Download CHAPTER 37 The human circulatory system consists of the heart, a

CHAPTER 37
The human circulatory system consists of the heart, a series of blood vessels, and the blood that flows through them.
The Heart
The heart is composed almost entirely of muscle. It is basically hollow and is about the size of your fist. It is enclosed by a
protective sac called the pericardium. The layer of muscle in the heart is called the myocardium- which contracts about 72
times a minute to pump about 70 ml of blood each time.
The septum separates the right side of the heart from the left side and prevents the mixing of oxygen-poor and oxygenrich blood. There are two chambers:
-Atrium- upper chamber that receives blood
-Ventricle- lower chamber that pumps blood out of the heart. There are 2 of each chamber, for a total of 4 chambers.
Circulation through the body is via two separate pathways:
-Pulmonary circulation- Right side of the heart that pumps oxygen poor blood from heart to lungs
-Systemic circulation- Left side of the heart that pumps oxygen rich blood from heart to rest of body.
Circulation through the heart happens when blood enters the right and left atria. A contraction pushes blood into the
ventricles and then out to the lungs or body. A system of valves keeps the blood from backflowing. Your heart beats faster
or slower depending on your need for oxygen-rich blood. This is controlled by neurotransmitters released by the
sympathetic nervous system (faster) and parasympathetic nervous system (slower).
Blood Vessels
As blood flows through the circulatory system, it moves through 3 types of vessels:
1. Arteries- Large vessels that carry oxygen rich blood from heart to tissues of the body.
2. Capillaries- Smallest of the vessels, many so narrow that only one blood cell can pass at a time! Where the exchange
of nutrients, waste, and oxygen/carbon dioxide occurs.
3. Veins- Returns oxygen poor blood to the heart. Large veins contain valves to push flow against the force of gravity.
Blood is under pressure produced by the pumping of the heart. The wave of pressure in the arteries is known as blood
pressure. It's regulated by the medulla oblongata- if it's too high, the autonomic nervous system releases
neurotransmitters to relax the vessel walls. Too low, the walls are constricted to increase pressure.
Blood pressure can be measured with a sphygmomanometer (sfig-moh-muh-NAHM-uh tur). Air is pumped into a cuff until
it cuts off flow thru an artery. Pressure is slowly released, and with a stethoscope the first pulse is the systolic pressureforce felt as ventricle contracts- second pulse is diastolic pressure- force felt as arteries relax. 120/80 is a normal reading.
High blood pressure causes the heart to work harder, which weakens the heart and vessels, increasing the risk of heart
attack and stroke. Another common disease is atherosclerosis, which is a build up of fatty deposits called plaque that build
up on the inner walls of arteries. This can block the arteries, leading to a heart attack. Or when a blood clot forms on the
plaque, it can break off, go to the brain and cause a stroke. Cardiovascular disease is easier to prevent than cure.
Exercise, eat a healthy diet, and avoid smoking!
Blood and the Lymphatic System
The human body has 4-6 liters of blood. 45% are cells that are suspended in the other 55%- a fluid called plasma. Plasma
is about 90% water, but also contains 3 groups of plasma proteins:
-Albumins: transport fatty acids, hormones, and vitamins. Help regulate osmotic pressure and blood volume.
-Globulins: same transport as albumins, but also fight infections.
-Fibrinogen: allows blood to clot
The cellular portion of blood consists of 3 types of cells:
1. Red Blood Cells: Most numerous. Transport oxygen. Get color from hemoglobin- an iron containing protein that binds to
oxygen in the lungs and transports it to tissues for release. Produced in red bone marrow. Circulate for an average of 120
days.
2. White Blood Cells: Guard against infection, fight parasites, and attack bacteria. Produced in red bone marrow. 1 to
every 1000 red blood cells. Body can increase numbers as needed to fight infection. Can live for days, months, or years.
3. Platelets: Fragments of cells that work in blood clotting. When a vessel is injured, platelets clump at the site and release
thromboplastin, which converts to thrombin. Thrombin then converts fibrinogen (plasma protein) into fibrin which causes a
clot and prevents further loss of blood.
The Lymphatic System collects the fluid (lymph) that is lost by the blood and returns it back to the circulatory system.
More than 3 liters of fluid leaks into the lymphatic system every day! If it weren't returned, the body would swell with fluid.
Lymph nodes are bean shaped enlargements that filter out microorganisms. When large numbers are trapped, you get
"swollen glands". Lymph vessels also line the intestines where they absorb fats and fat soluble vitamins from the digestive
tract and take them to the blood for redistribution throughout the body.
The basic function of the respiratory system is to bring about the exchange of oxygen and carbon dioxide between the
blood, air, and tissues. It consists of the nose, pharynx, larynx, trachea, bronchi, and lungs.
Air moves through the nose to a tube at the back of the mouth called the pharynx (throat). From there, it moves into the
trachea (windpipe). Before it reaches the lungs, air is filtered. Large particles are trapped by hairs in the nose. The
passages are lined with mucus which moistens the air and further filters it. Cilia (tiny hairs) sweep the trapped particles
and mucus away from the lungs and toward the pharynx where they are swallowed or spit out.
At the top of the trachea is the larynx, which contains two vocal cords. Air moving between them causes them to vibrate
and produce sound. Further down, the trachea splits into 2 branches called bronchi. Each bronchus subdivides into
smaller bronchi, which split into smaller bronchioles. At the end of bronchioles are tiny air sacs called alveoli that are in
clusters, like grapes, and surrounded by capillaries.
There are about 150 million alveoli in each healthy lung, providing a huge surface area for gas exchange. Oxygen diffuses
into the capillaries, carbon dioxide diffuses from the capillaries into the alveoli. The hemoglobin in red blood cells binds
with the oxygen to increase how much oxygen the blood can carry. Without hemoglobin, you would need 300 liters of
blood to get the same result!
The lungs are sealed in two sacs in the chest cavity. At the bottom of the cavity is a large, flat muscle called the
diaphragm. Contraction and relaxation of this muscle helps to pull air in and push air out of the lungs. The medulla
oblongata controls breathing. Cells in its breathing center monitor the amount of CO2 in the blood. As the level rises,
nerve impulses cause the diaphragm to contract, bringing air into the lungs.
The Effects of Tobacco
Tobacco smoke contains many dangerous substances, but the top three are:
1. Nicotine- a stimulant drug that increases heart rate and blood pressure
2. Carbon Monoxide- a poisonous gas that blocks the transport of oxygen by hemoglobin
3. Tar- contains compounds that cause cancer.
Nicotine and carbon monoxide paralyze the cilia, so inhaled particles stick to the walls of the respiratory tract (causes
smoker's cough) or enter the lungs. This can cause diseases such as bronchitis and/or emphysema. Smoking is also a
major cause of heart disease, due to the effects of nicotine.