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Blood: the story on HearLore | HearLore
Blood
Blood is the first word of this story, and it is also the first thing that defines life itself. This fluid, which accounts for 7% of the human body weight, is not merely a red liquid but a complex suspension of cells and plasma that performs the impossible task of keeping a human being alive. In an average adult, this substance fills roughly 1.3 gallons, yet it is so dense that its specific gravity of 1060 kg/m3 is nearly identical to pure water. The true miracle lies in the red blood cells, which number between 4.7 and 6.1 million per microliter in men, and 4.2 to 5.4 million in women. These cells lack a nucleus and organelles, a unique adaptation that allows them to carry hemoglobin, the protein responsible for binding oxygen. The collective surface area of all these red blood cells in a single human body is approximately 2,000 times greater than the body's exterior skin surface, creating a vast internal landscape for gas exchange. Without this intricate system, the heart's pumping action would be useless, and the cells of the body would suffocate within minutes.
The Silent War Inside
Beneath the surface of the red flow lies a constant, invisible battle for survival. White blood cells, numbering between 4,000 and 11,000 per microliter, serve as the immune system's primary defense force. These cells are responsible for destroying pathogens, removing old or aberrant cells, and attacking foreign substances that enter the body. When these cells malfunction, the result is leukemia, a cancer of the blood-forming tissues that can be fatal. The body also relies on platelets, or thrombocytes, which number between 200,000 and 500,000 per microliter. These cell fragments are the first responders to injury, forming a plug to stop bleeding and creating a fibrin mesh that seals the wound. The balance between these cellular components is delicate; a drop in red blood cell mass leads to anemia, while an overproduction of white blood cells can signal infection or cancer. The body maintains this equilibrium through a process called hematopoiesis, where cells are produced in the bone marrow. During childhood, almost every bone produces red blood cells, but as adults, this production is restricted to the vertebrae, sternum, ribcage, pelvic bones, and the bones of the upper arms and legs. The liver and spleen then work to clear out old cells, ensuring that the blood remains a dynamic, living fluid rather than a stagnant pool.
The Chemistry of Life
The chemical composition of blood is a marvel of biological engineering, designed to transport life-sustaining molecules while removing toxic waste. Plasma, which makes up 55% of blood volume, is 92% water and contains 8% blood plasma proteins, including albumin and immunoglobulins. The pH of blood is tightly regulated between 7.35 and 7.45, a narrow range that is essential for survival. A pH below 6.9 or above 7.8 is usually lethal, highlighting the precision required for homeostasis. Hemoglobin, the protein within red blood cells, has an oxygen binding capacity between 1.36 and 1.40 ml O2 per gram, which increases the total blood oxygen capacity seventyfold compared to if oxygen were solely carried by its solubility. This molecule also plays a crucial role in carbon dioxide transport, carrying about 23% of the gas as carbamino compounds and facilitating the exchange of hydrogen ions. The Bohr effect and the Haldane effect describe how changes in acidity and carbon dioxide levels influence the binding of oxygen and carbon dioxide, ensuring that oxygen is released where it is needed most and carbon dioxide is removed efficiently. This chemical dance allows the body to function under varying conditions, from rest to intense exercise, where oxygen saturation in venous blood can drop to less than 15% in a trained athlete.
Common questions
What percentage of human body weight is blood?
Blood accounts for 7% of the human body weight. This fluid fills roughly 1.3 gallons in an average adult and has a specific gravity of 1060 kg/m3.
How many red blood cells are in a microliter of blood?
Red blood cells number between 4.7 and 6.1 million per microliter in men and 4.2 to 5.4 million in women. These cells lack a nucleus and organelles to carry hemoglobin.
When was the circulation of blood first described by William Harvey?
William Harvey first described the circulation of blood in 1628. This discovery revolutionized the understanding of human physiology and the heart's pumping action.
Who discovered the ABO blood group system and when?
Karl Landsteiner discovered the ABO blood group system in 1900. Jan Janský is credited with the first classification of blood into four types in 1907.
What is the pH range of blood and why is it important?
The pH of blood is tightly regulated between 7.35 and 7.45. A pH below 6.9 or above 7.8 is usually lethal, highlighting the precision required for homeostasis.
Where are red blood cells produced in adults?
In adults, red blood cell production is restricted to the vertebrae, sternum, ribcage, pelvic bones, and the bones of the upper arms and legs. This process is called hematopoiesis and occurs in the bone marrow.
