The Circulatory System

THE CIRCULATORY SYSTEM AND LYMPHATIC SYSTEM

 

Most of the cells in the human body are Not in direct contact with the external environment.  The circulatory system acts as a transport service for these cells.   Two fluids move through the circulatory system: Blood and Lymph.  The blood, heart, and blood vessels form the Cardiovascular System.  The lymph, lymph nodes and lymph vessels form the Lymphatic System.  The Cardiovascular System and the Lymphatic system collectively make up the Circulatory System.

OBJECTIVES:

 List the parts of the circulatory system.  Describe the structure and function of the human heart.   Trace the flow of blood through the heart and body.  Distinguish between arteries, veins, and capillaries in terms of the structure and function. Distinguish between pulmonary circulation and systemic circulation. Describe the structure and function of the lymphatic system
1. Higher animals, including humans, usually have a CLOSED CIRCULATORY SYSTEM, meaning it is repeatedly cycled throughout the body.
2. It was in 1628, when the English physician William Harvey showed that BLOOD Circulated throughout the body in one-way Vessels.
3. According to Harvey, Blood was pumped out of the Heart and into the Tissue through ONE TYPE OF VESSEL and back to the Heart through ANOTHER TYPE OF VESSEL.  The Blood, in other words, moved in a CLOSED CYCLE through the body.
4. BLOOD IS THE BODY'S INTERNAL TRANSPORTATION SYSTEM.
5. PUMP BY THE HEART, BLOOD TRAVELS THROUGH A NETWORK OF VESSELS, CARRYING MATERIALS SUCH AS OXYGEN, NUTRIENTS, AND HORMONES TO AND WASTE PRODUCTS FROM EACH OF THE HUNDRED TRILLION CELLS IN THE HUMAN BODY.
6. BLOOD, THE HEART, AND BLOOD VESSELS MAKE UP THE CARDIOVASCULAR SYSTEM.
 
 
 
 
 
