Muscular Contraction PDF

b\. The Muscular Contraction The muscle is stimulated by impulses from the brain or spinal cord. The point of attachment of muscle and a motor neuron is known as neuromuscular junction. The point at which the nerve ending is attached to the muscle membrane is termed the motor end plate. When the im...

b\. The Muscular Contraction The muscle is stimulated by impulses from the brain or spinal cord. The point of attachment of muscle and a motor neuron is known as neuromuscular junction. The point at which the nerve ending is attached to the muscle membrane is termed the motor end plate. When the impulse reaches the motor end plate, it produces a potential difference in this region, thus stimulating muscle fibers. If the flow of impulses from the neuron to the muscle is inhibited, paralysis results, as the muscles are not stimulated. The muscles can only contract in the presence of a stimulus. No contraction or movement is observed in a muscle if it is excited by only a weak impulse. If the potential is gradually increased to the threshold level, an impulse is generated when this level is reached. Any excitement beyond this value has the same effect on muscle contraction. The energy for contraction is supplied by the muscle cells, not by the action potential. The response of the muscle at a certain threshold level is known as the all-or-none rule. Muscular contraction occurs in three phases Latent phase is the interval between the stimulation of muscle and the initiation of contraction. It lasts approximately 0.005 seconds. Contraction is the interval between the initiation of contraction and the initiation of relaxation. It lasts approximately 0.04 seconds. Relaxation is the interval between the initiation of relaxation and restoration of the original position. The Mechanism of Contraction The muscle contracts by actin and myosin fibers sliding over each other. Myosin is a protein which is 100--150 Å in width and 1.5 microns in length, whereas actin is 50--70 Å in width and 2 microns in length. Thus, myosin is shorter but thicker than actin. The muscle is in a relaxed state when the actin and myosin fibers move apart. The tips of each myosin fiber are thick with protruding branches (head of myosin). They give a folded appearance to each fiber. Actin fibers consist of three types of protein. The most abundant is the spherical actin protein. Approximately 300-400 are present in each fiber. There are also 40-60 tropomyosin molecules in addition to a small amount of circular tropin molecules. A muscle has repeating units called sarcomeres, which extend between two dark lines called the Z lines. The thick filaments are made up of a protein called myosin, and the thin filaments are made up of protein called actin. The I band is light colored because it contains only actin filaments attached to a Z line. The dark regions of the A band contain overlapping actin and myosin filaments, and its H zone has only myosin filaments. The plasma membrane of a muscle fiber is called sarcolemma, the cytoplasm is the sarcoplasm and the endoplasmic reticulum is the sarcoplasmic reticulum. A muscle fiber also has some unique structures, for example T tubules (transver tubules), which are extensions of sarcolemma that penetrate into the cell so that they come into contact with special portions of the sarcoplasmic reticulum, called calcium storage sacs, containing calcium ions (Ca++). Chemical changes during contraction Energy from ATP is required for the contraction of muscles. When the impulse reaches the sarcolemma, it is transmitted to the inner portions of muscles (to the calcium storage sacs) via the T tubules. Ca++ ions in the sarcoplasmic reticulum flow into the sarcoplasm, increasing its concentration. Calcium ions released from the storage sac combine with troponin. After binding occurs, the tropomyosin threads, which wind about an actin filament, shift their position, and the myosin binding side is exposed. By using ATP molecules, cross-bridges between actin and myosin are formed. The thin actin molecules are pulled to the center of the sarcomere. This is contraction. When another ATP molecule binds to a myosin head, the cross bridge is broken, as the head detaches from actin (relaxation). During the contraction process, the I bands shorten and the H zones almost or completely disappear If the muscle is stimulated again, after the refractory period (latent phase), but before it is fully relaxed, the second contraction adds to the remains of the first one, increasing the contraction strength. If the muscle receives repeated strong stimulations without a time to relax, the muscle has a continuous contraction called a tetanus. Cramps, for instance, are a type of tetanus occurring in human muscular tissue and result from continuous stimulation of the muscles. This condition is extremely painful and potentially dangerous if an attack occurs while an individual is swimming, for example. Tetanus ends when the muscle uses up its chemical energy reserves. One of the symptoms of tetanus caused by the bacterium *Clostridium tetani* is prolonged contraction of the muscles. The energy reserves in muscle can only supply energy for 0.5 seconds. After this period, creatine phosphate is used as an energy source. It is found in muscle cells and supplies 20 times more ATP energy. At rest, the muscle cells store the excess energy (ATP) that is unused, in the form of creatine phosphate. When it is needed, creatine phosphate donates its phosphate to ADP to form ATP, and ATP is used for muscular activity. When both ATP and creatine phosphate are consumed, glycogen and fatty acids stored in muscle cells are used as