Anatomy of Blood

Brief anatomy of the blood

• The body is a machine composed of interdependent systems, each with a specific function.
• Integrated function of these systems is essential for survival.

Anatomy, physiology, pathology

• Anatomy studies the structure of the body and the relationships between its systems.
• Physiology studies how these systems function and maintain life.
• Pathology studies abnormalities affecting body functions that can cause illness.

The Blood

• Blood is a fluid connective tissue that circulates, facilitating communication between distant tissues.
• Transports oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs.
• Carries nutrients from the alimentary tract to tissues and waste from cells to excretory organs such as kidneys.
• Transports hormones from endocrine glands to target tissues.
• Transports heat from active to less active tissues and antibodies to areas of infection.
• Carries clotting factors to areas that require coagulation of blood; minimizing bleeding from damaged blood vessels.
• Blood consists of plasma, a clear straw-colored fluid, and suspended blood cells.
• Plasma accounts for 55% of blood volume, with cells making up the remaining 45%.
• Components are separated by centrifugation or gravity, with cells sinking below plasma.
• Blood accounts for approximately 7% of body weight or ~5.6 liters in a 70 kg human.
• Volume is less in women and more in children, gradually decreasing until adulthood.
• Blood circulates in motion due to the heart pumping, keeping a steady environment for body cells.
• Homeostatic mechanisms maintain its volume and composition within narrow limits.
• In the plasma: includes water (90-92%), plasma proteins, inorganic salts, nutrients, waste, hormones, and gases.

Plasma Proteins

• Plasma proteins constitute about 7% of plasma, which are retained in the blood because of their size.
• They help create osmotic pressure, which keeps fluid circulating.
• Reduced plasma protein levels can lead to reduced osmotic pressure and, in turn, edema and body cavities.
• Plasma thickness is due to plasma proteins, most notably albumin and fibrinogen.
• Most plasma proteins are created in the liver, with exceptions of immunoglobulins.

Albumins

• Albumins are the most abundant plasma proteins at 60% of the total.
• Play a key role in sustaining plasma osmotic levels.
• Act as carrier molecules for free fatty acids, certain medicines and steroid hormones.

Globulins

• Globulins act as antibodies (immunoglobulins) that lymphocytes create for immunity.
• Functions include binding to and neutralizing substances and transportation of some hormones/minerals.
• Thyroglobulin transports the hormone thyroxine, and transferrin carries iron.
• Alpha-2 macroglobulin halts activity of trypsin.

Clotting factors

• Clotting factors facilitate blood coagulation, with fibrinogen as the most abundant.

Electrolytes, Nutrients, Waste Products and Gases in the blood

• Electrolytes play multiple roles, including muscle contraction (e.g. Ca2+), nerve transmission (e.g. Ca2+ and Na+), and acid-base balance (e.g. phosphate).
• Blood pH is maintained between 7.35-7.45 by buffering systems.
• Nutrients like glucose, amino acids, fatty acids, and glycerol are ingested from the alimentary tract, and used for energy, repair, and production activities.
• Waste products, including urea, creatinine, and uric acid, result from protein metabolism.
• These products are carried through the blood the liver, and then to the kidneys for excretion.
• Hormones synthesized by endocrine glands are transported in the blood to target tissues and organs to influence activity.
• Oxygen, carbon dioxide, and nitrogen are transported in the body while dissolved in plasma, and with hemoglobin in red blood cells.
• Most oxygen combines with hemoglobin, and most carbon dioxide exists as bicarbonate ions in plasma.
• Atmospheric nitrogen enters the body but has no known physiological function.

Cellular Content of Blood

• There are three types of blood cells: erythrocytes (red), thrombocytes (platelets), and leukocytes (white).
• Blood cells are synthesized mainly in bone marrow, while lymphocytes originate from pluripotent stem cells.
• Development follows different paths and the process of their formation is haemopoiesis.
• For the first years of life, red marrow occupies the entire bone capacity, and over the next 20 years it is replaced by yellow marrow.
• Haemopoiesis in the adult skeleton is confined to flat and irregular bones, and the ends of long bones, in sites such as the sternum, ribs, pelvis, and skull.