The circulation of blood is driven by the heart, a muscular organ that pumps blood through a vast network of vessels. In humans, blood is pumped from the left ventricle through arteries to peripheral tissues and returns to the right atrium through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs, where it releases carbon dioxide and picks up oxygen before returning to the left atrium through the pulmonary veins. This cycle was first described by William Harvey in 1628, revolutionizing the understanding of human physiology. The heart's pumping action is complemented by the movement of skeletal muscles, which can compress veins and push blood through valves toward the right atrium. The rate of blood flow varies greatly between different organs, with the liver receiving the most abundant supply at approximately 1350 ml/min, followed by the kidney and brain. The blood's ability to regulate body temperature is also a critical function, as increasing blood flow to the surface causes warmer skin and faster heat loss, while reducing flow to the extremities conserves heat for vital organs. This hydraulic system is so efficient that it can cause engorgement in specialized tissues, such as the penis and clitoris, and even power the jumping mechanism of spiders, which use blood pressure to straighten their legs.
The Color of Death and Life
The color of blood is a deceptive indicator of its state and function. Arterial blood is bright red because oxygen imparts a strong red color to the heme group within hemoglobin, while deoxygenated blood is a darker shade of red, visible in veins during blood donation. However, the color can change dramatically under different conditions. In carbon monoxide poisoning, blood remains bright red because carbon monoxide forms carboxyhemoglobin, which prevents oxygen from binding. In cyanide poisoning, the body cannot use oxygen, so venous blood remains oxygenated, increasing its redness. Some conditions, such as cyanosis, can make the skin appear blue due to issues with heme groups. Methemoglobin, formed when heme is oxidized, is brownish and cannot transport oxygen, while sulfhemoglobinemia can cause arterial hemoglobin to appear dark red with a bluish hue. Even the color of veins near the surface of the skin is an optical illusion caused by light-scattering properties of the skin and the processing of visual input by the visual cortex. In the genus Prasinohaema, skinks have green blood due to a buildup of the waste product biliverdin, a rare exception to the red norm. These variations in color reveal the complex chemistry at work within the circulatory system, where the slightest change can signal life or death.
The Blood of Belief
Throughout history, blood has been more than a biological fluid; it is a symbol of life, death, and the divine. In the Islamic, Jewish, and Christian religions, blood is considered sacred, with Leviticus 17:11 stating that the life of a creature is in the blood. This belief has led to dietary laws that forbid the consumption of blood, such as the Jewish practice of koshering meat by rinsing and salting it to remove blood. In Christianity, the blood of Jesus Christ is seen as the means for the atonement of sins, and the Eucharistic wine is believed to become the blood of Jesus for worshippers to drink. The Jehovah's Witnesses, based on their interpretation of scriptures, neither consume blood nor accept transfusions of whole blood or its major components. In indigenous Australian traditions, ochre and blood are applied to the bodies of dancers for ritual, attuning them to the invisible energetic realm of the Dreamtime. The Germanic tribes used blood in sacrifices, sprinkling it on walls and statues of the gods, a practice that influenced the term blessing. The ancient Greeks believed that the blood of the gods, ichor, was a substance that was poisonous to mortals. These cultural and religious beliefs have shaped the way humans understand and interact with blood, from the sacred to the profane.
The Science of Survival
The medical understanding of blood has evolved from ancient theories to modern science, transforming the way we treat disease and injury. The ABO blood group system was discovered in 1900 by Karl Landsteiner, and Jan Janský is credited with the first classification of blood into four types in 1907. The first blood transfusion using the ABO system was performed in 1907, and the first non-direct transfusion was performed on the 27th of March 1914. The Rhesus factor was discovered in 1937, further refining the ability to match blood types for transfusions. Today, blood banks store blood for transfusions, and crossmatching ensures that compatible blood products are used to prevent severe complications. Bloodletting, once a common treatment for various diseases, is now used only in rare cases such as hemochromatosis and polycythemia. Modern medicine has also developed treatments for blood disorders, such as anemia, leukemia, and hemophilia, using techniques like blood transfusions, chemotherapy, and clotting factor concentrates. The study of blood, known as hematology, continues to advance, with research into blood substitutes and the genetic basis of blood disorders. The ability to analyze blood through tests like the arterial blood gas test and the erythrocyte sedimentation rate has revolutionized the diagnosis and treatment of diseases, making blood a cornerstone of modern medicine.