fuel for muscular activity. Use of food as a source of energy can be done by using oxygen (aerobic respiration), which produces much more energy (38 ATP / glucose molecule), or by using only enzymes but not oxygen (anaerobic respiration or fermentation), at the end of which only 2 ATP net gain can be achieved for each glucose molecule. Slow-Twitch and Fast-Twitch Muscle Fibers Muscles provide energy by means of both aerobic and anaerobic respiration. Some fibers, however, utilize one method more than the other. Slow-twitch fibers tend to be aerobic and fast-twitch fibers tend to be anaerobic. Slow-twitch fibers are most helpful in sports like long-distance running, biking and swimming. Because they produce most of their energy aerobically, they tire only when their fuel supply is finished. These muscles have many mitochondria and are dark in color because they contain myoglobin, which store oxygen, like hemoglobin molecules in blood. Fast-twitch fibers seem to be designed for strength because their motor units contain many fibers. The end of the motor neuron connects to many fibers. As a result, many fibers can contract at once, which makes the muscles very strong. They provide explosions of energy and are most helpful in sports like sprinting, weightlifting, etc. These muscles are white in color because they have fewer mitochondria, little or no myoglobin and fewer blood vessels than slow-twitch muscles. Lactic acid, which is a by-product of anaerobic respiration, accumulates between the muscle fibers and causes them to fatigue quickly. Exercise and Size of Muscles Forceful muscular activity over a prolonged period causes muscle to increase in size. This is called hypertrophy and it occurs only if the muscle contracts to at least 75% of its maximum tension. Some people takes drugs, such as anabolic steroids, testosterone or related chemicals to promote muscle growth. This practice, however, can cause cardiovascular diseases, liver and kidney malfunctions and sterility. Rigor Mortis Any metabolic disorder in a muscle results in the loss of Ca++ ions from the sarcoplasmic reticulum. This process is irreversible, the muscles remaining contracted. This fact explains, why after death, all the muscles of the body contract. Rigor mortis first occurs in the heart, diaphragm, face, neck, jaw and eyelids which, in humans, do not close after death. The sequential hardening and relaxation of muscles during rigor mortis may be used as evidence in forensic medicine when a death is thought to be suspicious. If, however, a muscle is removed and placed into an oxygen-rich environment immediately after death, rigor mortis does not occur. The muscle fibers are autolyzed by catepsin after the process of rigor mortis is complete. SKIN All multicellular organisms have a skin composed of one or more layers. The skin functions as a protective layer for these organisms. The components of skin vary according to the type of organism, and may contain hairs, nails, scales, sweat glands or sensory receptors. The functions of skin can be listed as follows: - protection of the inner layers of the body from physical and chemical effects - prevention of the entry of microbes into the body - prevention of water loss in terrestrial organisms - protection of cells from the adverse effect of ultraviolet light - Regulation of body temperature in higher organisms. In hot conditions, capillaries in the skin dilate and radiate heat. The same capillaries constrict in cold conditions to prevent heat loss - a site of gas exchange - excretion of catabolic wastes via sweat glands - maintenance of a moist body surface - secretion of fat - absorption of some medicines Skin is composed of two completely different layers: Epidermis Dermis The epidermis is generated from ectoderm and is composed of epithelial cells. The dermis is generated from mesoderm and is located directly beneath the epidermis. Epidermis The outermost layer of the body is known as the epidermis. It is composed of keratinized epithelial cells and its surface is covered by either a layer of secretions from epidermal glands or by keratinized cells. This layer is involved in the protection of the body from physical and chemical hazards and forms a barrier against microbes. The epidermis contains no blood vessels, thus it is supplied with nutrients diffusing from blood vessels in the dermis. The epidermis is thickest in the palm of the hand and the sole of the foot. The epidermis is approximately 0.7 mm in thickness. The upper section of the epidermis is composed of nonliving epithelial cells containing a polypeptide known as keratin. This layer is shed over a period of time and is replaced by the rapidly dividing cells of the inner layer. The color of the skin is conferred by melanin, produced by granules located in melanocytes of the epidermis. Dermis It is a layer located directly beneath the epidermis and is 4 mm in thickness. It is composed of interconnected collagen and elastic fibers of connective tissue. The dermis is rich in blood vessels and nerve endings. The receptors located in the skin are connected to these nerve endings. Blood vessels are present in this layer and are involved in supplying nutrients to cells and in thermoregulation, the regulation of the temperature of the skin. Furthermore, the dermis contains smooth muscles, sweat and sebaceous (oil) glands, hair follicles, touch receptors and lymph vessels. Smooth muscle is connected to each hair follicle and is involved in hair movement. The skin is also connected to sympathetic nerves (that is why our hairs erect under panic situations). Touch receptors are located in the upper surface of the dermis and the lower surface of the epidermis, sometimes at the junction between them. Touch receptors, either Pacinian corpuscles or Meissner corpuscles are involved in sense reception. Pacinian corpuscles are sensitive to high pressure, while Meissner corpuscles are sensitive to light pressure. Other receptors in the dermis are sensitive to temperature. 