University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2019
Views:

359

Pages:

14
Author:

arborezmv

Erythrocytes (RBCs) • Biconcave Disc-shaped cells – 7.5 M diameter, 2.0 m thick at rim – Flexible center allows folding so cells can pass through narrow capillaries – More surface area than a simple disk facilitates diffusion rate of gasses in and out of the cell • Major function of RBCs is gas transport – Loses nucleus and most organelles during maturation resulting in increased surface area/volume ratio – 33% of cytoplasm is hemoglobin (Hb) – O2 delivery to tissues and CO2 transport back to lungs – contains the enzyme, carbonic anhydrase (CA) that produces carbonic acid from CO2 and water which plays an important role in gas transport and pH balance • RBC count normal values – men 4.6-6.2 million RBCs/L – women 4.2-5.4 million RBCs/L Erythrocytes on the tip of a Needle Capillaries are formed by cells called Endothelial Cells Flexible center of erythrocytes allows folding so cells can pass through narrow capillaries. Erythrocyte Production • Erythropoiesis from stem cells in bone marrow produces 2.5 million RBCs/second. • In response to erythropoietin (EPO), some stem cells develop into proerythroblasts which multiply and develop into erythroblasts. • Erythroblasts multiply, synthesize hemoglobin and develop into normoblasts. • Normoblasts synthesize more hemoglobin, discard their nucleus and develop into reticulocytes. • Reticulocytes have a reticulated pattern of nuclear debris and a welldeveloped network of endoplasmic reticulum that was used for hemoglobin synthesis. – a few reticulocytes enter the bloodstream before they fully mature into erythrocytes and can be 0.5% to 1.5% of the circulating RBCs • Development from a stem cell to a RBC takes 3-5 days and involves: – reduction in cell size, increase in cell number, synthesis of hemoglobin, loss of nucleus – blood loss speeds up the process increasing reticulocyte count in circulating blood which is called a “shift to the left” Erythrocyte Homeostasis • Negative feedback control – – – – kidney and liver hypoxia  EPO production bone marrow stimulation  RBC count in 3-4 days • Causes of Hypoxia – low levels of atmospheric O2 • high elevation or altitude – increase in exercise • Endurance-trained athletes have RBC counts as high as 6.5 million/ l (normal range is 4-6 million) – inhaling carbon monoxide – hemorrhage Nutritional Needs for Erythropoiesis • Iron is key nutrient for erythropoiesis – Iron is lost daily through urine, feces, and bleeding – on average, men lose 0.9 mg/day and women lose 1.7 mg/day – low absorption rate requires consumption of 5-20 mg/day – dietary iron comes in 2 forms: ferric (Fe+3) and ferrous (Fe+2) • stomach acid converts Fe+3 to absorbable Fe+2 • gastroferritin from stomach binds Fe+2 and transports it to the intestines • absorbed into blood & binds to transferrin to travel to bone marrow and liver • liver binds surplus iron to apoferritin to create ferritin for storage • erythrocytes use iron to make hemoglobin • muscle uses iron to make myoglobin which stores O2 • all cells use iron to make cytochromes in mitochondria • Other nutritional requirements include B12, Folic Acid, Vitamin C and Copper (Cu is used as a cofactor for enzymes) Iron Absorption, Transport & Storage Hemoglobin • Normal, adult hemoglobin (HbA) consists of 4 protein chains called globins (2 alpha chains and 2 beta chains). • Each globin has a heme group which binds oxygen to ferrous ion (Fe+2). • Each Hb molecule can carry four O2 molecules. • Fetal hemoglobin (HbF) has gamma instead of beta chains – HbF has a higher affinity for O2 than HbA enabling the fetus to extract O2 from the mother’s bloodstream Erythrocytes and Hemoglobin • RBC count and hemoglobin concentration determine the amount of oxygen the blood can carry. • Hemoglobin concentration of whole blood: – men 13-18g/dL – women 12-16g/dL (dL = deciliter = 100ml) • Why are values are lower in women? – androgens that stimulate RBC production are lower in women – women may have periodic menstrual loss of blood Erythrocyte Disorders • Polycythemia is an excess of RBCs – primary polycythemia is due to cancer of erythropoietic cell line in the red bone marrow • RBC count as high as 11 million/L (normal range is 4-6 million /L) • hematocrit of 80% – secondary polycythemia can result from dehydration, high altitude, or physical conditioning • RBC count of 8 million/L • Dangers of polycythemia – increased blood viscosity can lead to embolism, stroke or heart failure Anemia = Deficiency of RBCs or Hb • Types of anemia – iron-deficiency anemia – hemorrhagic anemias from loss of blood – hemolytic anemias from RBC destruction • Effects of anemia – tissue hypoxia and resulting necrosis – low blood osmolarity and resulting tissue edema – low blood viscosity and resulting increased heart rate Sickle-Cell Disease Sickle-Cell Disease • Sickle-Cell is a hereditary defect in Hemoglobin (HbS) – Recessive allele modifies hemoglobin structure • Caused by a single amino acid substitution (valine is inserted in the protein where a glutamic acid should be) • homozygous for HbS have sickle-cell disease • heterozygous for HbS have sickle-cell trait but rarely have symptoms • Most common in African Americans and people from Mediterranean region – Implications of sickle-cell disease: • HbS turns to gel (polymerizes) at low oxygen concentrations causing cell elongation and inflexible sickle shape that is prone to lysis • Sickle cells are sticky and agglutinate, blocking vessels, causing intense pain, kidney and heart failure, paralysis, and stroke • chronic hypoxia activates hematopoietic tissue producing more deformed cells – HbS gene persists despite its harmful effects to the homozygous individual because it protects from malaria • HbS indigestible by malaria parasites