THE HEART
1. The Central Organ of the Cardiovascular System is the HEART.
2. THE HEART IS A HOLLOW, MUSCULAR ORGAN THAT CONTRACTS AT REGULAR INTERVALS, FORCING BLOOD THROUGH THE CIRCULATORY SYSTEM.
3. The Heart is cone-shaped, about the size of a Fist, and is located in the Thoracic Cavity between the Lungs directly behind the Sternum (Breastbone).  The Heart is tilted so that the APEX (the pointed end) is oriented to the Left.
4. The walls of the Heart are made up of Three Layers of Tissue.
    A.  The Outer and Inner Layers are EPITHELIAL TISSUE.
    B. The Middle Layer (The walls of the four chambers of the Heart) is CARDIAC MUSCLE TISSUE CALLED THE MYOCARDIUM.
5. CARDIAC MUSCLE TISSUE IS NOT UNDER CONSCIOUS CONTROL OF THE NERVOUS SYSTEM.
6. Cardiac Muscle Tissue has a rich supply of Blood, which ensures that it gets plenty of Oxygen.
7. There is also a special connection between Cells that allow Impulses to travel from one cell to another.  The Cells that make up the Cardiac Muscle Tissue are loaded with MITOCHONDRIA, (POWERHOUSE OF THE CELL), guaranteeing the each Cell has a constant supply of ATP.
8. Our Hearts Contract or Beat about once every second of every day of our lives.   The heart beats more than 2.5 million times in an average life span.  The only time the Heart gets a Rest is Between Beats.
HOW THE HEART WORKS
1. The Heart can be thought of as TWO PUMPS sitting side by side.  The Human Heart, with a Right Atrium and Right Ventricle, as well as a Left Atrium and Left Ventricle, essentially has TWO Separate Hearts inside one. (Figure 47-1)
2. The RIGHT SIDE of the Heart pumps Blood From The BODY INTO THE LUNGS, WHERE OXYGEN POOR BLOOD (DEOXYGENATED, USUALLY SHOWN IN BLUE) GIVES UP CARBON DIOXIDE AND PICKS UP OXYGEN.
3. The LEFT SIDE of the Heart pumps OXYGEN RICH BLOOD (OXYGENATED, USUALLY SHOWN IN RED) FROM THE LUNGS TO THE REST OF THE BODY EXCEPT THE LUNGS.
4. The Heart is Enclosed in a Protective Membrane Sac called the PERICARDIUM.   The Pericardium surrounds the heart and secretes a fluid that Reduces Friction as the heart beats.
5. Our Heart has FOUR CHAMBERS: (Figure 47-1)
A.  The UPPER CHAMBERS of the Heart are the RIGHT AND LEFT ATRIA (ATRIUM), RECEIVE BLOOD COMING INTO THE HEART.
B. The LOWER CHAMBERS are the RIGHT AND LEFT VENTRICLES, PUMP BLOOD OUT OF THE HEART.  The Left Ventricle is the Thickest chamber of the heart because it has to do most of the work to pump blood to all parts of the body.
6. Vertically Dividing the Right and Left sides of the Heart is a Common Wall called the SEPTUM. The Septum Prevents the Mixing of Oxygen-poor and Oxygen-rich Blood.
THE RIGHT SIDE OF THE HEART (FROM BODY TO LUNGS, DEOXYGENTATED BLOOD -BLUE)
1. Oxygen-Poor Blood from the body enters the Right side of the Heart through TWO large blood vessels called VENA CAVA.
2. The SUPERIOR (UPPER) Vena Cava brings Blood from the UPPER PART OF THE BODY TO THE HEART.
3. The INFERIOR (LOWER) Vena Cava brings Blood from the LOWER PART OF THE BODY TO HE HEART.
4. Both VENA CAVA EMPTY INTO THE RIGHT ATRIUM.  When the Heart Relaxes (Between Beats), pressure in the circulatory system causes the Atrium to fill with blood.
5. When the Heart CONTRACTS, Blood is squeezed from the RIGHT ATRIUM INTO THE RIGHT VENTRICLE through flaps of tissue called a ATRIOVENTRICULAR (AV) VALVE, that prevents blood from flowing back into the Right Atrium.
6. The valve that separates the Right Atrium and Ventricle is called the TRICUSPID VALVE.
7. THE GENERAL PURPOSE OF ALL VALVES IN THE CIRCULATORY SYSTEM IS TO PREVENT THE BACKFLOW OF BLOOD.  They also ensure that BLOOD FLOWS IN ONLY ONE DIRECTION.
8. THE SPECIFIC PURPOSE OF THE TRICUSPID VALVE IS TO PREVENT BACKFLOW OF BLOOD FROM THE RIGHT VENTRICLE TO THE RIGHT ATRIUM WHEN THE RIGHT VENTRICLE CONTRACTS.
9. When the Heart CONTRACTS a second time, Blood in the RIGHT VENTRICLE IS SENT THROUGH THE A SEMILUNAR (SL) VALVE KNOWN AS THE PULMONARY VALVE INTO THE  PULMONARY ARTERIES TO THE LUNGS.  These are the Only Arteries to carry Oxygen-Poor Blood.   At the base of the Pulmonary Arteries is a valve (Pulmonary Valve) that prevents blood from traveling back into the Right Ventricle.
THE LEFT SIDE OF THE HEART  (FROM LUNGS TO BODY, OXYGENATED BLOOD-RED)
1. Oxygen-Rich Blood leaves the Lungs and Returns to the Heart by way of Blood Vessels called the PULMONARY VEINS.  These are the only Veins to carry Oxygen-Rich Blood.
2. Returning Blood enters the LEFT ATRIUM, IT PASSES THROUGH flaps of tissue called a ATRIOVENTRICULAR (AV) VALVE to the LEFT VENTRICLE.
3. The valve that separates the Left Atrium and Ventricle is called the MITRAL VALVE or BICUSPID VALVE.
4. FROM THE LEFT VENTRICLE, BLOOD IS PUMPED THROUGH A SEMILUNAR (SL) VALVE CALLED THE AORTIC VALVE INTO THE AORTA ARTERY THAT CARRIES IT TO EVERY PART OF THE BODY EXCEPT THE LUNGS.
5. At the base of the Aorta is a Valve (Aortic Valve) that prevents blood from flowing back into the Left Ventricle.
THE HEARTBEAT (CARDIAC CYCLE)
