Blood alone moves the wheels of history. ~ German friar Martin Luther
Fluids provide the transport medium for all living organisms. Plant blood is called sap, as is a human simpleton.
The derivation of a sap as a simpleton comes from sap as a shortened form of sapwood, which is the soft wood between the inner bark of a tree and the heartwood; that is, the wood that carries sap. Simpletons became saps around 1815, via contraction of sapskull, a synonym for simpleton dating to 1735.
While the vegetative version of blood morphed into denigration, blood itself runs quite the contrary. Blood is the only human body fluid that holds respect.
Many bodily fluids are regarded with varying levels of disgust in various cultures. That revulsion is typically codified in religion. Judaism, Hinduism, Christianity, and Islam all have disgusting things to say about various body fluids. But not blood. Christian sacrament pays homage to the symbolic “blood of Christ.”
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Blood is a carrier of oxygen and nutrients to cells, and hauls away metabolic wastes. This exchange involves an intermediary: lymph. Lymph is the tissue fluid (interstitial fluid) that bathes the cells of multicellular animals.
Hematopoiesis is the blood production process. Bone marrow is the factory for all blood cells: red and white. All red blood cells are alike, but there is a diversity of the white blood cells which comprise the immune system infantry.
Besides blood cells, bones produce hormones which communicate with other organs and tissues. Bones are much more active participants in bodily functioning than once thought. The reason for manufacture within bones is that they provide protection from ultraviolet radiation, and so preserve the quality of cells made within.
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A mammalian embryo has 3 germ cell layers which are differentiated early in development. The ectoderm is the outside layer, while the endoderm is on the inside. The middle layer, the mesoderm, further differentiates into bone, cartilage, muscle, connective tissue, and the reproductive and excretory systems.
Some cells in mesodermal tissues retain an ability to transform. Bone marrow (mesoderm) cells might become liver (endoderm) cells.
Blood and the immune system may generally be considered as connective tissue, though the functions of blood differ from other connective tissue. The extracellular fluid that baths many body cells is formed from blood.
Blood plasma is mostly water. Red blood cells (erythrocytes) make up about 40% of blood contents, with just 1% white blood cells (leukocytes). Blood accounts for 8% of human body weight.
An erythrocyte has a membrane-bound cytoplasmic structure with no nucleus. Erythrocytes lose their nuclei during formation in bone marrow, just before being released into the bloodstream.
Red blood cells have a ~4-month life span. Erythrocytes that lose their shape-shifting ability are broken down and partly recycled in the liver, spleen, and bone marrow.
Bone marrow is a cagey calculator. New erythrocytes are produced at the rate they are destroyed. Bone marrow has stem cells capable of cranking out all the different blood cell types. These hematopoietic stem cells are self-renewing. Loss of blood is accounted for by increasing red blood cell production and release until normal numbers are restored.
Blood circulates via the cardiovascular system, pumped by the heart. Blood is the road of cellular nourishment, waste, and invasion.
Cellular nourishment comes in numerous forms. The digestive tract contributes high-energy glucose, carbohydrates, fatty acids, lipids, and various proteins to the blood. Hormones and other supplies come from various glands and organs.
Oxygen is picked up at the lungs by hemoglobin. This precious molecule is carried in red blood cells; red because of the iron in those cells. The blood of most insects (hemolymph) doesn’t transport oxygen because insects are small enough for the air from breathing to suffuse and suffice as an oxygen supply.
300 million years ago, insects were able to supersize owing to rich atmospheric oxygen. That hurrah ended with the rise of modern birds, when being less conspicuous became advantageous.
Oxygen has a high affinity for iron where the oxygen concentration is high, such as the lungs. Highly reactive, oxygen leaks into regions where oxygen is sparse, which is most everywhere but the lungs. Liberated oxygen in the tissues is used in metabolic processes.
The reverse is true of carbon dioxide, a waste product from glucose metabolism. The globin in hemoglobin, a protein component, carries carbon dioxide, which is exhaled by the lungs.
Tissue cells grab what they need from passing blood and dump their waste back in. Besides gas outtake, cellular waste products also make their way out through various organs, including the skin. Some plasma and smaller transport molecules leave the blood stream via endothelial cells lining small blood vessels, bathing surrounding cells in tissue fluid.
Blood in tiny capillaries flows according to non-Newtonian viscoelastic fluid dynamics. There, blood flows effectively with less resistance than plasma by itself. Silly putty, ketchup, and chilled caramel topping are also non-Newtonian fluids, but they don’t flow with less resistance than blood plasma.
Blood can instantly change its viscosity and coagulate to prevent excess blood loss when blood vessels are damaged. Coagulation begins immediately after an injury to a blood vessel. A variety of blood rupture specialists recognize damage. This causes numerous changes in blood clotting agents, including platelets and fibrinogen, a plasma protein. The platelets form a plug, while the fibrin forms a mesh to assist the platelet plug.
The organic technology of rapid coagulation is highly conserved throughout biology. Blood is an exemplary application (as is frog spit, but that’s a different story).
If a pathogen gains entry it plies the blood pathway to spread. Lurking within are numerous immune system agents, waiting to coordinate their efforts to seek out and kill. These human cellular agents have friends.