“Submit” after answering each question.
Slides 11 through 22 –Negative Feedback Control Interactive Quiz
A non–graded assessment of your knowledge of negative feedback control.
Slide 23 – Positive feedback
The body does not have many examples of positive feedback. Usually the body doesn’t want
changes to occur. An example of positive feedback is the process of delivering a baby. The
bay’s head presses on the cervix, which cause it to dilate. This signals the brain to release the
hormone oxytocin. Oxytocin causes the uterine to contract. The contractions of the uterus
caused the cervix to dilate even further. The dilation of the cervix signals the brain to produce
more oxytocin. The cycle continues until the baby is born.
Slide 24 – Circulatory System
We just learned about feedback mechanisms and will we continue to study them throughout the
remainder of the course. Now we will switch gears to study the circulatory system.
Slide 25 – Purpose of the circulatory system
Let’s review a little from unit 1. Remember the process of cellular respiration? The cells of
your body perform cellular respiration in order to make the molecule ATP energy. In order to be
able to do this, these cells will need oxygen and glucose. The by–product of cellular respiration
was carbon dioxide. How do cells obtain oxygen and nutrients and get rid of waste products?
The answer is the circulatory system. The circulatory system brings cells nutrients, such as
oxygen and glucose and takes away wastes products such as carbon dioxide.
Slide 26 – Parts of the circulatory system
In order to perform its function, the circulatory system needs three parts: the blood which
contains the liquid parts of the blood and the blood cells; the arteries, veins, and capillaries or
blood vessels, which are the transportation tubes the blood travels through; and the heart, which
serves as the pump to move the blood around the body.
Slide 27 – Lymphatic system
The lymphatic system is another transport system of the body that is interconnected with the
circulatory system. We mentioned lymph vessels called lacteals during our discussion of the
digestive system. Recall that fatty acids were absorbed into the body through lymph vessels.
We will be discussing the lymph system in more detail when we examine the immune system.
Slide 28 – Transport function
The movement of oxygen and glucose through the blood to the cells and the removal of carbon
dioxide from the cells are two of the transport functions of the circulatory system. The blood
will also transport other needed materials to the cells such as vitamins, minerals, amino acids,
etc. These are all substances the cells need to perform their functions and to survive. The blood
transports waste materials away from the cells. Carbon dioxide is the waste product that you are
already familiar with, however cells produce other wastes some of which end up in urine and are
make from old proteins that are broken down. Another transport function of blood is the
movement of hormones. Remember the discussion of the regulation of blood glucose levels.
This regulation occurs through the hormones insulin and glucagons that travel from the pancreas
to the liver through the circulatory system.
Slide 29 – Defense and housekeeping
In addition to transport functions, the circulatory system is also responsible for housekeeping and
defense. These function are primarily carried out by white blood cells, which circulate
throughout the body via the circulatory system We will examine these cells more when we
explore the immune system.
Slide 30 – Distribution of body heat
The circulatory system is also responsible for distributing heat throughout the body. Remember
from our discussion of the maintenance of body temperature that some of the mechanisms the
body uses involved the circulatory system. Blood has the ability to carry heat to cooler areas of
the body and away from warmer areas of the body. This is why the blood vessels near the
surface of your skin dilate when you are hot and these same vessels constrict when you are too
Slide 31 – Blood contents
What is blood? Blood is a fluid composed of dissolved substances, water, and blood cells. You
can take blood and spin it very fast in a centrifuge and the blood will separate into three parts.
The top part will be a yellow liquid and the bottom will be red. In the middle is a thin white
layer. The yellow portion of the blood is the plasma. Plasma is made of water and dissolved
substances. The middle section will contain the white blood cells that are important for fighting
infection and platelets that are responsible for forming clots. The red portion at the bottom is
made up of red blood cells.
Slide 32 – Bone marrow
Blood cells are produced in the bone marrow. Bone marrow is the spongy material found inside
of bones. The next time you are at the grocery store look at a steak. Look at the yellowish
spongy part inside the bone, this is the marrow. The bone marrow is capable of producing all the
different types of blood cells. It knows which one to become because of the chemical
messengers it receives from the body. Let’s look at an example of how the bone marrow knows
to become a red blood cell.
