Blood!

Anatomy & Physiology

Introduction

Blood is a liquid connective tissue that serves as the transport medium in the circulatory system. A healthy adult has about 4-5 liters of blood.

Main Functions

Transport nutrients, oxygen, wastes, and hormones

Helps maintain the stability of interstitial fluid

Distributes heat throughout the body

Plays a major role in defending the body against disease

Major Components

Blood is composed of a liquid medium and blood solids.

SOLIDS (45% of the blood) - this solid part is referred to as the hematocrit (HCT), which is composed of red blood cells, white blood cells, and platelets

LIQUID (55% of the blood) which is composed of the plasma

Red Blood Cells

RBCs, or erythrocytes, have a very distinct shape: thick around the edges and thin in the center.

The shape of RBCs is due to the absence of nuclei.

Early in development, there is a nucleus in each red blood cell, but it is removed as time passes to make room for hemoglobin.

Hemoglobin, an iron-containing protein, makes it possible for RBCs to transport oxygen.

It makes up 1/3 of the volume of the RBCs and is responsible for the red color of blood

When hemoglobin is combined with oxygen, a component called oxyhemoglobin is formed and it is bright red.

When oxygen is released, a component called deoxyhemoglobin is formed and is a much darker shade of red.

Without nuclei, RBCs cannot make proteins or divide.  This limits the survival of RBCs to about 120 to 130 days.

Of the more than 30 trillion red blood cells circulating throughout the body at one time, 2 million disintegrate each second.

To replace them, new ones form at the same rate in the red bone marrow of the bones.

Old RBCs are recycled and the hemoglobin molecule is carried to the red bone marrow, where it is reused in new RBCs.

The number of RBCs circulating in the blood determines the amount of oxygen that can be transported through the blood, or the oxygen-carrying capacity.

Adult males: 4,600,000-6,200,000 per cubic millimeter

Adult females: 4,200,000-5,400,000 per cubic millimeter

RBCs are very flexible and easily pass through the smallest of blood vessels.

However, with age, the RBCs become much more fragile and can be damaged as they pass through the circulatory system.

Destruction of Red Blood Cells

A type of cell called macrophages is used to phagocytize and destroy these damaged blood cells.

The hemoglobin molecule is separated from the RBCs and is further broken down into its components:

heme - an iron-containing portion

globin - a protein

The blood transports the iron with a protein back to the blood forming tissues of the red bone marrow where it is reused.

RBCs are produced as they are needed, or in an attempt to maintain homeostasis.

A negative-feedback mechanism controls the release of erythropoietin (hormone that stimulates RBC formation).

Erythropoietin is released from the kidneys, and sometimes from the liver, when there is an oxygen deficiency.

A drop in oxygen stimulates the release of erythropoietin and in turn stimulates RBC production.

When oxygen supplies return to normal, the release of erythropoietin is decreased and RBC production slows.

Red Blood Cell Production

White Blood Cells

WBCs, or leukocytes, protect against disease. They are formed in the red bone marrow, lymph nodes, and spleen.

There are five main types of WBCs circulating in the blood and can be classified by two characteristics:

granulocytes - have granular cytoplasm

agranulocytes - lack cytoplasmic granules

Granulocytes are about twice the size of RBCs. They develop in the red bone marrow and only live about 12 hours.

1.Neutrophils - very fine granules that stain light purple

Nucleus is lobed and has 2-5 sections connected by thin strands of chromatin (resembles sausage links)

Composes 54-62% of leukocytes in a blood sample

2.Eosinophils - coarse cytoplasmic granules that stain deep red

nucleus usually has only two lobes composes 1-3% of leukocytes in a blood sample

3.Basophils - similar to eosinophils in size and nuclei shape but have fewer and irregularly shaped granules

granules stain deep blue

less than 1% of leukocytes in a blood sample

Agranulocytes

4. Monocytes - generally form in the red bone marrow; 3-9% of leukocytes

largest of the blood cells with nuclei that vary in shape (kidney, oval, or lobed)

may live for several weeks or months

5. Lymphocytes - usually slightly larger than RBCs; 25-33% of leukocytes

contains a large nucleus surrounded by a thin rim of cytoplasm

may live for years

Comparison in Size

Monocyte Neutrophil Eosinophil BasophilLymphocyte

Erythrocyte

The WBCC is the number of WBCs in a cubic millimeter of human blood, which normally includes 5,000-10,000 cells.