Erythrocytes (Red Blood Cells)

• Red blood cells are shaped as biconcave discs lacking a nucleus, and they measure approximately 7 micrometers in diameter.
• Primarily involved in gas transport, i.e. oxygen, and carbon dioxide.
• Biconcave shape increases surface area for gas exchange, thinness allows for quick movement of gases.
• Flexible cells lack intracellular organelles to accommodate hemoglobin needed for gas transport.

Life Span and Function of Erythrocytes

• Erythrocytes are produced in red bone marrow.
• They pass through stages prior to entering the blood, lasting about 120 days.
• Erythropoiesis, red blood cell development from stem cells, takes ~7 days.
• Immature cells, reticulocytes, mature into erythrocytes in a day/two.
• During maturation, they lose their nucleus and are therefore rendered incapable of division.
• Both vitamin B12 and folic acid are required for red blood cell synthesis.
• Vitamin B12 must bind to intrinsic factor for absorption, and both vitamins are present in dairy, meat and green vegetables.
• Signs of folic acid deficiency arise faster than vitamin B12 deficiencies.

Haemoglobin

• Haemoglobin has globin (a globular protein) and haem (iron).
• Each molecule has four globin chains and haem units, where each unit contains iron.
• One haemoglobin molecule carries up to four molecules of oxygen.
• Each red blood cell carries about 280 million haemoglobin molecules.
• Iron is transported by transferrin and deposited in the liver.

Oxygen transport

• Haemoglobin is saturated when all its sites bind to oxygen.
• Oxygen reversibly binds to haemoglobin and forms oxyhaemoglobin: its color depends on oxygen content.
• The bond, that is formed is not permanent so that it releases when specific conditions are met

Factors affecting Release of Oxygen

• Metabolically active tissues release acid waste products, which lowers local pH.
• This causes oxyhaemoglobin to release additional oxygen for tissue use.
• The more oxygen demand rises, the more oxygen is released to match.
• Oxyhaemoglobin releases oxygen in tissues with low oxygen.
• High oxygen levels, such as in the lungs, favor oxyhaemoglobin formation.
• Warmer temperatures increase oxygen dissociation in tissues with higher oxygen needs.
• Lower temperatures in the lungs promote oxyhaemoglobin formation.

Control of Erythropoiesis

• Red cell numbers remain constant.
• Bone marrow adapts production and destruction due to homeostasis via negative feedback.
• Erythropoietin (kidney) regulates red blood cell production.
• Hypoxia stimulates erythropoiesis, specifically; deficient oxygen supply to body cells.
• Hypoxia increases erythropoietin production; stimulating proerythroblast production and reticulocyte release while speeding up reticulocyte maturation.
• Normal lifespan of erythrocytes is about 120 days with breakdown carried out by phagocytic cells, and major sites of harmolysis occurring in the spleen, bone marrow, and liver.
• Break down of fragile erythrocytes release Iron that is retained and reused in the bone marrow, which forms biliverdin from part of the haemoglobin to be reduced to bilirubin and transported to the liver.

Blood Groups

• Individual blood groups are determined by inherited antigens on red blood cells.
• Individuals create antibodies against antigens, except their own, to prevent reactions.
• Antibodies circulate in the bloodstream, and the ability to produce them is genetically determined.
• Transfusion reactions occur when incompatible blood types with differing antigens mix.
• ABO and Rhesus systems are essential for red cell surface antigens.