Slide 33 – Red blood cell production
Suppose you went out this morning and donated blood at a blood drive. Your body will be able
to sense that you are a pint low on blood. Your kidneys will detect that you don’t have enough
red blood cells. The kidneys produce a hormone called erythropoietin, which tell the bone
marrow to start making more red blood cells. A stem cell in the bone marrow is a cell that can
become different types of cells depending on the type of signal it receives. The stem gets the
signal to become a red blood cell and transforms itself into a preliminary cell called an
erythroblast. The erythroblasts continue to transform until it becomes an erythrocyte or red
blood cell. An interesting component of this transformation is that the nucleus of the cell is lost.
Red blood cells do not have a nucleus. When the number of blood cells returns to normal, the
kidneys stop sending the signal to make more. This is another example of a negative feedback
Slide 34 – What do red blood cells do?
What do red blood cells do? The red blood cells are responsible for carrying oxygen to the cells
of the body. Each red blood cell contains hundreds of proteins called hemoglobin abbreviated
Hb. Each hemoglobin molecules can carry 4 oxygen molecules bound to the iron atoms within
them; therefore each red blood cell can carry thousands of oxygen molecules.
Slide 35 – Red blood cell design
Red blood cells have unique characteristics that allow them to be very successful in carrying
oxygen. First, red blood cells are flat in shape in the middle, similar to a piece of Certs candy.
The flat shape means that most of the hemoglobin is close to the surface of the cell where it is
easier to release and obtain oxygen. In addition, red blood cells do not have a nucleus. The
nucleus would just be wasted space. The cell needs to carry as much oxygen as possible.
Slide 36 – Bilirubin
The fact that red blood cells do not have nuclei means that they are not capable of reproduction.
Remember that the nucleus contains the DNA information needed for a cell to reproduce. Red
blood cells will eventually wear out and need to be replaced. Worn out cells are removed by the
liver. The iron is recycled and reused, but the protein is broken down and excreted out of the
body. The broken down hemoglobin becomes a substance called bilirubin, which is yellow.
People’s whose livers are not functioning properly develop a condition called jaundice, Which
gives these individuals a yellowish appearance to their skin, nails, and eyes. The yellow color is
a result of the circulating bilirubin.
Slide 37 – Check Your Understanding
Now that we have learned about the importance of red blood cells, let’s check your knowledge of
the subject. The following slides will have a series of questions on the topic. Be sure to click
“Submit” after answering each question.
Slides 38 through 43 – Importance of Red Blood Cells Interactive Quiz
A non–graded assessment of your knowledge of red blood cells.
Slide 44 – White blood cells
The white blood cells consist of many different types of cells. We will not discuss them in much
detail here, but will examine them further during our exploration of the immune system.
Slide 45 – Platelets
The platelets are not really cells, but rather parts of cells called a megakaryocyte. Platelets are
involved in clotting. At the site of an injury, platelets stick together to form a platelet plug,
which along with several proteins work to seal the injury. As we shall see in the immune system,
it is necessary to keep bacteria and viruses out of the body. A cut provides an entry into the body
for bacteria and viruses; therefore a clot is very necessary in not only from preventing blood
from escaping the body, but keeping pathogens out of the body as well.
Slide 46 – Clot formation
Let’s look at the formation of a clot in more detail. When a vessel is damaged, two things occur
simultaneously; first the platelets are attracted to the site and form a platelet plug. Secondly, a
blood protein called prothrombin is activated to form thrombin. This protein in turn activates the
protein fibrinogen to become fibrin. Fibrin acts like a net to hold the platelet plug in place over
the injury. This seals the break. Often time’s red blood cells get caught in the fibrin threads and
this is why the clot looks red.
Slide 47 – YouTube Video
An animation of blood clot formation after injury.
Slide 48 – Blood vessels
Now that we have talked about the components of blood, let’s turn our attention to blood vessels.
There are three types of blood vessels: arteries, capillaries, and veins.
Slide 49 – Arteries
You may have heard that arteries carry oxygenated blood. This is not entirely true. Arteries
carry blood away from the heart. One artery, the pulmonary artery carries blood from the heart
to the lungs and this blood is deoxygenated.