WBCC exceeding 10,000 per cubic millimeter is called leukocytosis (acute infections, such as appendicitis).

A total WBCC below 5,000 per cubic millimeter is called leukopenia (may accompany typhoid fever, influenza, measles, mumps, chicken pox, AIDS, or poliomyelitis).

White Blood Cell Counts

A differential white blood cell count (DIFF) lists the percentages of all the different types of WBCs in a sample. This can be very helpful in the identification of certain types of infections.

For example, HIV reduces the number of lymphocytes very rapidly.

Platelets, or thrombocytes, are not complete cells.

They are formed when extremely large cells, called megakaryocytes, break into several pieces and are released into circulation.

The larger fragments shrink and become platelets as they travel through the blood vessels of the lungs.

Platelets

Characteristics of Platelets

They do not have a nucleus and are about half the size of RBCs.

Platelets usually live about 10 days.

A person’s platelet count can vary from 130,000-360,000 per cubic millimeter of blood.

They function to close breaks in damaged blood vessels and begin the process of clotting.

Plasma is the liquid portion of the blood that is straw-colored and composed of about 90% water.

Plasma carries vitamins, minerals, amino acids, and glucose to the cells of the body.

Plasma also carries hormones and brings wastes from the cells to the kidneys or the lungs to be removed from the body.

Plasma

Certain proteins are carried by the plasma and have very specific functions such as blood clotting, etc.

Albumins - plays an important role in the regulation of pressure between the plasma and blood cells and between plasma and tissues

Globulins - alpha, beta, and gamma; transport lipids and vitamins

Fibrinogen - functions in blood coagulation (clotting)

Antibodies - help the body fight disease

Hemostasis

1.Blood vessel spasm - cutting or breaking a blood vessel causes the smooth muscles in its walls to contract, lessening the loss of blood immediately

vasospasm - ends of a severed blood vessel may close completely

Platelets release seratonin, which contracts smooth muscles in blood vessels.

2.Platelet plug formation - if a blood vessel breaks, platelets stick to the damaged area and to each other forming a plug in the blood vessel

Platelet plugs may stop bleeding in small breaks, but larger damage may require a clot.

3.Blood coagulation - formation of a blood clot

The plasma protein fibrinogen is converted into threadlike strands of fibrin.

Fibrin threads stick to the exposed surfaces of damaged blood vessels.

2. A meshwork begins to form, and blood cells and platelets become trapped to form a blood clot.

Antigens, also called agglutinogens, are surface molecules on RBCs.

Antibodies, also called agglutinins, are specific proteins responsible for protecting the body carried in the plasma.

The ABO blood groups are based on the presence or absence of either antigen A or B. These antigens also cause the blood to make certain antibodies.

Antigens and Antibodies

Antibodies will react with the antigen of the same type and cause RBCs to clump. This is why people can only receive transfusions of certain blood types:

Type A - A or O

Type B - B or O

Type AB - A, B, or O

Type O - only O

Type O is known as the universal donor, while type AB is known as the universal recipient.

In humans, this group includes several Rh antigens.

If any of the Rh antigens are present on RBCs, the blood is said to be Rh+. If RBCs lack the Rh antigens, the blood is called Rh-.

Rh Blood Groups

Antibodies against Rh antigens do not appear spontaneously, but are formed in response to certain stimulation.

If an Rh- person receives a transfusion of Rh+ blood, the person will begin producing anti-Rh antibodies.

There is usually no effect during the first transfusion, but if the person receives another Rh+ transfusion the new antibodies may make the RBCs clump.

A similar situation may occur when an Rh- woman becomes pregnant with an Rh+ positive fetus.

The first pregnancy normally proceeds without any problems, but if the woman becomes pregnant with another Rh+ fetus, the fetus will develop erythroblastosis fetalis.

Erythroblastosis fetalis is a condition in which the fetus’s RBCs are destroyed. The fetus will not survive.