Veterinary Hematology Reading

Introduction

In this chapter, we will explore blood—the fluid that flows through arteries and veins, transporting oxygen and nutrients to cells and removing carbon dioxide and other waste products. We will start by examining the components of blood, discuss blood's many functions, and subsequently explore the valuable clinical information about an animal that can be obtained from the analysis of blood. Later in the chapter, we will look at the lymphatic system and its role in keeping an animal's immune system healthy.

Blood Composition

Blood is a fluid connective tissue that flows throughout the entire body. Whole blood is the blood contained in the cardiovascular system. Peripheral blood is whole blood circulating in blood vessels carrying oxygen, nutrients, and waste materials. When you obtain an animal's blood sample from a vein or artery, you are taking peripheral blood.
Grossly, blood is an opaque, deeply red fluid. Microscopically, whole blood is a clear liquid, plasma, in which many cellular components are suspended. Plasma is primarily water in which various solutes are dissolved. The cellular components are red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). There are five types of white blood cells: neutrophils, eosinophils, basophils, lymphocytes, and monocytes.
In veterinary clinical practice, diagnostic blood tests are routinely performed on sick animals to determine the cause of illness. Several different blood samples may be obtained depending on which diagnostic tests are being performed. Whole blood samples are commonly obtained from an animal's vein using a vacuum tube and needle, a process called venipuncture. The vacuum tubes have differently colored stoppers or tops depending on which anticoagulant, if any, they contain.

Anticoagulants

An anticoagulant is a chemical that, when added to blood, prevents the blood from clotting after it is removed from the body. Blood clotting factors found in plasma need to be present in sufficient quantities for blood to clot. Substances that tie up clotting factors and prevent blood from clotting are called anticoagulants. If an anticoagulant is added to a blood sample, the blood will not clot. One of the most common anticoagulants is ethylenediaminetetraacetic acid (EDTA), which prevents clotting by tying up calcium (clotting factor IV). If even one clotting factor is absent, the blood will not clot—"No calcium, no clot."

If anticoagulant is added to a blood sample as it is drawn from an animal, the sample will not clot because all the clotting factors are not present. If the blood sample is then centrifuged (spun at high speed), the fluid that rises to the top of the tube is plasma. Conversely, if no anticoagulant is added to a blood sample, the blood will clot. If the clotted blood is centrifuged, the fluid that rises to the top of the tube is called serum. When blood clots, one of the dissolved plasma proteins, fibrinogen, is converted to insoluble fibrin, which precipitates out of solution as a meshwork of tiny fibers and helps make up the framework of the clot. Removing fibrinogen from plasma by allowing it to clot converts plasma to serum.

Clinical Application

Plasma, Serum, and Diagnostic Tests

Many of the diagnostic clinical chemistry tests performed on a patient sample are run on either plasma or serum. After the sample has been centrifuged, the plasma or serum can be drawn off and analyzed or frozen for analysis at a later date. Whole blood cannot be frozen because blood cells rupture easily during the freezing and thawing processes. Some blood samples will be collected using a syringe and needle and then transferred from the syringe to a specific tube to obtain a serum or plasma sample.
A commonly used diagnostic test in hematology is the complete blood count (CBC) and blood smear. This test uses blood samples that are not allowed to clot, so they are collected in a purple-top vacuum tube containing EDTA. Blood from these tubes is used to analyze blood from very small species, such as mice. If a blood sample in a blood tube with anticoagulant present is centrifuged, it separates into three layers based on the blood components' densities: the plasma layer on top containing the clotting proteins, the buffy coat layer in the middle composed of leukocytes and thrombocytes, and the erythrocyte layer on the bottom.

Function

Blood has three main functions: transportation, regulation, and defense.

Transportation

  • Erythrocytes (Red Blood Cells): Contain hemoglobin, which carries oxygen to every cell in the body.

  • Nutrients: Essential elements are dissolved in blood plasma and transported to tissues via arteries and capillaries.

  • Waste Products: Blood carries waste products from cellular metabolism via veins to the lungs and kidneys for elimination.

  • Hormones: Blood transports hormones from endocrine glands to target organs and also transports white blood cells to various sites of activity to defend the body from infection.

  • Platelets: Transport to sites of damage in blood vessel walls to form a plug that controls bleeding, known as hemostasis. Platelets are involved in activating the blood-clotting cascade.

Regulation

  • Body Temperature: Blood aids in regulating body temperature influenced by the thermal dynamics of circulating blood.

  • Tissue Fluid Content: The composition of body tissue fluid is maintained as constant as possible, with plasma leaving the bloodstream during dehydration and entering to compensate for fluid loss.

  • Blood pH (Acid-Base Balance): Normal blood pH falls within a range of 7.35 to 7.45, with the ideal being 7.4 (slightly alkaline).

Defense

  • Immune Response: Blood carries white blood cells to tissues exposed to foreign invaders to assist the immune system.

  • Hemostasis: Involves platelets to prevent excessive bleeding at sites of vessel damage.

