Blood Blood is a type of connective tissue consisting of several types of cell suspended in a liquid matrix called plasma. The average human body contains 4-6L of blood. When centrifuged the cellular elements which occupy about 45% of the volume of blood, settle to the bottom of the centrifuge tube. Above it is the transparent, straw-coloured plasma. Plasma is about 90% water and makes up 55% of whole blood. Among the variety of solutes within the plasma are inorganic salts, sometimes referred to as blood electrolytes. The combined concentration of these ions is important in maintaining the osmotic balance of the blood. Some of the ions also buffer the blood, which has a pH of 7.4. Another important class of solutes is the plasma proteins. Collectively they act as buffers against pH change, help maintain osmotic balance between blood and interstitial fluid, and contribute to the blood’s thickness. One class of these proteins, the immunoglobulins, are the antibodies that help combat viruses and other foreign agents that invade the body. Some of the proteins, called fibrinogens, are clotting factors that help plug leaks due to haemorrhage.

Erythrocytes

Constituents PLASMA CELLULAR ELEMENTS WATER-solvent for carrying other substances. IONS-Na,K,Ca,Mg,Cl,HCO3 -Osmotic balance,pH buffering and regulation of membrane permeability. PLASMA PROTEINS-Albumin,fibrinogen,immunoglobulins-osmotic balance, pH buffering, clotting and defence. SUBSTANCES TRANSPORTED- Nutrients (e.g., glucose,fatty acids,vitamins) waste products of metabolism. Respiratory gases(O2 and CO2). Hormones CELLULAR ELEMENTS ERYTHROCYTES-5-6 million per mm3-transport oxygen and help transport carbon dioxide. LEUKOCYTES-5000-10,000-defence and immunity (basophils,eosinophils,neutrophils, lymphocytes,monocytes) PLATELETS-250,000-400,000-Blood clotting.

Cellular elements Dispersed through blood plasma are 2 classes of cell, red blood cells and white blood cells. A third cellular element, platelets, are fragments of cells that are involved in blood clotting. Red blood cells are by far the most numerous and there are about 25 trillion in the 5L of blood in the body. Their structure is an excellent example of structure related to function. Their biconcave structure,flexible and strong sell membranes allow them to squeeze through capillaries and increase surface area. They lack nuclei and mitochondria therefore relying upon anaerobic respiration to generate ATP. The major function of erythrocytes is to carry oxygen so it would be inefficient if the oxygen they carried was to be used in their own respiration. They are also very small but still manage to contain 250-280,000,000 haemoglobin molecules. Oxygen diffuses into the erythrocytes and haemoglobin binds reversibly. There are 5 major types of white blood cell and their collective function is to combat disease and fight infection. They do this in a number of ways. White blood cells spend most of their time outside of the circulation patrolling interstitial fluid. The third cellular element, platelets, are fragments of cells. They have no nuclei and function in the process of blood clotting.

Replacement of cellular elements Erythrocytes, leukocytes and platelets wear out and are constantly replaced. For example, erythrocytes only circulate about 3-4 months and are then destroyed by phagocytic cells situated in the liver and the spleen. Most of the iron from the heme groups is recycled into new haemoglobin molecules. RBC,WBC and platelts all derive from a common source, a single population of cells called pluripotent stem cells in the red bone marrow of bones. Pluripotent means these cells have the potential to differentiate into any type of blood cell or into cells that produce platelets. Red blood cell production is controlled by a negative feedback mechanism that is sensitive to the amount of oxygen reaching the tissues from the blood.. If the tissues aren’t receiving enough then the kidneys convert a plasma protein to a hormone called erythropoietin which stimulates production of erythrocytes in the bone marrow.

Clotting Blood contains a self-sealing material that plugs leaks in our vessels. The sealant is always present in our blood in an inactive form called fibrinogen. A clot forms only when this plasma protein is converted to its active form fibrin , which aggregates into threads that form the fabric of the clot. The clotting mechanism usually begins with the release of clotting factors that ultimately transforms fibrinogen to fibrin. More than a dozen clotting factors have been discovered. An inherited defect in any step of the clotting process is called haemophilia and is attributed to the individual not possessing the correct gene to code for clotting factor VIII. Anticlotting factors present in the blood stop spontaneous clotting though sometimes, platelets clump and fibrin coagulates within a blood vessel, blocking the flow of blood. Such a clot is called a thrombus. These clots are more likely to form in individuals with cardio-vascular diseases. This is also the underlying cause of deep-leg thrombosis that can arise from long-haul airplane journeys due to sitting down for long periods of time.

Haemoglobin This is an oxygen-carrying protein which is also the pigment that gives whole blood it’s red colour

Platelets The arrow is pointing at a platelet which is a cell fragment that makes part of the formed element portion of blood. They play a very important role in blood clotting.

Monocytes These cells can migrate out of the blood to form macrophages, phagocytic cells that engulf bacteria and cell debris

Neutrophils These white blood cells are involved in phagocytosis

Lymphocyte These cells are responsible for releasing antibodies

Eosinophils These cells secrete a major basic protein in defence to certain parasitic worms.

Basophils These cells secrete heparin and histamine