1. The Cardiac Cycle is the Sequence of events in one heartbeat.  In its simplest form, the cardiac cycle is the Simultaneous Contraction of the TWO Atria, followed a fraction of a second latter by the Simultaneous Contraction of the TWO Ventricles.
2. The Heart consists of Muscle Cells that contract in Waves.  When the first group is Stimulated, they in turn stimulate Neighboring Cells. Those cells Stimulate more cells.  This chain reaction continues until all cells Contract.  The wave of activity spreads in such a way that the Atria and the Ventricles contract in a Steady Rhythm.
3. A Heartbeat has two Phases:
    A.  Phase 1 - SYSTOLE is the term for CONTRACTION. Occurs when the Ventricles contract, closing the AV Valves and opening the SL Valves to pump blood into two major vessels leaving the heart.
    B. Phase 2 - DIASTOLE is the term for RELAXATION. Occurs when the Ventricles relax, allowing the back pressure of the blood to closed SL Valves and opening AV valves.
4. The Cardiac Cycle also creates the HEART SOUNDS: each heartbeat produces TWO Sounds, often called LUBB-DUP, that can be heard with a stethoscope.
5. The First sound, the Loudest and Longest, is caused by the Ventricular Systole (Contraction) closing the AV Valves.
6. The Second sound is caused by the closure of the Aortic and Pulmonary Valves (SL).
7. If any of the Valves do not close properly, an extra sound called a HEART MURMUR may be heard.
8. Although the Heart is a SINGLE MUSCLE, it does NOT Contract in a Single motion.   The Contraction spreads over the Heart like a WAVE.
9. The Wave BEGINS in a Small Bundle of Specialized Heart Muscle Cells embedded in the RIGHT ATRIUM CALLED THE SINOATRIAL NODE (SA). (Figure 47-3)
10. The SA Node is the Natural PACEMAKER of the Heart.  It initiates each Heartbeat and sets the PACE for the HEART RATE.  (Figure 47-3)
11. The impulse spreads from the Pacemaker through the Cardiac Muscle Cells in the Right and Left Atrium, causing BOTH Atria to Contract almost Simultaneously.
12. When the impulse INITIATED by the SA Node reaches Another special area of the Heart known as  the ATRIOVENTRICULAR (AV) NODE. The AV Node is located in the Septum between the Right and Left Ventricles. The AV Node Relays the electrical impulse to the muscle cells that make up the Ventricles. The Ventricles Contract almost Simultaneously a Fraction of a second after the Atria, COMPLETING ONE FULL HEARTBEAT.
13. These Contractions causes the Chambers to Squeeze the Blood, Pushing it in the proper direction along its path.
14. The Heart Initiates its Own Stimulation from the Sinoatrial Node and Atrioventricular Node, and Does NOT require Stimulation from the Nervous System.
15. The Autonomic Nervous system does influence Heart Rate.  The Sympathetic Nervous System INCREASES HEART RATE and the Parasympathetic Nervous System DECREASES IT.
16. For most of us, at REST our Heart Beats between 60 and 80 beats per minute.   During Exercise that can increase to as many as 200 beats per minute.
BLOOD VESSELS  (ARTERIES, VEINS AND CAPILLARIES)
1. The Circulatory System is known as a CLOSED SYSTEM because the blood is contained within either the Heart or Blood Vessels at all times.
2. The blood Vessels that are part of the Closed Circulatory System of humans from a vast network to help keep the Blood flowing in One Direction.
3. After the Blood leaves the Heart, it is pumped through a network of Blood Vessels to different parts of the body.
4. The Blood Vessels that form this network and are part of the CIRCULATORY SYSTEM ARE THE ARTERIES, CAPILLARIES, AND VEINS.
5. With the exception of Capillaries and tiny Veins, Blood Vessels have WALLS made of THREE LAYERS OF TISSUE, that provides for a combination of Strength and Elasticity:   (Figure 47-4)
    A.  THE INNER LAYER IS EPITHELIAL TISSUE.
    B. THE MIDDLE LAYER IS SMOOTH MUSCLE TISSUE.
    C.  THE OUTER LAYER IS CONNECTIVE TISSUE.
ARTERIES AND ARTERIOLES (SMALL ARTERIES)
1. Arteries carry blood from the HEART TO CAPILLARIES AND THE REST OF THE BODY.  (Figure 47-4)
2. The Walls of Arteries are generally THICKER than those of Veins.
3. The Smooth Muscle Cells and Elastic Fibers that make up the Walls help make Arteries Tough and Flexible.  This enables Arteries to withstand the high pressure of blood as it is pumped from the Heart. The force that blood exerts on the walls of blood vessels is known as BLOOD PRESSURE.
4. EXCEPT FOR THE PULMONARY ARTERIES, ALL ARTERIES CARRY OXYGEN-RICH BLOOD.
5. The Artery that carries Oxygen-Rich Blood from the LEFT VENTRICLE to all parts of the body, EXCEPT THE LUNGS, is the AORTA.
6. THE AORTA WITH A DIAMETER OF 2.5 cm, IS THE LARGEST ARTERY IN THE BODY.
7. As the Aorta travels away from the Heart, it branches into smaller Arteries so that all parts of the body are supplied.
8. THE SMALLEST ARTERIES ARE CALLED ARTERIOLES.
CAPILLARIES
1. ARTERIOLES BRANCH INTO NETWORKS OF VERY SMALL BLOOD VESSELS CALLED CAPILLARIES. (Figure 47-5)
2. IT IS IN THE THIN-WALLED (ONE-CELL IN THICKNESS) THAT THE REAL WORK OF THE CIRCULATORY SYSTEM IS DONE.
 
