These notes will be covered in Tuesday’s lecture (April 23rd) – you can download the pdf version in the box to your right.
Blood is considered a specialized form of connective tissue with cells suspended in a liquid extracellular matrix
- origin in the bones
- Contains molecular fibers in the form of fibrinogen.
- The blood cells present in blood are mainly red blood cells (also called RBCs or erythrocytes) and white blood cells, including leukocytes and platelets (also called thrombocytes).
Blood performs many important functions within the body including:
- Supply of oxygen to tissues (bound to hemoglobin which is carried in red cells)
- Supply of nutrients such as glucose, amino acids and fatty acids (dissolved in the blood or bound to plasma proteins)
- Removal of waste such as carbon dioxide, urea and lactic acid
- Immunological functions, including circulation of white cells, and detection of foreign material by antibodies
- Coagulation, which is one part of the body’s self-repair mechanism
- Messenger functions, including the transport of hormones and the signaling of tissue damage
- Regulation of body pH (the normal pH of blood is in the range of 7.35 – 7.45)
- Regulation of core body temperature
- Hydraulic functions
- Blood accounts for 7% of the human body weight, with an average density of approximately 1060 kg/m³, very close to pure water’s density of 1000 kg/m3.
- The average adult has a blood volume of roughly 5 litres, composed of plasma and several kinds of cells (occasionally called corpuscles);
- these formed elements of the blood are erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets).
- By volume the red blood cells constitute about 45% of whole blood, the plasma constitutes about 55%, and white cells constitute a minute volume.
Erythrocytes (Red Blood Cells)
- 4.7 to 6.1 million (male), 4.2 to 5.4 million (female) 600 RBC for each WBC
- Biconcave shape
- In mammals, mature red blood cells lack a nucleus and organelles.
- They contain the blood’s hemoglobin and distribute oxygen.
- The red blood cells are also marked by glycoproteins that define the different blood types.
- The proportion of blood occupied by red blood cells is referred to as the hematocrit, and is normally about 45%.
Red cell formation:
- occurs in the yolk sac, liver and spleen initially;
- after birth formation occurs exclusively in bone marrow
- The various cells of blood are made in the bone marrow in a process called haematopoiesis, which includes erythropoiesis, the production of red blood cells
- During childhood, almost every human bone produces red blood cells; as adults, red blood cell production is limited to the larger bones: the bodies of the vertebrae, the breastbone (sternum), the ribcage, the pelvic bones, and the bones of the upper arms and legs.
- 3 substances involved:
- vitamin B12: DNA synthesis; cell growth and division
- folic acid: DNA synthesis; cell growth and division
- erythropoietin – hormone that controls rate of red blood cell formation
- Iron: Hemoglobin synthesis
Breakdown of RBCs:
- more likely with age as cells become more fragile
- Lifespan of the average RBC is 120 days
- Over the years they experience wear and tear
- Undergo phagocytic breakdown: Fe + haem + globin
- Squeezing through capillaries of active tissues damages RBCs
- Macrophages phagocytize damaged RBCs
- Hemoglobin is broken down into heme and globin
- Heme is broken down into iron and biliverdin
- Iron is either reused in synthesis of new haemoglobin or stored in the liver as ferritin
- Some biliverdin (green pigment) is converted to bilirubin (orange pigment)
- Both biliverdin and bilirubin are excreted in bile
- Globin is broken down into amino acids that are either metabolized by macrophages or released into the plasma
White blood cells (also called leukocytes)
- 4,000-11,000 leukocytes
- White blood cells are part of the immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances.
- The cancer of leukocytes is called leukemia.
- The various cells of blood are made in the bone marrow in a process called haematopoiesis,
- which includes myelopoiesis, the production of white blood cells and platelets.
