Episode 33 - Cardiovascular system - Blood

- Welcome to episode 33 of Pulse of Physiology. I'm your host, Dr. Jim Davis, and today we're exploring blood, one of the most vital components of the cardiovascular system. Blood plays a central role in transporting oxygen, nutrients and waste products, while also contributing to immune defense, clotting and homeostasis. In our next episode, we'll discuss cardiac pacemaker cells and how they generate the electrical impulses that regulate heartbeat. But for now, let's take a closer look at the composition, function, and key characteristics of blood. Blood is a fluid, connective tissue made up of plasma-informed elements, which include red blood cells, white blood cells and platelets. A typical adult has about five liters of blood, which accounts for seven to 8% of total body weight. Now, plasma makes up about 55% of blood volume and contains mostly water, about 92%, along with proteins, electrolytes and nutrients. This means that the remaining 45% is made up of formed elements, which includes erythrocytes, which are known, which are red blood cells, leukocytes, which are white blood cells and platelets. Now, plasma serves as the transport medium for cells and dissolved substances. It contains three major plasma proteins, each with distinct functions. There are albumins, which are produced by the liver, and they help maintain blood, osmotic pressure, which prevents excessive fluid loss from blood vessels into the tissues. It also, or these albumins, also act as carriers for hormones and other molecules. Then there are globulins. There are alpha and beta globulins, which transport lipids, vitamins and metal ions. Then there are gamma globulins, which are antibodies and are produced by B lymphocytes and play a key role in the immune response. Finally, we have fibrinogen. Now, our precursor, fibrinogen is a precursor to fibrin, which forms the mesh work of blood clots to prevent excessive bleeding. Now, the formed elements include red blood sites or erythrocytes, white blood cells, or leukocytes and platelets. Each type of blood cell, or each of the types of formed elements, serve a specialized role in circulation and immune defense. So let's start by talking about erythrocytes. Now, erythrocytes or red blood cells are the most abundant formed element in the blood. Their key characteristics include having a biconcave shape, which allows for greater surface area for oxygen transport. They don't have a nucleus or organelles, which allows for more space to carry hemoglobin. And their primary function is to transport oxygen from the lungs to the tissues and carbon dioxide from tissues to the lungs. Red blood cells contain hemoglobin, which is a protein responsible for binding oxygen. Now, we have leukocytes or white blood cells. Now, these are part of the immune system that can help protect the body from infections and foreign invaders. They are categorized into granular and agranular leukocytes. Granular leukocytes are made up, or there are three types of granular leukocytes. So first, we have neutrophils. Neutrophils are the first responders to infections, and they attack bacteria and release enzymes, and they are the most common type of white blood cell. Then we have the eosinophils. These are involved in allergic reactions and parasitic infections. Finally, we have basophils, and there are not very many basophils, but they release histamine to trigger inflammatory responses. Then we have the agranular leukocytes. The most common type of agranular leukocytes are lymphocytes, which make up 20 to 30% of all white blood cells. These include B cells, which produce antibodies, and T cells, which destroy virus-infected cells. Finally, we have monocytes, and monocytes make up about one to 6% of white blood cells, and these differentiate into macrophages, which engulf and digest pathogens and debris. Finally, we have platelets. Platelets are cell fragments that are derived from cells called mega-karyocytes, which exist in the bone marrow. They play a crucial role in blood clotting by releasing clotting factors when blood vessels are damaged, by forming a platelet plug at injury sites, and by converting fibrinogen into fibrin, which creates a stable blood clot. We'll talk more about this later on in the semester when we get to hemostasis. Now hemoglobin is the oxygen-carrying protein found in red blood cells. Each hemoglobin molecule consists of four protein subunits, or globins. There are four heme groups, each containing an iron atom that binds oxygen molecules. Hemoglobin can also bind to carbon dioxide and transport it back to the lungs for exhalation. When red blood cells are broken down, hemoglobin is metabolized into globin, which is recycled for protein synthesis, iron, which is stored and reused for new red blood cell production, and then there are the non-iron heme components, which are converted into bilirubin and excreted in the bile. Hematocrit refers to the percentage of blood volume that's composed of red blood cells. A typical range for males is 42 to 52%. Females, on the other hand, have a much lower hematocrit, 37 to 47%, due to menstrual blood loss. Now, in general, a low hematocrit may indicate anemia, while a high hematocrit may suggest dehydration or polycythemia, which means you're producing more red blood cells than you need. So to summarize with our key takeaways, when we've got six of them here, blood is composed of plasma and formed elements, including red blood cells, white blood cells, and platelets. Plasma proteins include albumins, which are used for osmotic balance, globulins, which are used for immune function, and fibrididogen, which is used for clotting. Red blood cells transport oxygen and contain hemoglobin, which binds oxygen and carbon dioxide. White blood cells fight infections and are divided into granular, which includes neutrophils, eosinophils, and basophils, and agranular, which are lymphocytes and monocytes. Platelets assist in blood clotting by forming a platelet plug and stabilizing fibrin clots. Hematocrit measures red blood cell concentration with normal levels differing between males and females. And so that's it for episode 33 of Pulse of Physiology. I hope this episode has given you a better understanding of blood and its critical functions in the body. Join me next time for episode 34, where I'll discuss cardiac pacemaker cells and how they initiate the electrical activity that keeps the heart beating. Until then, keep questioning, keep exploring, and stay curious.