3. The Walls of the Capillaries consist of only one layer of cells, making it easy for Oxygen and Nutrients to DIFFUSE FROM THE BLOOD INTO THE TISSUE.
4. Forces of Diffusion drive CO2 and waste products from the tissue into the Capillaries.
5. Capillaries are extremely NARROW; Blood Cells moving through them must pass in Single file.
 
 
 
 
VEINS
1. THE FLOW OF BLOOD MOVES FROM CAPILLARIES INTO THE VEINS. (Figure 47-6)
2.  Veins form a system that COLLECTS Blood from every part of the Body and CARRIES it Back to the HEART.
3. The smallest Veins are called VENULES.
4. LIKE ARTERIES, VEINS ARE LINED WITH SMOOTH MUSCLE.  Vein walls are thinner and less elastic than Arteries.  Veins though are more FLEXIBLE and are able to stretch out readily.
5. This flexibility reduces the Resistance the flow of blood encounters on its way back to the Heart.
6. Large Veins contain Valves that maintain the one direction flow of Blood.  This is important where Blood must flow against the Force of Gravity. (Figure 47-7)
7. The flow of Blood in Veins is help by Contractions of Skeleton Muscles, especially those in the legs and arms.  When muscles contract they squeeze against Veins and help force Blood Toward the Heart.
PATTERNS OF CIRCULATION
1. Blood moves through the body in a continuous pathway, of which there are TWO MAJOR PATHS; THE PULMONARY AND SYSTEMIC CIRCULATION. (Figures 47-8 & 9)
2. THE PULMONARY CIRCULATION CARRIES BLOOD BETWEEN THE HEART AND THE LUNGS.  THIS CIRCULATION BEGINS AT THE RIGHT VENTRICLE AND ENDS AT THE LEFT ATRIUM.   (Figure 47-8)
3. Oxygen-Poor blood is pumped out of the Right Ventricle of the Heart into the Lungs through the Pulmonary Arteries.  These are the only Arteries in the Body to Carry Deoxygenated Blood.
4. Blood returns to the Heart through the Pulmonary Veins, the only Veins to carry oxygen-rich blood.
5. THE LUNGS ARE THE ONLY ORGANS DIRECTLY CONNECTED TO BOTH CHAMBERS OF THE HEART.
6. THE SYSTEMIC CIRCULATION, STARTS AT THE LEFT VENTRICLE AND ENDS AT THE ATRIUM, CARRIES BLOOD TO THE REST OF THE BODY.  (Figure 47-9)
7. Oxygen-rich blood leaving the Heart passes through the Aorta and into a number of Arteries that supply blood to every part of the body.
8. SYSTEMIC CIRCULATION SUPPLIES EACH MAJOR ORGAN WITH BLOOD, INCLUDING THE HEART.
9. The Heart receives its supply of Blood from a PAIR of CORONARY ARTERIES leading from the Aorta.  Blood enters into Capillaries that lead to Veins through which blood returns to the Right Atrium.
 
