Physiology of Blood

In this post, we’ll be discussing the fifth topic in our Veterinary Physiology 1 unit - blood. Today we’ll look at the main components of blood and their functions, what a haematocrit is and the major steps in the life cycle of a red blood cell (RBC). We’ll discuss what RBCs do and what factors are required for them to be produced by the body. Other topics that we’ll cover include: anaemia, blood coagulation, and blood groups. 

The Components of Blood

Mammalian blood contains:

·         Proteins: mammalian blood contains a fairly high concentration of proteins. The principle proteins found in blood are carrier proteins such as albumin and the globulins as well as proteins involved in blood clotting. Albumin helps to maintain plasma oncotic pressure.

·         Erythrocytes (RBCs): These cells are the most abundant in blood and transport oxygen using haemoglobin. They have a biconcave disc-like structure with a diameter of 7µm.

·         Leukocytes:

o   Granulocytes: A granulocyte is any white blood cell that contains granular material and lysosomes in its cytoplasm, they include:

§  Neutrophils: these are also called PMNs or polymorphonuclear leukocytes. They have a segmented nucleus and finely granular cytoplasm. They adhere to the margins of vessel walls, migrate out of the blood stream and engulf foreign particles within tissues. They are the immediate cellular defence against bacteria.

§  Eosinophils: which have pink staining granules in the cytoplasm and a bi-lobed or segmented nucleus. They control infection of parasites and regulate allergic and inflammatory reactions.

§  Basophils: they have dark purple-staining acidic granules which contain histamine, heparin and serotonin. They are extremely rare in peripheral blood.

o   Mononuclear cells:

§  Lymphocytes: these are the most common of the leukocytes in the adult bovine. They are part of the immune system and combat infection by releasing toxic molecules. Most lymphocytes found in circulation are T cells which are involved in cell mediated immunity.

§  Monocytes: these transform into macrophages (which are large phagocytic cells) within tissues and consume particles, dead cells and unwanted debris.

·         Platelets: these are anucleate cell fragments which are produced by megakaryocytes in bone marrow. Their function is to maintain the reliability of blood vessel walls by aggregating at the site of injury and creating a platelet ‘plug’, acting in the first stage of blood clotting.

This website has some useful descriptions and diagrams about the cell types mentioned above,
 
The Functions of Blood

The functions of whole blood include:

• Transport to tissues:
• hormones, nutrients, proteins, cytokines
• Oxygen
• White blood cells to fight infection in tissues
• platelets and coagulation factors
• Transport from tissues:
• CO2 and nitrogenous wastes
• It acts as a buffer system using proteins and carbonic anhydrase to deal with the levels of CO2.
• The protein component maintains colloidal osmotic pressure to keep fluids in circulation. Other proteins help with blood clotting and in fighting infection.

The Haematocrit

A haematocrit is the fraction of the blood that is made up of erythrocytes. This is calculated by centrifuging a sample of anti-coagulated blood. This results in three major components:

·         Plasma: This contains water, proteins, electrolytes and solutes etc.

·         Buffy coat: this contains platelets and white blood cells

·         Red blood cells: this is the haematocrit which is also known as a PCV (packed cell volume).

The normal average haematocrit for an dog is 45% and 30% in cats.

             

Blood Components

The Production of Erythrocytes

In adult mammals, blood cells are produced continuously within bone marrow from self-replicating stem cells. The production of blood cells is called haematopoiesis and is tightly regulated. The production of blood cells in other tissues such as the spleen, liver and lymph nodes, which provide the correct conditions for this process is called extra-medullary haematopoiesis. A normal 20kg dog releases 1 000 000 mature RBCs per second from the bone marrow into peripheral blood. 

Erythropoesis, which is the production of erythrocytes, is regulated by the hormone erythropoietin. A decreased amount of oxygen in the blood stimulates the release of erythropoietin which stimulates the red bone marrow to produce more RBCs. The increased amount of RBCs in the blood leads to an increase in the oxygen carrying capacity which leads to higher levels of oxygen in the blood. This produces a negative feed back control on red blood cell production.

Anaemia

Anaemia is a condition that occurs when there are too few erythrocytes, the erythrocytes do not contain enough haemoglobin, or the erythrocytes are abnormal in other respects. Anaemia may result from a loss of blood or from a deficiency in the factors required to make haemoglobin (eg. iron) or erythrocytes (eg. folic acid or vitamin B12). The shape of the erythrocytes may be abnormal because of a disease (such as sickle cell anaemia).

Consequences of Anaemia  

Generally, consequences include pale mucous membranes, anoxia to tissues and a reduced blood viscosity. A reduction in viscosity leads to less resistance to blood flow which causes more blood to flow to the heart, resulting in an increased cardiac output and heart rate. An increased rate in respiration may also occur. The animal is likely to compensate for all of this by decreasing its activity.

That's about all we'll need to know about blood for now. If you have any questions please don't hesitate to ask :)