32.4 Blood

In vertebrates, it is considered a connective tissue Composed of various types of cells suspended in a liquid matrix called the plasma (more on that later) The average human body contains between 4-6L of it Average pH of 7.4 in humans

Separation If a blood sample is taken, the cells can be separated from the plasma by spinning the whole blood in a centrifuge whole blood is blood with all its components intact Formed elements, the cells that occupy 45% of the volume of blood, settle to the bottom and leave a red pellet at the bottom with the plasma suspended above

Plasma Water accounts for 90% of the plasma and has a variety of solutes dissolved in it Electrolytes- inorganic salts in the blood present in the plasma in the form of ions The concentration of these ions is important factor in maintaining osmotic balance between the blood and interstitial fluid Nerves and muscles require a concentration of key ions in the interstitial fluid in order to function properly, which reflects the concentration in the plasma The kidney is responsible for maintaining plasma electrolytes at precise concentrations, which is an example of homeostasis

Plasma (cont’d) Plasma proteins are another important class of solutes and have several functions Act as buffers to help maintain constant pH Help determine osmotic strength of blood Contribute to blood viscosity Some also have specific functions Some serve as escorts for lipids (remember?)

Plasma (cont’d) Immunoglobulins are a class of proteins that serve as antibodies that help combat viruses and other foreign agents that invade the body Fibrinogens are a class of proteins that that serve as clotting agents that help to plug leaks when blood vessels are injured Blood plasma that has the fibrinogens removed is called serum

Plasma (cont’d) Also contains various substances that are traveling from one part of the body to another Nutrients Metabolic waste products Respiratory gases Hormones NB: blood plasma and interstitial fluid are similar in composition, but blood plasma has a higher concentration of proteins Because capillary walls are not permeable to proteins

Blood Cells 3 classes of cells red blood cells, which transport oxygen white blood cells, which function in defense platelets, which are involved in blood clotting

Red Blood Cells Red cells, also known as erythrocytes, are the most numerous blood cells Each cubic mm of human blood contains about 5 million of these cells, and there are about 25 trillion of them in the average 5L of human blood

RBC Structure Structure fits function Biconcave disc, flatter at the center than at its edges NB: mammalian erythrocytes have no nuclei, which is unusual for living cells (other vertebrate classes have nucleated erythrocytes) Lack mitochondria and generate ATP only through anaerobic metabolism

RBC Structure (cont’d) Small -> great for oxygen transport For oxygen to be transported, it needs to be able to diffuse across the plasma membrane of the RBC The smaller the cell, the greater the total area of plasma membrane in a given volume of blood Biconcave shape also adds to the surface area

RBC: Small but mighty! Contains 250 million molecules of hemoglobin, a protein containing iron has heme’s that the oxygen bind to Heme’s are the deep red, nonprotein, ferrous component of hemoglobin As red blood cells pass through the capillary beds of lungs, gills, or other respiratory organs, oxygen diffuses into the erythrocytes and hemoglobin binds the oxygen Reversed in the capillaries of the systemic circuit with hemoglobin unloading the oxygen

RBC Life Formed in the red marrow of bones, esp. the ribs, vertebrae, breastbone, and pelvis Marrow has stem cells (remember those?) that can develop into any type of blood cell Red cell production is stimulated by a negative feedback mechanism that is sensitive to the amount of oxygen reaching the tissues via blood If the tissues aren’t receiving enough oxygen, the kidney secretes a hormone called erythropoietin, which stimulates the production of erythrocytes in the marrow If blood is supplying more oxygen than the tissues are using, the level of erythropoietin is reduced and RBC production is slowed

RBC end On average, RBC’s live 3-4 months before they are destroyed by phagocytic cells primarily in the liver The hemoglobin is digested and the amino acids are incorporated into other proteins made in the liver Most of the iron in the hemoglobin is cycled back to bone marrow, where it is reused in erythrocyte production

White Blood Cells White cells, aka leukocytes, fight infections There are five types: basophil, neutrophil, eosinophil, lymphocyte, monocyte Some of these are phagocytes, which eat bacteria and debris from our own dead cells Lymphocytes give rise to the cells that produce antibodies, the plasma proteins that react against foreign substances

WBC (cont’d) The leukocytes we see in blood are in transit White cells spend most of their time outside of the circulatory system, battling pathogens in the interstitial fluid Lots of white cells, esp lymphocytes, in lymph nodes and other parts of the lymphatic system (more later)

WBC Life Arise in bone marrow from the same stem cells that can differentiate into erythrocytes Some lymphocytes, after leaving the marrow, mature in the spleen, thymus, tonsils, adenoids, and lymph nodes – all of which are called lymphoid organs Normally a cubic mm of human blood has 5000 to 10,000 leukocytes, but this number increases when the body is fighting an infection

Platelets NB: NOT ACTUALLY CELLS Chips of cells about 2-3 µm in diameter No nuclei Originate as pinched-off cytoplasmic fragments of larger cells in the bone marrow Enter the blood and help with clotting

Blood Clotting Fibrinogen is the inactive form of the sealant our body uses to plug leaks in our vessels Its active form, fibrin, aggregates into threads that form the fabric of the clot Clotting begins with the release of clotting factors from platelets and involves a complex chain of reactions More than 12 clotting factors have been discovered and they are still not fully understood

Blood Clotting (cont’d) Hemophilia is a disease characterized by excessive bleeding from even minor cuts and bruises Caused by an inherited defect in any step of the clotting process Anticlotting factors in the blood normally prevent spontaneous clotting in the absence of injury Sometimes platelets clump and fibrin coagulates within a blood vessel, blocking the flow of blood This clot is called a thrombus More likely to form in someone with cardiovascular disease

The Lymphatic System Returns fluid to the blood and aids in body defense Fluids and some blood proteins that leak from the capillaries into the interstitial fluid are returned to the blood via the lymphatic system Fluid enters this system by diffusing into tiny lymph capillaries intermingled among capillaries of the cardiovascular system Once inside the lymphatic system, the fluid is called lymph, with a composition similar to the interstitial fluid The lymphatic system drains into the circulatory system near the junction of the venae cavae with the right atrium

The Lymphatic System (cont’d) Lymph vessels, like veins, have valves that prevent the backflow of fluid toward the capillaries Rhythmic contraction of the vessel walls helps draw fluid into lymphatic capillaries Like veins, lymph vessels depend mainly on the movement of skeletal muscle to squeeze fluid toward the heart Along lymph vessels are organs called lymph nodes The lymph nodes filter the lymph and attack viruses and bacteria Inside a lymph node is a honeycomb of connective tissue with spaces filled with white blood cells specialized for defense When the body is fighting an infection, these cells multiply, and the lymph nodes become swollen

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