10. The Systemic System can be divided into THREE SUBSYSTEMS: (Figure 47-9)
    A. CORONARY CIRCULATION - SUPPLIES BLOOD TO THE HEART.
    B. RENAL CIRCULATION - SUPPLIES BLOOD TO THE KIDNEYS.   Nearly one-forth of the blood that is pump into the Aorta by the Left Ventricle flows to the Kidneys.  The Kidneys Filter Waste From the Blood.
    C. HEPATIC PORTAL CIRCULATION - Nutrients are picked up by capillaries in the small intestines and are transported to the Liver.  Excess nutrients are stored in the Live for future needs.  The Liver receives oxygenated blood from a large Artery that branches of the Aorta.
BLOOD PRESSURE
1. Blood moves through our Circulation System because it is under Pressure.
2. This Pressure is caused by the Contraction of the Heart and by Muscles that surround Blood Vessels.
3. A MEASURE OF FORCE THAT BLOOD EXERTS AGAINST A VESSEL WALL IS CALLED BLOOD PRESSURE.
4. Blood Pressure is Always highest in the Two Main Arteries that leave the Heart.
5. Blood Pressure is maintain by TWO WAYS:  (1)  The Nervous System, which can speed up or slow down the Heart Rate; (2)  The KIDNEYS, which regulate blood pressure by the amount of fluid in our Blood.
6. When our pressure is too high, kidneys remove water from blood, lowering the total amount of fluid in the Circulatory System.
7. Both High and LOW Blood Pressure can cause our bodies problems.
8.  Blood Pressure is Usually Measured in the Artery Supplying the upper Arm.
9. To measure Blood Pressure:
  A. A Cuff is inflated around a persons arm - stopping the flow of blood through the artery.
    B. Air Pressure in the Cuff is slowly released- the first sounds of blood passing through the artery means that the Ventricles have pump with enough force to overcome the pressure exerted by the cuff.
    C. This measurement is known as the SYSTOLIC PRESSURE, or the pressure of the blood when it leaves the Ventricles.  NORMAL PRESSURE IS ABOUT 120 mm Hg FOR MALES, AND 110 mm Hg FOR FEMALES.
    D. Air pressure is continued to be released - listening for the disappearance of Sound, which indicates a steady flow of blood.  This known as the DIASTOLIC PRESSURE, or the pressure of the blood is sufficient to keep arteries open constantly even with the Ventricles Relax.  NORMAL PRESSURE IS ABOUT 80 mm Hg FOR MALES AND 70 mm Hg FOR FEMALES.
    E. YOUR BLOOD PRESSURE IS GIVEN TO AS THE SYSTOLIC NUMBER OVER THE DIASTOLIC NUMBER.
THE LYMPHATIC SYSTEM
1. As Blood Circulates throughout the body, Fluid from the Blood LEAKS into tissue.
2. A NETWORK OF VESSELS KNOWN AS THE LYMPHATIC SYSTEM COLLECTS THE FLUID AND RETURNS IT TO THE CIRCULATORY SYSTEM. (Figure 47-10)
3. The loss Fluid is known as LYMPH, a transparent yellowish fluid, and is collected in Lymphatic Capillaries and moves to larger Lymph Vessels.  Like Veins Lymph Vessels contain valves to prevent the back flow of lymph. Lymph vessels form a one-way system that returns fluids collected in tissues back to the bloodstream.
4.  The Lymphatic system has no pump like the heart, lymph must be moved through vessels by the squeezing of skeletal muscles.
5. These Lymph Vessels Pass Through small bean-shaped enlargements (organs) called LYMPH NODES, WHICH ACTS AS FILTERS AND PRODUCERS OF SPECIAL WHITE BLOOD CELLS CALLED LYMPHOCYTES THAT ARE SPECIALIZED TO FIGHT INFECTION.
6. The Fluid is returned to the Circulatory System at an opening in a Vein located under the Left Clavicle, or Collarbone, just below the shoulder.
 