White Blood Cells are divided into:
- Granulocytes: contains visible granules in cytoplasm
- Neutrophils – first to arrive at infection site (54% to 62% of leukocytes)
- Eosinophils – moderate allergic reactions & defend against parasitic worm infections (1% to 3% of leukocytes)
- Basophils – release histamine and heparin (<1% of leukocytes)
- agranylocytes: contains no granules
- lymphocytes- single large nucleus (25% to 33% of leukocytes)
- recognizes foreign substances (bacteria and cancer cells)
- monocytes (3% to 8% of leukocytes)
- mobile and phagocytic
- circulating precursors of macrophages (exist as monocytes before they turn into macrophages)
- 200,000-500,000 thrombocytes
- Platelets are responsible for blood clotting (coagulation).
- They change fibrinogen into fibrin.
- This fibrin creates a mesh onto which red blood cells collect and clot, which then stops more blood from leaving the body and also helps to prevent bacteria from entering the body.
- About 55% of whole blood is blood plasma,
- a fluid that is the blood’s liquid medium, which
- by itself is straw-yellow in color.
- The blood plasma volume totals of 2.7-3.0 litres in an average human.
- It is essentially an aqueous solution containing 92% water, 8% blood plasma proteins (albumins, globulins, fibrinogen), gases (oxygen, carbon dioxide, nitrogen) and trace amounts of other materials.
- Plasma circulates dissolved nutrients, such as, glucose, amino acids and plasma lipids (fatty acids, cholesterol, phospholipids — dissolved in the blood or bound to plasma proteins),
- Removes waste products, such as, carbon dioxide, urea and lactic acid.
Functions of plasma proteins
- Maintain Colloidal osmotic pressure
- Acts as transport medium for fat, fatty acids, hormones, calcium, iron, etc.
- Y-globulins- act as antibodies
- Fibrinogen + globulins- acute phase reactions
- Regulate the pH of blood (acting as buffer)
- Acts as reserve supply of amino acids in the blood
Electrolytes – important in maintaining plasma pH and osmotic pressure
- Sodium, Potassium, Calcium, Magnesium
- Chloride, Bicarbonate, Phosphate, Sulphate
Other important components include:
- Serum albumin
- Blood clotting factors (to facilitate coagulation)
- Immunoglobulins (antibodies)
- Various other proteins
- Various electrolytes (mainly sodium and chloride)
The term serum refers to plasma from which the clotting proteins have been removed.
- Hemoglobin determines the color of blood in vertebrates.
- Each molecule has four heme groups, and their interaction with various molecules alters the exact color.
- In vertebrates and other hemoglobin-using creatures, arterial blood and capillary blood are bright red as oxygen impacts a strong red color to the heme group.
- Deoxygenated blood is a darker shade of red; this is present in veins, and can be seen when blood is taken from the vein (blood donation; venous blood samples)
General medical disorders
Disorders of volume
- Injury can cause blood loss through bleeding.
- A healthy adult can lose almost 20% of blood volume (1L) before the first symptom, restlessness, begins,
- and 40% of volume (2L) before shock sets in.
- Thrombocytes are important for blood coagulation and the formation of blood clots which can stop bleeding.
- Trauma to the internal organs or bones can cause internal bleeding, which can sometimes be severe.
Disorders of circulation
- Shock is the ineffective perfusion of tissues, and can be caused by a variety of conditions including blood loss, infection, and poor cardiac output.
- Atherosclerosis reduces the flow of blood through arteries, because atheroma lines arteries and narrows them. Atheroma tends to increase with age, and its progression can be compounded by many causes including smoking, high blood pressure, excess circulating lipids (hyperlipidemia), and diabetes mellitus.
- Coagulation can form a thrombosis which can obstruct vessels.
- Problems with blood composition, the pumping action of the heart, or narrowing of blood vessels can have many consequences including hypoxia (lack of oxygen) of the tissues supplied.
- The term ischaemia refers to tissue which is inadequately perfused with blood, and
- infarction refers to tissue death (necrosis) which can occur when the blood supply has been blocked (or is very inadequate).
