Lecture 1- Blood Plasma (PINK)

Composition:

  • About 92% water, which acts as a solvent for many substances.

  • Contains various ions and small molecules such as sodium, chloride, urea, oxygen (O2), and carbon dioxide (CO2), as well as hormones like insulin and cortisol that play crucial roles in metabolism and homeostasis.

Functions:

  • Holds heat effectively and aids in cooling the body through sweating, a thermoregulatory process that emits heat and moisture.

  • Nutrient transfer mechanism providing essential nutrients such as glucose for cellular respiration and vitamins critical for various bodily functions.

  • Plays a pivotal role in the elimination of waste products like urea and bilirubin, the latter contributing to the yellow coloration of plasma and bile, which is important for digestion.

Plasma proteins (7% of plasma weight):

  • Albumins: Comprising about 5% of plasma volume; they are critical for maintaining osmotic pressure and thus fluid balance; they also serve as carriers for fatty acids, bilirubin, and various drugs.

  • Globulins: Making up about 2% of plasma volume; they consist of antibodies (immunoglobulins) that are essential for immune response and proteins that transport hormones and other substances in the blood.

  • Fibrinogens: The largest plasma protein, involved in blood clotting by converting to fibrin; this process is essential for wound healing and maintaining hemostasis.

Roles of Plasma Proteins:

  • Maintain blood pH and viscosity, crucial for the proper functioning of the circulatory system.

  • Bind to poorly soluble substances such as steroid hormones and cholesterol, facilitating their transportation in the bloodstream.

  • Participate in defense against pathogens via immune responses.

  • Responsible for colloidal osmotic pressure, crucial for fluid recirculation between blood and tissues, preventing excessive fluid accumulation in the tissues.

Capillary Exchange

Mechanisms:

  1. Diffusion: A primary mechanism occurring between plasma and interstitial fluid, governed by concentration gradients. The rate of diffusion is contingent upon the size and permeability of the capillary membrane; for example, gases like oxygen and carbon dioxide can easily traverse, while larger proteins cannot.

  2. Bulk Flow: Refers to the movement of water and solutes between capillaries and interstitial fluid, consisting of:

    • Filtration: Movement of fluid from capillaries into the interstitial fluid, influenced by blood pressure.

    • Reabsorption: The process by which fluid moves back from interstitial fluid into plasma, mainly driven by osmotic pressure created by plasma proteins.

Factors Influencing Bulk Flow:

  • Hydraulic (blood) pressure: The force exerted by the blood flow pushes fluid out of capillaries

  • Colloidal osmotic pressure: This pull originates from the presence of blood proteins such as albumin, drawing fluid back into the capillaries from interstitial spaces.

Exchange along Capillary Length:

  • At the arteriole end, blood pressure typically exceeds osmotic pressure, resulting in net ultrafiltration where fluid leaves the capillary.

  • Conversely, at the venule end, osmotic pressure is greater than blood pressure, leading to net reabsorption of fluid back into the capillaries, establishing a crucial balance necessary for homeostasis.

  • The average net bulk flow of approximately 3 L/day into the lymphatic system serves to maintain fluid balance and immune function.

Lymphatic System

  • Lymphatic capillaries feature larger pores allowing interstitial fluid entry without a pump; instead, they rely on smooth muscle contraction and unidirectional valves to propel lymph fluid toward larger lymphatic vessels and ultimately the thoracic duct, where it drains into the venous circulation.

  • Blocked lymph vessels, often due to infections or parasitic infestations, can result in conditions like elephantiasis, characterized by extreme swelling in affected regions due to fluid accumulation.

Edema

  • Edema refers to the excessive accumulation of interstitial fluid, which occurs when fluid balance is disrupted due to several factors:

    • High blood pressure: Increases filtration beyond the absorption capacity of capillaries.

    • Decreased blood protein levels (hypoproteinemia): Often due to malnutrition, liver disease, or kidney failure, leading to insufficient plasma oncotic pressure to draw fluid back into the capillaries.

    • Increased interstitial proteins due to injury or inflammation (e.g. histamine release), resulting in increased vascular permeability and fluid leakage into surrounding tissues.

    • Nasal Decongestant: OxyMetazoline (Afrin, Synex) works by constraining blood vessels and reducing fluid filtration; however, an overdose can lead to down-regulation of alpha receptors, causing rebound congestion.

Blood Cells

  • Types:

    1. Leukocytes: These cells are integral to the immune response, functioning to identify and fight off pathogens.

    2. Platelets: Small cell fragments crucial for hemostasis, contributing to blood clotting and wound healing.

    3. Erythrocytes (RBCs): Comprising approximately 99% of blood cells, they primarily facilitate oxygen transport via hemoglobin, a protein capable of binding and releasing oxygen molecules efficiently.

Erythrocytes (Red Blood Cells)

Structure:

  • Erythrocytes possess a unique biconcave disc shape that maximizes surface area for gas exchange; this design permits a rapid diffusion of oxygen and carbon dioxide across their membrane.

  • They lack a nucleus and other organelles to allow more room for hemoglobin, which binds oxygen for transportation. Furthermore, their lack of DNA ensures they cannot undergo division, necessitating a continuous supply from erythropoiesis.

  • Their average lifespan is about 120 days, after which they undergo senescence, leading to their breakdown, which produces bilirubin (yellow) and biliverdin (green), pigments indicative of hemoglobin degradation.

Erythrocyte Production (Erythropoiesis)

  • This process primarily occurs in red bone marrow from hematopoietic stem cells. The production is stimulated when low oxygen levels trigger the release of erythropoietin (EPO) from the kidneys, signaling the marrow to increase RBC production.

  • Factors affecting production include testosterone, which enhances EPO production, and higher altitudes, which have lower oxygen levels and stimulate erythropoiesis.

  • Iron transport is crucial for hemoglobin synthesis, sourced from diet, transported by transferrin in circulation, and subsequently stored as ferritin in the liver for later use.

Erythrocyte Disorders

  • Anemia: A condition characterized by reduced oxygen transport capacity due to low RBC counts, which can result from blood loss, iron deficiency, chronic diseases, and cancers or kidney failure.

  • Polycythemia: An elevated RBC count often seen as an adaptation to high altitude, where oxygen levels are limited, enabling increased red cell production to improve oxygen delivery to tissues.

  • Jaundice: Occurs due to elevated bilirubin levels from excessive breakdown of hemoglobin, leading to yellowing of the skin and eyes, and can indicate liver dysfunction or hemolytic anemia.

  • Sickle Cell Anemia: A genetic disorder caused by a mutation in the hemoglobin gene, leading to malformed (sickle-shaped) RBCs that can obstruct blood flow in capillaries, resulting in pain and possibly organ damage; management includes pain relief and treatments like hydroxyurea to promote production of fetal hemoglobin.

Conclusion

  • The interstitial space acts as a buffer for blood volume and plays a critical role in stabilizing blood pressure; baroreceptors located in major blood vessels are essential for detecting changes in blood pressure and activating reflexes to maintain homeostasis.