 
SECTION 47-2 BLOOD
Blood is a Liquid Connective Tissue that constitutes the transport medium of the circulatory system.  The Two main functions of blood are to transport nutrients and oxygen to the cells and carry carbon dioxide and waste materials away from the cells.   Blood also transfers heat to the body surface and plays a role in defending the body against disease.
OBJECTIVES:  List the components of blood. Distinguish between red blood cells, white blood cells, and platelets in terms of structure and function.  Summarize the process of blood clotting.  Explain what determine the compatibility of blood types for transfusion.
1. The Main Function of the Circulatory System is to Transport Material in a FLUID Medium throughout the body.
2. THIS FLUID MEDIUM IS CALLED BLOOD.  BLOOD IS A TYPE OF LIQUID CONNECTIVE TISSUE THAT HAS MANY FUNCTIONS. Blood is composed of a Liquid Medium and Blood Solids.  The liquid makes up about 55 percent of the blood, and blood solids make up the remaining 45 percent.
3. BLOOD TRANSPORT NUTRIENTS, DISSOLVED GASES (O2, CO2), ENZYMES, HORMONES, AND WASTE PRODUCTS.
4. BLOOD REGULATES BODY TEMPERATURE, pH, and ELECTROLYTES.
5. BLOOD PROTECTS THE BODY FROM INVADERS, AND BLOOD RESTRICTS THE LOSS OF FLUID.
6. Our Bodies contains 4 to 5 liters of Blood.
BLOOD PLASMA
1. Approximately 55 percent of Blood in made up of a Fluid Portion called PLASMA.
2. Plasma is the Straw-Colored Liquid portion of Blood and is 90 Percent Water and 10 percent dissolved fats, salts, sugars, and Proteins called PLASMA PROTEINS.
3. THE PLASMA PROTEINS ARE DIVIDED INTO THREE TYPES:
    A.  ALBUMINS - HELP REGULATE OSMOTIC PRESSURE (MAINTAIN NORMAL BLOOD VOLUME AND BLOOD PRESSURE). THIS IS THE MOST ABUNDANT PLASMA PROTEIN.
    B.  GLOBULINS OR ANTIBODIES - INCLUDE ANTIBODIES THAT HELP FIGHT OFF INFECTION.  ANTIBODIES INITIATE THE DESTRUCTION OF PATHOGENS AND PROVIDE US WITH IMMUNITY.
    C.  FIBRINOGEN - RESPONSIBLE FOR THE ABILITY OF BLOOD TO CLOT.
BLOOD CELLS OR SOLIDS
THE CELLULAR PORTION OF BLOOD MAKE UP THE OTHER 45 PERCENT AND  INCLUDES SEVERAL TYPES OF HIGHLY SPECIALIZED CELLS AND CELL FRAGMENTS.  THEY ARE RED BLOOD CELLS (RBC), WHITE BLOOD CELLS (WBC), AND PLATELETS.
RED BLOOD CELLS (RBC) ERYTHROCYTES
1. RBC are the most numerous of the Blood Cells. One microliter of blood contains approx. 5 million RBCs.  (Figure 47-11)
2. RBC are BICONCAVE, or shaped so that they are narrower in the center than along the edges.
3. RBC are produced from cells in the Bone Marrow, they are gradually filled with HEMOGLOBIN which forces out the nucleus and other organelles.
4. Mature RBC do not have a Cell Nucleus and Organelles.  The Mature RBC becomes little more than a membrane sac containing Hemoglobin.
5. Hemoglobin is the iron-containing protein that gives RBC the ability to carry Oxygen.  Hemoglobin gives the RBC their color.
6. RBC stay in circulation for about 120 days before they are destroyed by special WBC in the liver and spleen.  RBC in your body are dying and being replace at a rate of about 2 million per second.
WHITE BLOOD CELLS (WBC) LEUKOCYTES
1. Outnumbered by RBC almost 500 to 1.
2. WBC are produced in the Red Bone Marrow, The Lymph Nodes, and the Spleen.  They are larger than RBC, almost Colorless, and do NOT Contain Hemoglobin. (Figure 47-12)
3. WBC have a Nucleus and can live for many months or years.
4. THE MAIN FUNCTION OF WBC IS TO PROTECT THE BODY AGAINST INVASION BY FOREIGN CELLS OR SUBSTANCES.
5. WBC called PHAGOCYTES can destroy bacteria and foreign cells by Phagocytosis (engulfed and digested), some produce special proteins called ANTIBODIES, and some release special chemicals that help the body fight off disease and resist infection.
6. Doctors are able to detect the presence of infection by counting the number of WBC in the blood.  When a person has an infection, the number of WBC can Double.
PLATELETS AND BLOOD CLOTTING
1. Platelets are NOT Cells; they are tiny Fragments of other Cells that were formed in the bone marrow.
2. Platelets are formed when small pieces of Cytoplasm are pinched off the large cells in the Red Bone Marrow called MEGAKARYOCYTES, which are found in the Bone Marrow.  Platelets lack a nucleus and their life span is about 7 to 11 days.
3. Platelets play an important role in Blood Clotting.
4. Platelets help the Clotting process by Clumping together and forming a Plug at the site of a wound and then releasing proteins called CLOTTING FACTORS.
5. Clotting Factors start a series of Chemical Reactions that ends with a sticky meshwork of Fibrin Filaments that stop bleeding by producing a clot. (Figure 47-14)
6. A genetic disorder of Clotting Factors is called HEMOPHILIA, suffers may bleed uncontrollably from even a small cut or scrape.
7. Clotting of blood in Vessels can block the flow of blood, if this happens in the brain, brain cells may die, causing a STROKE.
BLOOD TYPES
1. Blood type is determined by the Type of ANTIGEN present on the Surface of RBC.
2. An ANTIGEN is a protein or carbohydrate that acts as a signal, enabling the body to recognize foreign substances in the body.
3. Blood from Humans is Classified into FOUR GROUPS, based on the Antigens on the Surface of RBC. (Table 47-1)
4. BLOOD TYPING involves identifying the Antigens in a Sample.
5. THREE of the most important human antigens are called A, B, and Rh.
6. The A-B-O System is based on the A and B Antigen. It is a means of classifying blood by the Antigens located on the surface of RBC and the Antibodies circulating in the Plasma.
7. An Individual's RBC may carry an A ANTIGEN, a B ANTIGEN, both A and B ANTIGENS, OR NO ANTIGEN AT ALL. These Antigen patterns are called BLOOD TYPES A, B, AB, O RESPECTIVELY.   (Table 47-1)
8. Type AB is known as a Universal Receiver, meaning that they can receive any type blood.
9. Type O is known as a Universal Donor, meaning they can donate blood to anyone.
 
