Blood is a connective tissue that flows through the body of many animals, transporting gases, nutrients, waste products and hormones around the body. Is it also important for a number of other functions such as regulating the fluid that surrounds cells, reducing fluid loss after injury, regulating body temperature and immunity defenses.
Relative to water, blood is a viscous fluid due to the amount of proteins, red blood cells and other compounds it contains. It owes its vibrant red color to haemoglobin, a protein found in the red blood cells that binds to oxygen and increases the efficiency of oxygen transport around the body.
The contents of blood can be separated into two groups; one group is called the “formed elements” which is 99.9% red blood cells, but also includes white blood cells and platelets (important components of the immune system and the clotting of blood). The other half of the blood is known as plasma and contains around 92% water, plasma proteins and other solutes such as electrolytes and organic wastes.
Red blood cells
Red blood cells (RBC) are responsible for the transportation of oxygen around the body and their significance is proven by the fact that they account for almost half of the entire blood volume. They are chocked full with haemoglobin which makes up approximately 95% of the proteins found in red blood cells.
Structurally red blood cells are shaped like a doughnut without the hole. This shape creates a large surface area which helps to increase the efficiency of oxygen exchange between the blood and tissue cells. Their shape also makes it easier for them to travel through thin capillaries as they can bend more and stack together.
Another important feature of RBCs in mammals is that they don’t have a nucleus or any organelles, one of the only animal or plant cells to lack such features. There is a certain level of variation between mammal species but generally the nucleus and organelles are absent in the red blood cells.
Haemoglobin
Haemoglobin is one of the most important and common proteins in the body. It is a globular protein – shaped like a globe – and is formed from four sub-unit proteins, each with a haem group in the middle. The haem group is a molecule in the center of the protein and has an iron ion, Fe2+, in its center. The haem group is able to reversibly bond to oxygen which is why haemoglobin is so helpful for transporting oxygen around the body.
The Fe2+ ion attracts oxygen but but the protein surrounding the Fe2+ ion prevents the oxygen from bonding and becoming FeO, or rust. Changes in the shape of the protein affect how tightly or loosely oxygen binds to the haem depending how close the O2 gets to the iron within the haem molecule.
White blood cells
The white blood cells or leukocytes show a much greater variation than the red blood cells and they perform a wide range of functions, more often than not, that help boost the immune system. They differ significantly from red blood cells in that they have nuclei and other organelles and do not have any haemoglobin.
There are a number of different types of white blood cells such as neutrophils, basophils, eosinophils and lymphocytes. Each different type of white blood cell performs a different set of functions. Neutrophil cells are the most common and make up to 70% of the white blood cells. They are an important component of the inflammatory system and are the cells responsible for the formation of pus. Basophil cells release compounds, such as histamine that help the repairing process of damaged tissue.
Eosinophils are a type of cell known as phagocytes, which basically means they engulf substances, often foreign to the body, such as bacteria, but also the break down components of bodily compounds, such as dead cells. Each eosinophil has particular anti-bodies, compounds on the cells exterior that attract the cell to specific compounds, which may be found on the cells of bacteria or the break-down components of damaged tissue. Macrophage cells are large generalist phagocytes.
Lymphocytes are very specific defense cells and are crucial to the adaptive immune system of mammals and higher animals. Lymphocyte cells include T cells, B cells and Natural Killer cells.
Blood plasma
The blood plasma contain a number of important compounds such as proteins, water and electrolyes. The most common plasma proteins are the albumins which are responsible for maintaining the osmotic pressure of the blood. Without albumins the blood would be more like the consistency of water. Increasing the thickness of blood reduces the amount of fluid that enters into the bloodstream from outside the capillaries.
Globulins are the second most common protein in the blood plasma. These include the immunoglobins which are an important part of the immune system and are also important for transporting hormones and other compound around the body. Fibrinogen makes up the majority of the remaining proteins in the blood and is the compound responsible for the clotting of blood to help prevent blood loss.
Transportation of blood
Blood is transported around the body through arteries, capillaries and veins. Arteries carry the blood away from the heart, and veins carry it back. Capillaries are very fine blood vessels that transport blood through the different tissues of the body. Pressure and osmotic gradients between the capillaries and the fluid outside of the capillaries allow for the transfer of blood between the two.
When blood is pumped from the heart, the pressure within the capillaries is much greater than the external pressure and blood is forced out of the capillaries to reduce the pressure. As the blood moves through the body, the pressure gradually reduces due to the movement of blood out of the capillaries. The osmotic pressure forces fluid into the capillaries once the pressure within the capillaries is reduced.
New blood cells
Haematopoiesis is the formation of new blood cells. It begins with stem cells, known as hemocytoblasts, which have the potential to form any type of blood cell. The process occurs mostly in the bone marrow but some final differentiation can occur in the blood and tissue.
Each stem cell undergoes a number of phases, each phase producing a different precursor cell than the previous phase. The pathway that any given cell might take depends on the compounds present, such as hormones, which influence how a cell will differentiate. At the end of the process a fully differentiated red, white or thrombocyte cell is formed.
Last edited: 25 May 2015