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 Updated: 16 September 2008 | |||
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Updated: 16 September 2008 | |||
BloodBlood consists of a liquid portion called plasma (about 55%) and formed elements (about 45%) that include red blood cells, white blood cells, and platelets. Plasma is a complex mixture of water (about 92%), amino acids, proteins, carbohydrates, lipids, vitamins, hormones, electrolytes, and cellular wastes. Functions of plasma constituents include transporting nutrients, gases, and vitamins; helping regulate fluid and electrolyte balance; and maintaining a favourable pH.Platelets (thrombocytes) are small cell fragments without a nucleus. They are capable of amoeboid movement and may circulate for about ten days. The main function of platelets is to stop the loss of blood from wounds (hematostasis). They help close breaks in damaged blood vessels and initiate formation of blood clots. White Blood Cells (leukocytes) are responsible for the defense of the organism by protecting against infection in various ways. Normally, five types of white blood cells are in circulating blood. They differ in size, certain characteristics and thus their functions. Some phagocytize (“cell-eating”) bacterial cells in the body. Others produce proteins (antibodies) that destroy or disable foreign particles, an important function of our immune system. Certain leukocytes help control inflammation and allergic reactions by removing bio-chemicals associated with these reactions. More contain a blood-clot-inhibiting substance, which helps prevent intravascular blood clot formation. Others release histamines which increase blood flow to injured tissues. |
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| Red Blood Cells (erythrocytes) are the most numerous blood cells. They are produced in the red bone marrow of long bones, and circulate for about 120 days, after which they are broken down in the liver and spleen. The absence of a nucleus gives them the shape of a biconcave lens. They are rich in hemoglobin (about 1/3 by volume), a protein able to bind in a faint manner to O2. Hence, red blood cells are responsible for providing O2 to tissues and partly for recovering CO2 produced as waste. However, most CO2 is carried by plasma, in the form of soluble carbonates. The level of CO2 in the blood (and not the level of O2) serves as a stimulus to breathing. Hyperventilation i.e., breathing faster or deeper than necessary to delay the urge to breathe, washes CO2 out of the lungs in a larger proportion than it increases O2 availability. When the available O2 is used up, but the CO2 levels in the blood have not yet reached the level to trigger the breathing response, the body starts suffering from O2 starvation. Hypoxia, i.e., lack of O2 to the brain can lead to loss of consciousness, and result in brain damage within only minutes and is often irreversible. Hemoglobin and red blood cell production require a small supply of iron from food. Too little hemoglobin (for example due to a lack of iron in the diet) or too few red blood cells cause anemia which reduces the O2-carrying capacity of blood. An affected person may appear pale and lack energy. The hormone erythropoietin (EPO) is the main stimulus for red blood cell formation. Hypoxia – decreased O2 levels in the tissues due to smoking or a sojourn at high altitude spurs the release of this hormone in larger-than-normal quantities. Consequently, red blood cell production is increased up along with the formation of hemoglobin. The process takes about 30 days to complete. On average, your body has about 5 liters (5.3 quarts) of blood continually traveling through it by way of the circulatory system. | |
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| Blood is pumped through the body by the heart. It follows a winding course through the right chambers of the heart, into the lungs, where it picks up O2, and back into the left chambers of the heart (pulmonary circulation). From these it is pumped into the main artery, the aorta, which branches into increasingly smaller arteries, through the smallest arterioles through a vast amount of tiny, thin-walled structures called capillaries. Here, the blood gives up its O2 and its nutrients to the tissues and absorbs from them CO2 and other waste products of metabolism. The blood completes its circuit by passing through small veins that join to form increasingly larger vessels until it reaches the largest veins, the inferior and superior venae cavae, which return it to the right side of the heart (systemic circulation). Valves in the heart and in the veins ensure blood flow in only one direction. The heart is composed primarily of cardiac muscle tissue, that continuously contracts and relaxes. Like the smooth muscle tissue of other internal organs, cardiac muscles are involuntary muscles that cannot be consciously controlled. (see Chapter: Skeletal Muscle). As the cardiac muscle cells can never rest in their rhythmic contractions, the heart must have a constant supply of O2 and nutrients. The coronary arteries are the network of blood vessels that carry O2- and nutrient-rich blood to the cardiac muscle tissue (cardiac circulation). Blockage in those coronary arteries can trigger a cardiac event (heart attack). Contractions of skeletal muscle (e. g., of the legs) also contribute to circulation by rhythmically "massaging" the one-way (back towards the heart) veins, thus supporting the work done by the heart. That is why it is important to keep moving (engaging skeletal muscles) during “rest periods” in our aerobic workout.... The contractions of the heart are triggered by electrical impulses from the Sino-atrial (SA) and atrioventricular (AV) nodes. The SA node causes contraction of the atria which sends the blood from the atria to the ventricles, and activates the AV node which initiates contraction of the ventricles, expelling the blood from the ventricles into the aorta. In adults the normal resting heart rate ranges from 60 to 100 beats per minute. The resting heart rate in children is much faster. Heart rate increases during emotional stress, exercise or strenuous activity as a normal physical response. | |
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Abnormal heart rates - cardiac arrhythmia - may be merely annoying, even go undetected; cause aggravating symptoms such as an abnormal awareness of heart beat (palpitations), or even develop into life-threatening medical emergencies that can result in sudden arrhythmia death syndrome (SADS), a term used to describe sudden death due to cardiac arrest brought on by an arrhythmia. An abnormally slow rhythm, (less than 60 beats/min), is labelled brachycardia, (or also bradycardia), an abnormally fast rhythm tachycardia. Ventricular fibrillation (V-fib) occurs when the AV node misfires in a chaotic fashion, causing the heart to quiver, precluding effective pumping. An individual suffering from V-fib will not survive the next few minutes unless cardio-pulmonary resuscitation (CPR) and defibrillation using an Automated External Defibrillator (AED) are provided immediately. CPR can prolong the survival of the brain in the lack of a normal pulse, but defibrillation is the only intervention that can restore a healthy heart rhythm by applying an electric shock to the heart, which resets the cells, permitting a normal beat to re-establish itself. Look at a step-by-step overview about how to administer Emergency Care during a cardiac event.
 Artificial pacemakers or implantable cardioverter-defibrillators (ICD) are small battery-powered electrical impulse generators with one or many wires. Electrodes at the tips of the leads are screwed into the muscle of the heart. These electrodes can sense existing electrical activity as well as generate necessary additional stimulus. They are programmed to detect cardiac arrhythmia and correct it by delivering a jolt of electricity. Such a device might be implanted in patients at risk for recurring life-threatening episodes of certain forms of cardiac arrhythmia.
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