- Insufficient red cell mass
- (anemia) can be the result of bleeding, blood diseases like thalassemia, or nutritional deficiencies; and
- may require blood transfusion.
- Several countries have blood banks to fill the demand for transfusable blood.
- A person receiving a blood transfusion must have a blood type compatible with that of the donor.
Types of anaemia:
– Aplastic – caused by toxic chemicals; result in damaged bone marrow
– Hemolytic – caused by toxic chemicals/radiation; result in destroyed blood cells
– Iron deficiency – caused by dietary lack of iron; result in haemoglobin deficient
– Sickle cell – caused by defective gene; result in red blood cells abnormally shaped
– Thalassemia – caused by defective gene; result in red blood cells short-lived/haemoglobin deficient
– Pernicious anemia – caused by inability to absorb vitamin B12; result excess of large, fragile cells –
N.B – Vitamin B12 is absorbed from the small intestine- needs intrinsic factor produced in the stomach- and stored in the liver. Deficiency of intrinsic factor causes pernicious anaemia – can cause permanent brain damage
Disorders of coagulation
Hemophilia is a genetic illness that causes dysfunction in one of the blood’s clotting mechanisms. This can allow otherwise minor wounds to be life-threatening, but more commonly results in hemarthrosis, or bleeding into joint spaces, which can be crippling.
Ineffective or insufficient platelets can also result in coagulopathy (bleeding disorders).
Thrombophilia (hyper-coagulable state) results from defects in regulation of platelet or clotting factor function, and can cause thrombosis.
Blood is an important vector of infection. Owing to blood-borne infections, bloodstained objects are treated as a biohazard.
– HIV, the virus which causes AIDS, is transmitted through contact between blood, semen, or the bodily secretions of an infected person.
– Hepatitis B and C are transmitted primarily through blood contact.
– Bacterial infection of the blood is bacteremia or sepsis.
– Viral Infection is viremia.
– Malaria and trypanosomiasis are blood-borne parasitic infections.
Carbon monoxide poisoning
– Substances other than oxygen can bind to hemoglobin, and can cause irreversible damage to the body.
– Carbon monoxide is extremely dangerous when carried to the blood via the lungs by inhalation,
– Carbon monoxide irreversibly binds to hemoglobin to form carboxyhemoglobin,
– less hemoglobin is free to bind oxygen, and less oxygen can be transported in the blood.
– Can cause suffocation insidiously (creeps up).
– A fire burning in an enclosed room with poor ventilation presents a very dangerous hazard since it can create a build-up of carbon monoxide in the air.
– Some carbon monoxide binds to hemoglobin when smoking tobacco.
– Blood for transfusion is obtained from human donors by blood donation and stored in a blood bank.
– There are many different blood types in humans, the ABO blood group system, and the Rhesus blood group system being the most important.
– Transfusion of blood of an incompatible blood group may cause severe, often fatal, complications, so crossmatching is done to ensure that a compatible blood product is transfused.
– Other blood products administered intravenously are platelets, blood plasma, cryoprecipitate and specific coagulation factor concentrates.
– Many forms of medication (from antibiotics to chemotherapy) are administered intravenously, as they are not readily or adequately absorbed by the digestive tract.
– After severe acute blood loss, liquid preparations, generically known as plasma expanders, can be given intravenously, either solutions of salts (NaCl, KCl, CaCl2 etc…) at physiological concentrations, or colloidal solutions, such as dextrans, human serum albumin, or fresh frozen plasma.
– In these emergency situations, a plasma expander is a more effective life-saving procedure than a blood transfusion, because the metabolism of transfused red blood cells does not restart immediately after a transfusion.
- In modern evidence-based medicine bloodletting is used in management of a few rare diseases, including haemochromatosis and polycythemia.
- However, bloodletting and leeching were common unvalidated interventions used until the 19th century, as many diseases were incorrectly thought to be due to an excess of blood, according to Hippocratic medicine.