Function and Composition of Blood

Function and Composition of Blood (10.1)

Functions of Blood

• Transports oxygen, carbon dioxide, waste, hormones, enzymes, nutrients, blood cells, and plasma proteins.
• It regulates body temperature and pH.
• It protects against infections via white blood cells; antibodies detect foreign material, and clotting factors prevent excessive bleeding.

Formed Elements

• Plasma is the liquid component of blood, making up 55% of blood volume.
• Formed elements are the solid components, which constitute 45% of blood.
• Erythrocytes carry oxygen to tissues.
• Leukocytes protect the body from infections.
• Platelets aid in blood clotting.
• Spinning blood in a centrifuge separates it into three layers.
• Liquid plasma forms the top layer.
• The buffy coat is a thin layer between plasma and blood cells, containing white blood cells and platelets, comprising less than 1%.
• Red blood cells settle at the bottom.
• Separating blood reveals the hematocrit, which is the proportion of total blood volume composed of red blood cells, expressed as a percentage.
• Hematocrit averages 45% in men and 40% in women.
• Dehydration and living at high altitudes can increase hematocrit levels.

Properties of Blood

• Blood accounts for approximately 8% of total body weight.
• Both men and women typically have around 4-5 liters of blood, depending on body size, muscle mass, and physical fitness.
• The pH of blood ranges between 7.35 and 7.45.
• Blood is five times thicker than water; factors such as decreased blood temperature, prolonged exposure to high altitudes, and elevated hematocrit levels increase blood viscosity.
• Higher oxygen levels in the bloodstream result in brighter red blood.

Plasma

• Plasma is a pale yellow fluid consisting of 90% water and 8% plasma proteins.
• It contains fibrinogen, albumin, and globulin.
• It has various functions, including transporting lipids and fat-soluble vitamins, regulating blood pressure and volume, and assisting in blood clot formation

Blood components

• Albumin, comprising 58% of plasma, regulates osmotic pressure and blood volume and transports lipids, hormones, and other solutes.
• Globulin, at 38%, aids in blood clot formation and transports lipids and fat-soluble vitamins.
• Gamma globulins help fight infection.
• Fibrinogen (4%) aids in blood clot formation.
• Electrolytes maintain fluid and electrolyte balance.
• Buffers regulate blood pH.
• Bicarbonates are the main transporters of carbon dioxide in plasma.
• Plasma composition changes during gas exchange and substance exchange.

Manufacturing Blood Cells

• New blood cells are created through hematopoiesis in bone marrow.
• The bone marrow contains fat, blood, and stem cells.
  • Red bone marrow produces stem cells and blood cells.
  • Yellow bone marrow contains fat.
• Each formed element in blood starts as a hematopoietic stem cell, leads to a blast cell, and then develops into a red blood cell, white blood cell, or platelet.

Erythrocytes

• Erythrocytes conduct gas exchange, carrying oxygen to all cells and carrying carbon dioxide away.
• Mature red blood cells are disk-shaped.
• As red blood cells develop, the nucleus is forced out, causing the center of the cell to collapse.
  • This increases the surface area for oxygen and carbon dioxide binding.
  • It also increases the cell flexibility, allowing it to fit into small capillaries.
  • Nucleus removal limits the cell's lifespan because the cell cannot replicate.

Hemoglobin

• Hemoglobin is a molecule within red blood cells that binds to oxygen and carbon dioxide, composed of globin, iron, and heme.
• Oxygen plus Hemoglobin forms oxyhemoglobin, which travels to tissue capillaries, unloading oxygen into oxygen-deprived tissues.
• Carbon dioxide plus Hemoglobin, attaches to the globin part, and form carbaminohemoglobin
  • Carbaminohemoglobin transports carbon dioxide to the lungs.

Erythropoiesis

• Erythropoiesis is the process by which red blood cells are produced.
• Erythropoietin (EPO) is a hormone secreted by the kidneys that stimulates red blood cell production when oxygen levels are low.
  • EPO stimulates cell production of red blood cells in red bone marrow.
• When additional red blood cells are produced and blood oxygen rises, erythropoietin levels lower, slowing red blood cell production.
• Iron, folic acid, Vitamin B12, and protein are needed for red blood cell production.

