Anatomy Lecture Notes Unit 7 Circulatory System

Unit 7 – Circulatory System - The Blood - Class Lecture Notes

Normally, 7 to 8% of human body weight, the volume of blood in the body is about six liters. Blood is about 22% solids and 78% water. The temperature of blood in the body is 38^oC (100.4^oF), slightly higher than body temperature. Blood is slightly alkaline, with a pH between 7.35 and 7.45.

Blood takes these to cell tissues	Blood takes these away from cell tissues
Nourishment Electrolytes Hormones Vitamins Antibodies Oxygen Heat	Waste matter Carbon dioxide

Blood is a highly specialized tissue composed of many different kinds of components produced in bone marrow. Four of the most important ones are red cells, white cells, platelets, and plasma. All humans produce these blood components - there are no racial or regional differences.

Red blood cells, or erythrocytes, RBCs, are relatively large microscopic cells without nuclei. These cells normally make up 40-50% of the total blood volume. They transport oxygen from the lungs to all of the living tissues of the body and carry away carbon dioxide. Hemoglobin, Hb is the gas transporting protein molecule that makes up 95% of a red cell. Each red cell has about 250 million iron-rich hemoglobin molecules. The number of RBCs varies, but the average is about 5 million cells per cubic centimeter (cm³). Although the numbers are important, it is the amount of hemoglobin in the blood at any time that really determines how well oxygen is transported.

Developing RBCs divide many times and then begin synthesizing huge amounts of hemoglobin. Suddenly, when enough hemoglobin has been accumulated, the nucleus and most organelles are ejected and the cell collapses inward. Because they are anucleate, without a nucleus, the mature cells are unable to sunthesize proteins, grow, or divide. As they age, RBCs become more rigid and begin to fragment in 100 to 120 days. Their remains are removed from the blood by phagocytes in the spleen and liver and their components recycled.
White blood cells, or leukocytes, WBCs exist in variable numbers and types but make up a very small part of blood's volume - normally only about 1%. Some white cells (lymphocytes) provide a physiological defense against infection by seeking out microscopic parasites and destroying them. Their numbers increase when the body is under attack by bacteria, viruses, fungi, or other parasites. Some white cells (macrophages) are the blood's disposal units. They have the function of getting rid of old, unneeded blood cells as well as foreign matter (dust and bacteria). A total WBC count above 11,000 cells/cm³ is referred to as leukocytosis, and generally indicates a bacterial or viral infection. Individual white cells remain viable for only 18 to 36 hours.

The several types of white blood cells are classified into two major groups, depending on whether or not they contain visible granules in their cytoplasm.

Granulocytes are granule-containing WBCs:

Neutrophils have a multilobed nucleus and very fine granules. They are avid phagocytes at sites of acute infection.
Eosinophils have a blue-red nucleus and large brick-red granules. Their numbers increase rapidly during allergies.
Basophils, the rarest of all WBCs, contain large histamine-containing granules. Histamine is an inflammatory chemical that makes blood vessels leaky and attracts other WBCs to the inflammatory site.

Agranulocytes do not have visible cytoplasmic granules:

Lymphocytes have a large dark purple nucleus that occupies most of the cell volume. Lymphocytes reside in lymphatic tissues and are the first immune response of the body.
Monocytes are the largest of WBCs. When they migrate into the tissues, they change into macrophages with an important role in fighting chronic infections.

Platelets, or thrombocytes, are cells that clot blood at the site of wounds. Platelets are not cells in the strict sense. The are fragments of multinucleated cells called megakaryocytes, which rupture, releasing thousands of "pieces" that quickly seal the leak in the blood vessel. There are more than a dozen types of platelets that need to interact in the blood clotting process. Individual platelets are about ¹/₃ the size of red cells. The normal platelet count in blood is about 300,000/cm³. Platelets have a lifespan of 7 to 10 days.
Plasma is the relatively clear liquid medium which carries the red cells, white cells, and platelets. Most of our blood's volume is made up of plasma. About 95% of it consists of water that is as salty as the oceans. As the heart pumps blood to cells throughout the body, the plasma brings them nourishment and removes the waste products of metabolism.
Other Blood Components:

Chemical substances, including; fat, carbohydrates, proteins, and hormones.
Gases, including; oxygen, carbon dioxide, and nitrogen.

Blood Types:

Sometimes when the blood of two people is mixed together, the red blood cells clump together in the liquid plasma. This is agglutination. This is not the same thing as clotting. When agglutination occurs, the blood mostly remains liquid. With clotting, however, it does not.

The difference in composition between blood types is in the specific kinds of antigens found on the surface of the red cells. Antigens are relatively large protein molecules that provide the biological signature of an individual's blood type.

Within blood there are substances called antibodies which distinguish particular antigens from others, causing hemolysis, bursting, or agglutination of the red cells when alien antigens are found. The antibodies bind to the antigens. In the case of agglutination, the antibodies glue together the antigens from different red cells thereby sticking the red cells together.

The specific type of antigens on red blood cells determine blood types. There are 27 known human blood groups, for which each of us can be typed.

Long before the blood antigen-antibody interaction was discovered, surgeons experimented with transfusions in an attempt to save the lives of patients who were dying from severe blood loss and the resulting shock. The first attempt may have been an English physician during the mid-17th century who infused a wounded soldier with sheep blood. Not surprisingly, the soldier suffered a painful death. During the 19th century, European and American doctors used transfusions in a last ditch attempt to save soldiers and other patients with severe wounds. They usually transferred blood directly from a healthy individual to their patient via a rubber tube with hypodermic needles at each end. This occasionally resulted in success but more often than not killed the recipient. The results seemed to be random. Doctors in the 19th century also experimented with a variety of blood substitutes, including milk, water, and even oils. It was the discovery of the ABO blood types in 1900 that finally led us to understand how to consistently use transfusions to save lives.