Rh SYSTEM
1. An antigen that is sometimes on the surface of RBC is the Rh FACTOR, named after the rhesus monkey in which it was first discovered.
2. Eighty-five percent of the U.S. population is Rh-positive (Rh+), meaning that Rh Antigens are present.
3. People who do not have Rh Antigens are called Rh-negative (Rh-).
4. If an Rh- person receives a transfusion of blood that has Rh+ antigens, Rh- antibodies will react with the Antigen and Agglutination (clumping) will occur.
5. The Rh Factor is the reason there are blood test before marriage.  The most serious problem with Rh incompatibility occurs during pregnancy.
6. If the mother is Rh- and the father is Rh+, the child may inherit the Dominant Rh+ allele (gene) from the father.
7. If the babies Rh+ blood gets into the mother during delivery, the mother will develop Antibodies to the Rh Factor.
8. If a second Rh+ child is conceived later, the mother's antibodies can cross the placenta and attack the blood of the fetus.
9. This condition is called ERYTHROBLASTOSIS FETALIS.
10.  To prevent this condition, an Rh- mother of an Rh+ child can by given Antibodies to destroy and Rh+ cells that have entered her bloodstream from the fetus.
11. The antibodies, a substance called RHOGAM, must be administered to the mother within Three Days after the birth of her first Rh+ child to remove from her bloodstream any Rh+ antibodies.

12.  By destroying any Rh+ cells in her bloodstream, any danger to a second child is prevented because the mother will not make any Antibodies against the blood cells of the Rh+ fetus.