Slide 50 – Artery properties
Arteries have thick muscular walls that allow them to contract and keep blood flow moving.
These vessels are then able to regulate blood flow. The largest artery I the body is the aorta,
which comes right off the heart. Smaller arteries are called arterioles. Arterioles connect with
Slide 51 – Capillaries
Capillaries are very small blood vessels that are found in every tissue. The capillaries are the site
of gas exchange between cells and the blood. Capillaries have thin walls and have very thin
Slide 52 – Diameter size
The small diameter of capillaries means that the flow of blood in these vessels is slow. This is
beneficial for the body, because it allows for the exchange of materials between the body’s cells
and the blood. Many capillaries are so small that red blood cells travel through them in a single
file line. Think about the beltway. If there are four lanes of traffic moving you can move much
faster than if there was on one lane of traffic open.
Slide 53 – Capillary exchange
So what exchange is occurring in the capillaries? It is from the capillaries that oxygen, glucose,
and other nutrients get into the cells. The blood in the capillaries will take the carbon dioxide
and waste products. In the lungs the process is reversed; the red blood cells take in oxygen and
carbon dioxide is given off by the blood.
Slide 54 – Blood Pressure
Whenever you visit the doctor’s office your blood pressure is checked. What is blood pressure
and why is it important to maintain your blood pressure within a specific range? Blood pressure
is the force of the blood exerted against the walls of the arteries as the heart pumps blood
throughout the circulatory system. There are two aspects of blood pressure; systolic and
diastolic pressure. The systolic pressure is the force that is exerted when the heart contracts,
while diastolic pressure is the force of the blood against the arteries when the heart relaxes.
Blood pressure is measured in mm of mercury (mmMg) and is shown as a fraction with the
systolic pressure on the top and the diastolic pressure on the bottom. A healthy blood pressure is
120/80 or less. Hypertension is a condition in which the blood pressure is elevated above
140/90. Untreated hypertension can lead to stroke, heart attack, kidney damage, or eye damage.
Slide 55 – Hypertension
YouTube video – Hypertension
Slide 56 – Lymph vessels
The capillaries are some what leaky and materials get out into the space around the cells in the
capillary beds. Capillaries are surrounded by lymph vessels which collect this leaked material
and return it to the circulatory system.
Slide 57 – Veins
One the blood has passed through the capillary bed, it will return back toward the heart. Veins
are blood vessels that carry blood toward the heart. Not all veins carry deoxygenated blood. The
pulmonary vein carries blood from the lungs to the heart. This blood is oxygenated.
Slide 58 – Vein valves
In contrast to arteries, veins have very thin walls. The veins do not have layers of muscles that
force blood back toward the heart. The movement of skeletal muscle is what returns blood to the
heart. Veins do contain valves, which prevent the back flow of blood. The valves work like a
gate so that blood can not go back from where it just came.
Slide 59 – Varicose Veins
Because the veins do not have thick muscular walls, and only contain valves, a problem can
develop in people who stand still for long periods of time called varicose veins. Varicose veins
develop when blood pools behind a valve in the vein and the person does not move enough to
force that blood forward. The pooled blood stretches the vein. The vein looses its elasticity
when this happens.
Slide 60 – Edema
As we mentioned previously, the lymph system consists of a series of vessels. Smaller lymph
vessels are found in the capillary bed to collect material leaked from the capillaries. This fluid is
eventually returned to the blood when the two systems merge. Lymph vessels flow into lymph
nodes that where many cells involved in the immune system are found. The lymph nodes you
are most familiar with are the tonsils. Lymph vessels are structured similar to veins in that they
contain valves and have fluid moved by skeletal contractions. Another consequence of standing
still for long periods of time is the collection of lymph in the feet and ankles; that is swelling of
Slide 61 – Check Your Understanding
Now that we have learned about the circulatory system, let’s check your knowledge of the
subject. The following slides will have a series of questions on the topic. Be sure to click
“Submit” after answering each question.
Slides 62 through 74 – Review of the Circulatory System Interactive Quiz
A non–graded assessment of your knowledge of the circulatory system.
Slide 75 – Summary
This slide is a summary of all of the “Check Your Understanding” questions from this lecture.
Be sure to review the questions you answered incorrectly.
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