Blood is connective tissue that never stops moving
Roughly seven percent of adult body weight is blood—about five liters on average. Plasma makes up most of the volume as straw-colored fluid; red cells pack hemoglobin that ferries oxygen. The heart pushes this mixture through arteries and veins in a loop William Harvey described in 1628.
Blood is specialized connective tissue: cells and fragments suspended in plasma. Plasma is mostly water with proteins, glucose, ions, and hormones; by one accounting it is about fifty-five percent of blood fluid and ninety-two percent water by volume. Red cells dominate the formed elements, white cells police infection, and mammalian platelets seal leaks. Arterial blood looks bright when hemoglobin is oxygenated and darker when deoxygenated. Functions range from nutrient and waste transport to clotting, hormone delivery, and immune surveillance.
An average adult holds roughly five liters. Red cells occupy about forty-five percent of whole blood (the hematocrit), plasma about fifty-four percent, and white cells under one percent. Typical red-cell counts run millions per microliter, with sex-specific ranges; white cells number thousands and platelets hundreds of thousands. Healthy erythrocytes last about one hundred twenty days before spleen and liver Kupffer cells clear them. Blood pH stays tightly near seven point three five to seven point four five; values below six point nine or above seven point eight are usually lethal.
About ninety-eight point five percent of arterial oxygen at sea level rides on hemoglobin; only about one point five percent dissolves freely. Hemoglobin can raise oxygen-carrying capacity roughly seventyfold versus solubility alone. In adults at rest, blood leaving the lungs is about ninety-eight to ninety-nine percent saturated, delivering roughly nine hundred fifty to eleven hundred fifty milliliters of oxygen per minute while the body uses about two hundred to two hundred fifty. Carbon dioxide travels mostly as bicarbonate—about seventy percent—with smaller shares dissolved or bound as carbamino compounds. Platelets are a mammalian specialty; other vertebrates use nucleated thrombocytes instead.
Source: Blood
