Blood Composition and Functions

Blood I

• Blood is the life-sustaining transport vehicle of the cardiovascular system
• Blood is the only fluid tissue in the body and a type of connective tissue

Learning Objectives

• Define the meaning of plasma, formed elements, hematocrit, and hematopoiesis
• List the functions of blood and its normal pH
• Know the composition of blood as well as the protein in highest abundance in plasma
• List the formed elements of blood
• Describe the 3 structural features that make RBCs efficient at gas transport
• State what hemoglobin does and how many O₂ it can bind
• State how long it takes an RBC to form and how long they live

Functions of Blood

• These include transport, regulation, and protection
• Transport delivers O2 and nutrients to body cells, transports metabolic wastes to the lungs as well as kidneys for elimination, and transports hormones from endocrine organs to target organs.
• Regulation maintains body temperature by absorbing and distributing heat, maintains normal pH using buffers, uses alkaline reserve of bicarbonate ions, and maintains adequate fluid volume in circulatory system
• Protection prevents blood loss, plasma proteins, and platelets in blood which initiate clot formation. Also prevents infection- utilizes agents of immunity carried in blood (antibodies, complement proteins, and white blood cells)

Composition of Blood

• Blood is composed of plasma, which is non-living fluid matrix, and formed elements, which are living blood cells suspended in plasma
• Spun tube of blood yields 3 layers: Plasma on top (~55%), Buffy coat in middle (< 1%) which contains WBCs and platelets, and Hematocrit on bottom (45%) made up of RBCs
• Blood accounts for ~8% of body weight with males having 5-6 L and females having 4-5 L
• Color varies with O₂ content: High O₂ levels show a scarlet red and Low O₂ levels show a dark red.
• pH is between 7.35-7.45

Blood Plasma

• Plasma is a straw-colored sticky fluid that is about 90% water
• Plasma has over 100 dissolved solutes, including nutrients, gases, hormones, wastes, proteins, and inorganic ions
• Plasma proteins are the most abundant solutes and stay in the blood, not taken up by cells. They are mostly produced by the liver.
• Albumin makes up 60% of plasma proteins, functions as carrier of other molecules, as blood buffer, and contributes to plasma osmotic pressure.

Formed Elements

• These include RBCs, WBCs, and platelets
• Only WBCs are complete cells and RBCs have no nuclei or organelles
• Platelets are cell fragments and most formed elements survive in bloodstream for only a few days with most blood cells originating in bone marrow

Erythrocytes (RBCs)

• These are small-diameter cells that contribute to gas transport and are filled with hemoglobin (Hb)
• They have a biconcave disc shape and no nucleus or organelles
• RBC diameters are larger than some capillaries
• Membrane protein spectrin provides flexibility to change shape

Erythrocytes Adaptations

• Structure and function demonstrate a complementary relationship
• Provide gas transport
• Have a biconcave shape which provides Large SA:V for gas exchange.
• Hemoglobin makes up 97% of cell volume without water.
• They have no mitochondria with ATP production being anaerobic to avoid consuming the transported O₂.

Erythrocyte Function

• Erythrocytes dedicated to gas transport through hemoglobin
• Hemoglobin reversibly bind with oxygen
• Hemoglobin is made of red heme pigment bound to the protein globin
• Globin composed of four polypeptide chains, with two alpha and two beta chains
• A heme pigment is bonded to each globin chain, giving blood red color
• Each heme's central iron atom binds one O₂
• Each Hb molecule can transport 4 O2 so each RBC contains 250 million Hb molecules
• In the lungs O₂ loading produces oxyhemoglobin (ruby red) and, in the tissues O₂ unloading produces deoxyhemoglobin (dark red)
• 20% of CO2 in blood binds to Hb, producing carbaminohemoglobin

Production of Formed Elements

• Hematopoiesis is the formation of all blood cells
• Occurs in red bone marrow in axial skeleton, girdles, and proximal epiphyses of humerus and femur
• Hematopoietic stem cells (hemocytoblasts) give rise to all formed elements with hormones and growth factors pushing cell toward specific pathway of blood cell development
• Committed cells cannot change

Erythrocyte Formation

• Erythropoiesis is the formation of RBCs that takes about 15 days
• RBCs have a lifespan of 100-120 days and, because they have no nucleus, they cannot make new proteins or grow/divide
• Old RBCs become fragile and Hb degrades, and this can lead to trapping in smaller circulatory channels, especially in spleen
• Macrophages in spleen engulf and breakdown dying RBCs

Erythrocyte Destruction

• During RBC breakdown, heme, iron, and globin are separated
• Iron binds to ferritin or hemosiderin and is stored for reuse
• Heme is degraded to yellow pigment bilirubin
• The liver secretes bilirubin in bile and releases it into intestines where it is degraded to pigment urobilinogen
• Urobilinogen is transformed into brown pigment stercobilin that leaves body in feces
• Globin is metabolized into amino acids