ABO System

• Approximately 55% of the population has A-type, B-type, or both A and B antigens, making them blood type A, B, or AB, respectively.
• About 45% have neither A nor B antigens, making them blood type O.
• The complement antibodies that correspond are called anti-A and anti-B.
• Blood group A individuals cannot create anti-A, but can create anti-B.
• Similarly, group B produces anti-A only; AB produces neither; and O produces both.
• Type AB: Universal recipients can likely receive any blood.
• Type O: Universal donors can safely donate to any ABO type.

Rhesus System

• The Rhesus (Rh) antigen, or Rhesus factor determine the blood group in the Rhesus system.
• 85% of the population has this antigen (Rh+) and don't make anti-Rhesus antibodies.
• 15% don't have this antigen (Rh-) and can make anti-Rhesus antibodies, i.e. pregnancy.

Leukocytes (White Blood Cells)

• Leukocytes play a vital role in defense and immunity despite accounting for only about 1% of the blood volume.
• Leukocytes are larger than red blood cells but less abundant.
• All leukocytes contain nuclei and have granules in the cytoplasm.
• Two main types: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (monocytes and lymphocytes)
• Normal leukocyte counts in adult blood are specified in a table.
• Rising numbers typically signify physiological problems like infection, trauma, etc.

Granulocytes

• During formation, granulocytes share development before differentiating, and all have multi lobed nuclei in cytoplasm.
• They are named based on how they respond to lab stains: Eosinophils take up red acid dyes, basophils take up alkaline dyes, and neutrophils take purple dyes.

Neutrophils

• Neutrophils serve as scavengers that fight bacterial invasions protecting the body.
• Chemotaxins attract them to sites with infection.
• They quickly rise in infected/damaged areas to perform phagocytosis and die.
• Pus forms from dead tissue cells, microbes, and phagocytes killed by microbes.

Eosinophils

• Eosinophils are less active in phagocytosis than neutrophils, but their role is in the elimination of parasites/worms.
• They promote tissue inflammation to eliminate parasites, and dampen inflammation by breaking down histamine.
• They are found at sites of allergic inflammation.

Basophils

• Basophils, primarily located in allergic reactions, are comprised of cytoplasmic granules that also consist of heparin, histamine and that further promote inflammation.

Haemostasis

• When blood vessels get damaged, blood loss stops through platelets, in processes where is healing occurs.
• Severity of vessel damage affects how fast coagulation begins.

Vasoconstriction

• Exposed platelets adhere to exposed walls of damaged blood vessels; the surfaces of these vessels are now sticky.
• Platelets release serotonin constricting reducing blood flow, as well as vasoconstrictors after damage.

Platelet plug formation

• Adherent platelets release substances like adenosine diphosphate (ADP), attracting platelets more to the site.
• Additional platelets join forming a temporary seal (platelet plug); normally completed by 6 minutes after injury.

Coagulation - Blood Clotting

• Clotting factors activate to form prothrombin, which activates thrombin to convert fibrinogen to insoluble fibrin.
• Platelet plug stabilizes by fibrin meshwork, trapping blood cells and is way stronger than platelet components.
• The final common pathway can be induced by two processes: extrinsic and intrinsic pathways with their respective activators
• Damaged tissue released thromboplastin or tissue factor.
• Clot shrinks (retracts) through platelets, releasing serum.
• Clot shrinkage pulls the damaged vessel together.

Fibrinolysis

• Fibrinolysis is the removing of the clot.
• Plasminogen gets converted to plasmin with activators from endothelial cells.
• Plasmin breaks down the fibrin clot to products, which are removed by phagocytosis, and the blood vessel restores integrity.

Control of coagulation

• Thrombin is a simulator of its production.
• Body possessed mechanisms involving smoothness, the binding of thrombin to bind with receptors located on the blood vessels; the presence of anticoagulants (heparin)

Overview of blood anatomy, physiology, and pathology. Blood is a fluid connective tissue that circulates, facilitating communication between distant tissues. It plays roles like transporting oxygen, carbon dioxide, nutrients, waste, hormones, heat, and antibodies.