Structure and Function of the Blood in Dogs
Dr. Bari Spielman
Neutrophils are the most numerous of all white cells. In the normal dog there are usually 3,000 to 11,500 of these cells per microliter of blood. They form a primary defense against bacterial infections. They move out of blood vessels into infected or inflamed tissue in order to attack the infection or injury.
What is Blood?
Blood is the life-maintaining transport fluid that circulates oxygen and nutrients throughout the body, carries away waste products, and helps defend against disease. Blood consists of numerous components, such as cells and proteins, that circulate in a fluid called plasma. The millions of cells that circulate in blood include erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets). Besides transporting vital components, the blood plays an important role in the immune functions of the body and is vitally important to coagulation (ability of blood to clot properly).
Various components of blood are produced in several tissues and organs. Red blood cells, platelets and some white blood cells are manufactured in the bone marrow, which is present in the central cavity of the long bones of the body. Other white blood cells are formed in the lymph nodes. Proteins that circulate in the blood are manufactured in the liver and by circulating white blood cells. At times the spleen is also a site of the production of some blood components.
Where is Blood Located?
Blood is located in almost every part of the body, because it circulates through the body's heart, arteries, veins and capillaries (the tiny vessels that connect arteries and veins). Any tissue containing blood vessels normally contains blood.
What is the General Structure of Blood?
Blood consists of four main parts: red blood cells, white blood cells, platelets and plasma.
Red blood cells (RBCs). These are the most numerous cells found in the blood. In the normal dog, there may be 5.5 to 8.5 million RBCs in a microliter of blood. RBCs are disc-shaped cells that contain hemoglobin, an important protein that transports oxygen. Mature red blood cells are unique in that they do not contain a nucleus. (The nucleus is the small, oval area in the cell that contains DNA genetic material.)
White blood cells. Several classes of white blood cells (WBCs) circulate in the blood. At any given time, in the normal dog, there are roughly 6,000 to 17,000 of these cells per microliter of blood. The types of white blood cells are as follows:
Lymphocytes are an active component of the immune system and are manufactured in the bone marrow, lymph nodes, spleen and other lymphatic tissues. In the normal dog, approximately 1000 to 5000 of these cells are present in each microliter of blood. A major function of many lymphocytes is to produce antibodies.
Monocytes circulate in the blood until they are needed in tissues that are inflamed or infected. They then leave the blood and enter such tissues where they mature into cells called macrophages. Macrophages are capable of engulfing and destroying harmful organisms and other materials. At any one time, there are usually 150 to 1,350 monocytes present in each microliter of blood in the normal dog.
Eosinophils play an important role in the response of the body to allergic and inflammatory reactions, and to parasitic infestations. In the normal dog, only about 100 to 750 eosinophils are seen per microliter of blood, but their numbers may be dramatically increased if parasites or other foreign protein are present in the body.
Basophils are the rarest of all white blood cells and are not usually seen in blood samples. They participate in many of the same reactions that eosinophils are involved in. Finding basophils in the circulating blood is usually significant.
Platelets. Platelets are not cells; they are very tiny disks that look like flat plates and are produced primarily in the bone marrow. Their major function is to plug any leak that develops in the walls of blood vessels and to start the process of blood clotting. In normal dogs, there are often 175,000 to 500,000 platelets per microliter of blood.
Plasma. Plasma is the fluid portion of blood. It is a watery mixture of proteins, minerals and other chemicals vital to the body. It contains substances such as sodium, potassium, chloride, magnesium, glucose (blood sugar), and various enzymes that are produced in the liver, pancreas, kidneys, and muscles. Plasma also contains important clotting factors and many body waste products.
What are the Functions of Blood?
Each component of blood has very specialized and important functions.
Red blood cells contain hemoglobin, which is a red, iron-rich protein. Hemoglobin enables red blood cells to carry oxygen from the lungs to all parts of the body. Red blood cells give blood its color. When the blood is rich in oxygen it is red, and when there is little oxygen in the blood, the blood is blue. Because blood traveling from the lungs to the body usually contains lots of oxygen, blood in the arteries is normally red. Much of the oxygen is removed from the small capillaries by the body tissues, so blood in the veins tends to be blue in color.
The white blood cells defend the body against disease. They destroy bacteria and foreign material, they stimulate inflammation and assist in the healing process, and they produce proteins called antibodies that destroy bacteria, viruses, and other diseases. WBCs move in and out of the blood stream, depending upon where they are needed.
Platelets help the blood to clot. They group together to form clumps, plugging any holes that develop in blood vessels. Clumps of platelets form a scaffolding upon which a blood clot may form. Formation of a blood clot is a complicated process called coagulation.
Plasma is the watery material that carries all other components of the blood within the blood vessels. If water is lost through dehydration, wounds or burns, then the blood can become thickened, almost like sludge, and circulation will be adversely affected.