14.1: Introduction Blood: Is connective tissue Transports vital substances Maintains stability of interstitial fluid Distributes heat Blood cells: Form mostly in red bone marrow and are: Red blood cells (RBCs) White blood cells (WBCs) Platelets (cell fragments) The amount of blood varies with body size, changes in fluid concentration, changes in electrolyte concentration, and amount of adipose tissue Blood is about 8% of body weight Adult blood volume is about 5 liters

14.2: Blood Cells Blood cells originate in red marrow from hemocytoblasts or hematopoietic stem cells Stem cells can then: Give rise to more stem cells Specialize or differentiate

Characteristics of Red Blood Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Red blood cells are: Erythrocytes Biconcave discs One-third hemoglobin Able to readily squeeze through capillaries Lack nuclei and mitochondria Top view 7.5 micrometers 2.0 micrometers Sectional view (a) (b) b: © Bill Longcore/Photo Researchers, Inc.

Red Blood Cell Counts RBC counts is the number of RBCs in a cubic millimeter or microliter of blood It may vary depending on age and health Typical ranges include: 4,600,000 – 6,200,000 in males 4,200,000 – 5,400,000 in adult females 4,500,000 – 5,100,000 in children RBC counts reflects blood’s oxygen carrying capacity

Red Blood Cell Production and Its Control Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Low blood oxygen causes the kidneys and the liver to release erythropoietin (EPO) which stimulates RBC production This is a negative feedback mechanism Within a few days many new blood cells appear in the circulating blood Low blood oxygen Release into bloodstream Liver Kidney Stimulation Inhibition – Erythropoietin Increased oxygen- carrying capacity Bloodstream Increased number of red blood cells + Red bone marrow

Dietary Factors Affecting Red Blood Cell Production Vitamin B12 and folic acid are necessary They are required for DNA synthesis making them necessary for the growth and division of all cells Iron is also necessary It is required for hemoglobin synthesis