Blood is the fluid that moves through the vasculature and performs gas exchange with the tissues. It also plays a role in transporting nutrients, hormones, and wastes. Blood is composed of three main components: plasma, white blood cells or leukocytes, and red blood cells or erythrocytes. Plasma constitutes the majority of blood volume. It is the liquid portion of blood and sorts itself above other components when spun in a test tube due to being less dense. It mainly comprises water, dissolved electrolytes, organic compounds, and gases. The smallest portions of blood consist of white blood cells and platelets. White blood cells aid in fighting and identifying infections in the immune system. Platelets are small cell fragments essential for blood clotting, providing a rapid response to plug any holes and assist other factors in sealing the wound. However, problematic clots, known as thrombi (singular: thrombus), can form and obstruct blood flow, leading to severe consequences.
Red blood cells are notable for their role in oxygen transport, attributed to the presence of the protein hemoglobin. Mature red blood cells lack nuclei and organelles to maximize space for hemoglobin. These cells have a characteristic donut-like disk shape with a central depression observable from both sides, and they are produced in the bone marrow. The production of red blood cells is stimulated by erythropoietin, a hormone secreted by the kidneys. Hemoglobin's quaternary structure consists of four polypeptide subunits, each containing a heme group bound to an iron molecule within a porphyrin ring. The iron alternates between reduced and oxidized states to bind and release oxygen. This porphyrin structure is a notable example of nature's tendency to conserve functional structures across different biological systems, as seen with chlorophyll in plants which utilizes a similar structure for photosynthesis.
Another important respiratory pigment is myoglobin, found primarily in skeletal muscles. It binds oxygen more tightly than hemoglobin and contains only one heme group. Myoglobin's function is crucial in muscles, particularly in oxygen dissociation and binding, which will be explored further in later discussions.
Last, a mention of sickle cell disease, caused by a mutation in the hemoglobin protein which leads to abnormal aggregation in red blood cells, distorting their shape and impairing function. Although sickle cell disease can be life-threatening, it has persisted in certain populations because heterozygotes show increased resistance to malaria, providing a genetic advantage in malaria-endemic areas.
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