Recycling Erythrocytes

• Red blood cells are flexible and can bend and twist to fit into capillaries.
• Constant movement damages the red blood cell membrane; lack of cycles prevents self-repair.
• Macrophages in the liver and spleen phagocytize and recycle old red blood cells.
• Hemolysis is the rupture of red blood cells

Blood cell breakdown

• The red blood cell membrane breaks down easily, except for hemoglobin molecules.
• Hemoglobin is separated into globin and heme for recycling.
  • Globin breaks down into amino acids for new proteins.
  • Heme breaks down into iron and bilirubin, stored in the liver until needed.

Leukocytes (White Blood Cells)

Classifying Leukocytes

• Red blood cells outnumber white blood cells, but white blood cells can leave the blood.
• Diapedesis occurs when white blood cells squeeze through spaces in capillary walls to move from the blood to sites of infection.
• White blood cells fight infections through phagocytosis or antibody production.

Neutrophils

• These are the most abundant type of white blood cells and the "first responders."
• They are active phagocytes that kill foreign invaders like bacteria, viruses, and fungi.

Eosinophils

• They participate in inflammatory processes, like allergic reactions, and are capable of phagocytosis.
• Eosinophils are active with parasites and worms.

Basophils

• These are the least abundant white blood cells.
• Basophils produce histamine, triggering an inflammatory response that summons more white blood cells to the site of injury.
• They also produce heparin, an anticoagulant that prevents blood clotting.

Lymphocytes

• There are two types: T cells and B cells.
• These produce antibodies to fight antigens and cancer cells.

Monocytes

• These are the largest white blood cells.
• They are produced in red bone marrow and then move to body tissues.
• They develop into macrophages that engulf microorganisms.
• Granulocytes include neutrophils, eosinophils, and basophils.
• Agranulocytes include lymphocytes and monocytes.

Thrombocytes (Platelets)

• Platelets are part of the formed elements of blood.
• They are small, irregularly shaped cell fragments.
• They lack a nucleus.
• Old platelets are removed from the body by phagocytosis in the spleen and liver.
• Platelets are derived from multinucleated megakaryocytes, which develop from hematopoietic stem cells.
• Thrombopoietin is the hormone that regulates platelet production from megakaryocytes.
• Platelets play a crucial role in homeostasis, the sequence of events that forms blood clots and stops bleeding.

Homeostasis

• Homeostasis involves four key steps:

Vessel Wall Injury + Constriction

• When the endothelium (inner lining of blood) is injured, it releases endothelin, causing the blood vessel to constrict and spasm.
• Construction aids in reducing blood loss.
• Platelets do not typically attach to the blood vessel wall because the wall is coated with a platelet-repellent, called prostacyclin.
• Collagen fibers along the vessel wall are exposed.

Platelet Aggregation

• Platelets stick to collagen fibers and rough edges of the blood vessel wall.
• Platelets repair the hole/tear, and release chemicals that maintain constriction of blood vessels, attracting more platelets

Platelet Plug Formation + Coagulation

• The gathering of platelets forms a platelet plug, a small, loose mass at the site of injury.
• Injured blood vessels release a chemical, tissue factor, to initiate coagulation.
• Coagulation involves the enzyme thrombin a protein fibrinogen = fibrin
• Clotting factors, thrombin, and fibrinogen create fibrin, a long, thread-like fiber.
• Fibrin strands wave in and around the platelet plug, creating a strong woven mesh

Blood Clot Formation + Retraction

• Red blood cells and white blood cells are trapped in the fibrin mesh, forming a red-colored blood clot.
• Once coagulation is complete, a blood clot has formed.
• Shortly after the fibrin mesh is in place, the blood clot begins to retract in size.
• As the blood clot retracts, platelets pull fibrin threads together, drawing the edges of the ruptured vessel together.
• Factors initiating the hemostasis sequence are rapidly inactivated, and coagulation inhibitors are released to prevent further clot formation.
• Eventually, other chemicals dissolve the fibrin strains, dissolving the blood clot completely.