ABO Blood Groups
Blood Group	RBC Antigen	Plasma Antibody	Blood that can be received
AB	A and B	None	A, B, AB, and O
B	B	Anti-A	B and O
A	A	Anti-B	A and O
O	None	Anti-A and Anti-B	O

Frequency of Blood Groups, % of U. S. Population
Blood Group	White	Black	Asian
AB	4	4	5
B	11	20	27
A	40	27	28
O	45	49	40

The Rh blood groups are so named because one of the eight Rh antigens (agglutinogen D) was originally identified in Rhesus monkeys. Most Americans are Rh⁺ (Rh positive), meaning that the RBCs carry the Rh antigen. Blood without the antigen is Rh ^- (Rh negative).

Before a transfusion today, blood is typed and cross-matched, involving 3 basic steps:

Determine the ABO group
Determine the Rh factor
Test for agglutination of the donor RBCs by the recipient's serum and of the recipient RBCs by the donor's serum.

When blood is donated for future use, the usual blood bank procedure involves mixing it with an anticoagulant to prevent clotting. The treated blood can be stored (regrigerated at 4^oC) for about 35 days.

Because of its absolute importance to life and its relatively short "shelf-life", blood is a priceless commodity. The U.S. Food and Drug Administration has encouraged the development of artificial blood for over 20 years. While no marketable product has been produced, several companies are close to developing effective human blood substitutes. These substitutes consist of either synthetic chemicals called perfluorocarbons or modified hemoglobin extracted from cows' blood and unused human blood that is too old for transfusing. The advantage of these blood substitutes is that they do not have antigens that would cause rejection. The disadvantage is that blood substitutes are filtered out by kidneys in only a few days and patients may then need a whole blood transfusion.

Blood cells are made in the bone marrow. Bone marrow is the spongy material in the center of the bones that produces about 95 percent of the body's blood cells. In adults, the blood-producing marrow is found mainly in the flat bones of the skull, pelvis, ribs, and sternum.

The production and development of new blood cells is a process called hematopoiesis. All blood cells formed in the bone marrow start out as a stem cell or hemocytoblast. Stem cells differentiate into either lymphoid stem cells (which produce lymphocytes) or myeloid stem cells (which can produce all other classes of blood cells). The entire development process from hemocytoblast to mature blood cell takes 3 to 5 days.

Farther differentiation of stem cells is controlled by four hormones:

Erythropoietin controls the rate of RBC production. Normally a small amount of this hormone circulates in the blood at all times, and RBCs are formed at a fairly constant rate. Although the liver produces some, the kidneys play the major role in producing erythropoietin.
Colony stimulating factors, CSFs and interleukins stimulate the production of WBCs.
Thrombopoietin accelerates the rate of platelet production.

Hemostasis is the process by which blood flow is stopped at the site of a break in a blood vessel.
When a blood vessel breaks, three things happen in rapid sequence:

Platelet plug formation

1. Collagen fibers are exposed when a vessel breaks. This causes platelets to rupture, sticking to the damaged site and releasing chemicals that attract more platelets, forming a plug.

Vascular spasms

2. Once anchored, the platelets release serotonin, causing the blood vessel to go into spasms. The spasms narrow the blood vessel at the site, decreasing blood loss until clotting can occur.

3. At the same time, the injured tissues release thromboplastin in preparation for clotting.

Coagulation (blood clotting)

4. PF₃, a chemical coating the surfaces of platelets, interacts with thromboplastin, and Ca⁺² to form a prothrombin activator that triggers the clotting cascade.

5. The activator converts prothrombin, present in the plasma, to thrombin, an enzyme.

6. Thrombin joins soluble fibrinogen proteins into long hairlike molecules of insoluble fibrin, which forms a mesh that traps RBCs and forms the base of the clot. Within an hour, the clot begins to retract, squeezing serum (plasma minus the clotting proteins) from the mass and pulling the ruptured edges of the blood vessel closer together.

Normally, blood clots within 3 to 6 minutes. As a rule, once the clotting cascade has started, the triggering factors are rapidly inactivated to prevent widespread clotting. Once the clotting cascade was understood, it became clear that placing a sterile gauze over a cut or applying pressure to a wound would speed up the clotting process. The gauze provides a rough surface to which the platelets can adhere, and the pressure fractures cells, increasing the release of thromboplastin locally.

A complete blood cell count, CBC is a measurement of size, number and maturity of the different blood cells in a specific volume of blood. A complete blood cell count can be used to determine many abnormalities with either the production or destruction of blood cells. Variations from the normal number, size, or maturity of the blood cells can be used to indicate an infection or disease process.

Often with an infection, the number of white blood cells will be elevated.
Many forms of cancer can affect the bone marrow production of blood cells.
An increase in the immature white blood cells can be associated with leukemia.
Anemia and sickle cell disease will have abnormally low hemoglobin.

Normal red blood cells are flexible and disk-shaped, thicker at the edges than in the middle. In several inherited disorders, red blood cells become spherical, hereditary spherocytosis, oval hereditary elliptocytosis, or sickle-shaped sickle cell disease.