What Is Blood Flow?
Blood flow is a physiological phenomenon. The driving force of blood flow is the pressure generated by the contraction of the heart. Blood can flow in the order of ventricles, arteries, capillaries, veins, and atriums, relying on the pressure difference between each part. The blood volume flowing through the blood vessels per unit time is directly proportional to the pressure difference within the segment of the blood vessel and the fourth power of the vessel diameter; the blood flow velocity is directly proportional to the square of the vessel diameter. Blood flow can be directly measured by inserting a flowmeter into the blood vessel. Additionally, blood flow can also be indirectly measured based on the amount of gas absorbed by the blood. Blood flow is usually expressed as the blood volume flowing through in one minute, known as minute volume. Blood flow is influenced by various factors such as myocardial contractility, ventricular filling, heart rate, vascular resistance, blood viscosity, blood volume, as well as factors unrelated to the vascular system such as gender, body posture, external temperature, and physical labor. The normal plasma pH value is relatively constant, relying on buffering substances in the blood and normal lung and kidney function. A pH value less than 7.35 can lead to acidosis, and a pH value greater than 7.45 can lead to alkalosis.
Blood is a tissue that circulates within the heart and blood vessels. In adults, blood accounts for approximately one-thirteenth of body weight, with a relative density of 1.050 to 1.060, a pH value of 7.3 to 7.4, and an osmotic pressure of 313 mm/L. Blood is composed of plasma and blood cells. Plasma contains various nutrients such as plasma proteins (albumin, globulin, fibrinogen), lipoproteins, inorganic salts, oxygen, hormones, enzymes, antibodies, and cellular metabolites. Blood cells include red blood cells, white blood cells, and platelets. Physiological and pathological changes in the body often lead to alterations in blood composition, making the detection of blood components clinically significant. ABO blood type is the primary classification of human blood types, which can be divided into type A, type B, type AB, and type O. The functions of blood include transportation, regulation of body temperature, defense, regulation of osmotic pressure, and acid-base balance. Red blood cells primarily function to transport oxygen and carbon dioxide, while white blood cells play a role in killing bacteria, resisting inflammation, and participating in immune responses. Platelets are mainly responsible for hemostasis in the body, while plasma functions to provide nutrition, transport lipids, buffer, form osmotic pressure, participate in immunity, and coagulation and anticoagulation.
The total amount of blood in the human body is called blood volume, which is the sum of plasma volume and blood cell volume. However, except for red blood cells, the number of other blood cells is relatively small and is often neglected. The amount of blood in each individual is determined by their body weight. In normal adults, the total blood volume accounts for approximately 8% of body weight in men and approximately 7.5% in women. There are differences in blood volume between genders, and even in the same individual, blood volume can vary under different conditions. Physically fit individuals tend to have slightly more blood than thinner individuals, athletes have more blood than the average person, and pregnant women experience a significant increase in blood volume. However, under normal conditions, the blood volume of a person is relatively constant, with general increases or decreases not exceeding 10%.
Safe blood refers to blood that does not contain any viruses, parasites, drugs, alcohol, chemicals, or other foreign substances that can harm, endanger, or cause disease to the recipient. Blood donors must be healthy and have never had any serious illnesses. Recipients should not be harmed by the transfusion, and donors should not be exposed to any risks. There are differences in the color of blood, which is primarily due to the hemoglobin in red blood cells. Hemoglobin appears bright red when it contains a high amount of oxygen (arterial blood) and dark red when it contains less oxygen (venous blood). Usually, the blood collected for donation is venous blood, which appears dark red. If the blood contains a high concentration of methemoglobin or other hemoglobin derivatives, it may appear purple-black. Plasma (or serum) appears transparent and yellowish due to the presence of a small amount of bilirubin. If it contains chylomicrons, it appears milky and cloudy. If hemolysis occurs, it appears as red plasma.
