Physiology of Blood Plasma and Red Blood Cells(Page 1 of study guide)

Basic Composition: Blood is composed of cells (red and white blood cells) and a liquid called plasma.
Plasma Composition:
- Buffy Coat: Contains white blood cells and platelets.
- Formed Elements: Includes red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).
Blood Flow Pathway:
- Artery → Arteriole → Precapillary Sphincters → Capillary → Venule → Vein.

pH Maintenance: Helps maintain blood pH and affects viscosity.
Colloidal Osmotic Pressure: Plasma proteins contribute to the maintenance of colloidal osmotic pressure, especially due to laminar proteins.
- Viscosity: Influenced by the concentration of proteins.

Mechanisms:
- Filtration: The process by which blood components move from plasma into the interstitial space at the arteriole end due to blood pressure.
- Reabsorption: Movement of fluid back into the plasma from the interstitial fluid at the venule end.
Factors Influencing Exchange:
1. Blood Pressure: Drives fluid out of blood vessels into the interstitial space.
2. Colloidal Pressure: Creates a concentration gradient that pulls fluid back into the blood.
Constant Dynamics: Typically, filtration exceeds reabsorption, necessitating lymphatic capillaries to balance fluid loss.

Role: Lymphatic capillaries help recover fluid lost from capillaries to prevent accumulation in the extracellular space.
Mechanism: Lymphatic capillaries have valves that open to allow fluid influx; they rely on muscle contractions to facilitate flow, as there is no central pump.

Regular Dynamics: Blood pressure is high at the arteriole side of capillaries, promoting movement (filtration) into the interstitial space. As blood progresses, pressure drops and reabsorption occurs at the venule end due to lower blood pressure.
Pressure Relationships: Blood pressure and colloidal pressure balance results in no net fluid movement when they equalize.
Abnormal Conditions:
- Hypertension: Leads to excess filtration causing fluid accumulation (edema).
- Low Blood Pressure: Results in less filtration and more reabsorption, potentially causing dehydration.

Function: Main role is the diffusion of oxygen ($O2$) and carbon dioxide ($CO2$). Their biconcave shape maximizes surface area-to-volume ratio, ideal for gas exchange.
Composition: About 40-45% of blood volume, contains hemoglobin, lacks organelles and nucleus, can’t divide but are metabolically alive.
Production and Lifespan:
- Location: Produced in the red bone marrow due to the presence of stem cells.
- Lifespan: Old RBCs are broken down in the spleen; damaged or senescent cells rupture when squeezed through narrow capillaries.

Pathways:
- Hemoglobin decomposes into heme and globin; globin is broken down into amino acids, while iron from heme is recycled back to the spleen via transferrin.
- Heme converts to bilirubin, excreted through the liver and bile.

Erythropoietin Production:
- Stimulated by low $O_2$ levels; kidneys produce erythropoietin (EPO) which promotes RBC production in the bone marrow.
- High Altitude: Decreased $O_2$ availability can stimulate increased RBC production.
Clinical Conditions:
- Anemia: Caused by low iron, reduced RBC count, or dysfunction in hemoglobin.
- Polycythemia: Characterized by increased RBC count.
- Jaundice: Caused by excess bilirubin due to rapid hemolysis.
- Sickle Cell Anemia: A single amino acid change in hemoglobin causes deformity in RBCs, impacting their passage through capillaries.
- Treatment may involve drugs like hydroxyurea (droxia) that promote fetal hemoglobin synthesis, which does not have the problematic beta chain.

Overall Process Control: Baroreceptors and feedback mechanisms regulate blood pressure, fluid balance, and red blood cell production.