Hematopoiesis

Before exploring the individual cellular components of blood (erythrocytes, leukocytes, and platelets), it's critical to examine their origins. Hematopoiesis is the production of all blood cells occurring continuously throughout an animal's life.
In the fetus, hematopoiesis occurs in the liver and spleen. In newborns, it takes place primarily in red bone marrow located in most of the body's bones. As the animal matures, the rate of production slows with less active marrow dominated by fat (yellow bone marrow). However, yellow marrow can be reactivated by increased physiological demands.

Sources of Blood Cell Production

  • Adult animals have red bone marrow in various sites, including the skull, ribs, sternum, vertebral column, pelvis, and proximal ends of femurs. Red bone marrow produces billions of each cell type daily, continually replacing aged or damaged blood cells to maintain health.

  • The liver and spleen can participate in hematopoiesis when necessary, but not at the same capacity as bone marrow.

  • Pluripotential Stem Cells: All blood cell types derive from a single primitive stem cell, with the fate of these cells being determined by specific stimuli called poietins, colony-stimulating factors, or interleukins.

  • Hematopoiesis is an ongoing process with maturation occurring in the bone marrow and peripheral bloodstream.

Erythropoiesis

Erythropoiesis specifically refers to red blood cell production. Unipotential stem cells are stimulated to differentiate into proerythroblasts, which undergo division through multiple stages, leading to mature red blood cells (RBCs).

  • The process takes about a week in dogs, 4 to 5 days in cows, and 36 hours in birds.

  • The key hormones regulating erythropoiesis are erythropoietin (EPO) and the availability of iron, folic acid, vitamin B12, and protein.

  • Erythropoietin is produced in the kidneys and is stimulated by hypoxia, which increases erythropoiesis demand during low oxygen availability.

  • In cases of severe anemia, immature nucleated RBCs may be released which are less efficient than fully mature RBCs.

Clinical Application

Polychromasia and Nucleated RBCs

In emergency situations, the bone marrow may produce and release immature RBCs in response to a sudden loss of red blood cells, creating polychromasia or the presence of nucleated RBCs in circulation, indicating a regenerative response from the bone marrow.

Thrombopoiesis

Thrombopoiesis refers to platelets' production beginning with unipotential stem cells differentiating into megakaryocytes. These large, multinucleated cells release cytoplasmic fragments into peripheral blood, which are known as platelets. The process can take up to 7 days.

Leukopoiesis

Leukopoiesis is the formation of white blood cells starting from pluripotential stem cells. Each white blood cell type has a specific production stimulus.

  • Granulopoiesis: Formation of granulocyte types (neutrophils, eosinophils, basophils) with distinct and specific granules in their cytoplasm.

  • Lymphocytes and monocytes are classified as agranulocytes and develop in response to specific stimuli.

Cellular Components of Blood

Red Blood Cells

Structure

Mature RBCs are non-nucleated, biconcave discs, composed primarily of hemoglobin for oxygen transport. Their hemoglobin content imparts their red appearance and allows them to efficiently carry oxygen and carbon dioxide across the body.

Function

  • Transport Oxygen: Facilitated by hemoglobin.

  • Transport Carbon Dioxide: Carried from the tissues to the lungs.

  • Maintain Shape: The biconcave shape enhances surface area for gas exchange and allows for deformability as they traverse small capillaries.

Life Span and Destruction

The average life span varies by species, with typical durations being 120 days for dogs, 68 days for cats, and longer for certain others. Aging RBCs undergo senescence which reduces enzyme activity and flexibility, leading to their removal from circulation by macrophages, primarily in the spleen (extravascular hemolysis).

Hemolysis Processes

  • Extravascular Hemolysis: Predominantly occurs (90%), where macrophages in the spleen digest senescent cells.

  • Intravascular Hemolysis: A minority (10%) occurs within blood vessels under oxidative stress, leading to fragmented RBCs.

Clinical Applications in RBC Evaluation

Jaundice/Icterus

This condition arises when there is an excess of unconjugated bilirubin due to increased RBC breakdown from hemolysis, leading to yellow-colored tissues and waste products.

Complete Blood Count (CBC)

The CBC evaluates plasma proteins, RBCs, WBCs, and platelets, yielding insights regarding an animal's health status and potential pathologies a affecting these components.

Anemia

Anemia could arise from various mechanisms:

  • Hemorrhage: Loss of blood volume,

  • Destruction: Excessive hemolysis or immune-mediated hemolytic anemia,

  • Production Failures: In cases such as iron deficiency or bone marrow disorders. Diagnosing involves checking reticulocyte counts for regenerative responses or antioxidant psi-ometric measures in blood samples.

Other Blood Component Disorders

Conditions like thrombocytopenia (low platelet counts), leukocytosis or leukopenia (abnormal WBC counts) indicate diverse pathologies, from infections to immune disorders necessitating clinical inquiry and testing according to the observed abnormalities.

By examining variations in blood analysis, understanding the function of each component, and recognizing interrelations in disease states, one can ascertain insights useful in veterinary clinical practice for diagnostic and therapeutic approaches. This chapter sets the groundwork necessary for deeper explorations into hematopathology and clinical pathology practices.