3\. Accesory Structures of the Skin a. Skin glands The skin includes two types of glands: sebaceous glands and sweat glands. Sebaceous glands: The sebaceous glands are scattered through the skin. They are present in all areas of the body except the palms and soles. They secrete sebum, a mixture of fatty material and cell debris, either into hair follicles through a short duct, or directly to the surface of the skin. Furthermore, sebum functions as a barrier against infectious bacteria and fungi. There are between 400 and 900 sweat glands per cm2 on the face and head. Sweat glands: They are present in nearly all regions of the skin and consist of tiny tube-like structures. The sweat glands of the dermis are coiled and ball-shaped. They open onto the surface of the skin via the pores. They are involved in the removal of water, minerals, urea and other substances by sweating. The composition of sweat resembles dilute urine. The sweat glands are therefore considered a third kidney. Approximately 500-600 ml of liquid is sweated in a 24 hour period. A heavy laborer working in high temperatures may lose 15 litres of moisture per day. Furthermore, odors are secreted from apocrine glands, a type of sweat gland. They are concentrated in the armpits, the groin and around the nipple region. The main function of sweat glands is the regulation of body temperature by evaporation of water. The body temperature remains constant since excess heat is released by evaporation of water through sweating. If the skin temperature were unable to be regulated, for example in the event of third degree burns, this would prove fatal. Dogs have no sweat glands in their skin, only on their tongue. This fact explains why dogs protrude their tongue when they are overheated. b. Hair follicles This feature is unique to mammals. Hair follicles cover the whole body except for the palms, soles, lips, etc. The root of the hair in the dermis is termed the hair follicle, whereas the visible portion of it is termed the hair shaft. The diameter of the hair in the follicle is wider than that of the shaft. Hair is formed from keratinized epithelial cells. In this process, living hair cells are impregnated with keratin from epithelial cells. Each hair follicle has a sebaceous gland in addition to smooth muscle. During contraction of the smooth muscle, the hair is raised from its normal position at the surface of the skin to an erect position. Hair color is determined by pigment produced by melanocytes located at the base of the hair follicle. The greater the amount of melanin produced, the darker the hair color. Trichosiderin is an iron pigment that gives red color to hair. Blonde hair color results from less melanin production, the amount of production being genetically controlled. If the gene that controls melanin synthesis is nonfunctional, the individual becomes albino, with colorless hair and skin. The hair loses its color due to aging or stress as the production of melanin decreases. The distribution, quantity, length, diameter and appearance of hair varies in different regions of the body. c. Nails They form protective coverings of the toes and fingers. Each nail is composed of a nail plate and a nail bed. The nail plate is a continuation of the epithelium of the skin, and is therefore composed of epithelial cells. The base of the plate resembles the shape of a half-moon and contains rapidly dividing epithelial cells. After division, impregnation with keratin forms a horny structure. Growth of the base pushes the nail plate forward. During normal use, the extended portion is worn away. The thickness of a nail is between 0.5-0.7 mm. Nails grow between 0.5-1 mm per week. The amount is affected, however, by hormones, disease and diet. d. Skin Pigments Skin color is produced by granular pigments. Body color has different functions in vertebrates and invertebrates. Some, for example, use color in adaptation to the environment. Color can be used in defense and in the attraction of a mate. The main function, however, is protection from harmful rays of sunlight. It is interesting that some insects mimic the color and design of their environment when they are forming their new exoskeleton. The production of color results from the distribution or accumulation of pigments in the body. The body darkens in color when the pigments are accumulated at the core of the cell. The pigments located in the star-like cells of amphibia are known as chromatophores, and are rare in mammals. Only albinos lack melanin in their bodies. The quantity of melanin in various races of humans, the quantity increasing from the poles to the equator, is an adaptation to protect the body from intense sunlight. Pigmentation is regulated by both the endocrine and nervous system. Sunlight entering the eye stimulates the secretion of melanocyte stimulating hormone (MSH) from the pituitary and is involved in the accumulation or distribution of pigments. 4\. Touch Receptors The receptors of the skin are involved in the perception of stimuli from touch, pain, temperature, pressure and vibration. These receptors are categorized into two groups: Paccinian corpuscles and Meissner corpuscles. Paccinian corpuscles are involved in the reception of heavy pressure, whereas Meissner corpuscles are involved in the reception of light pressure. Pain is detected only by nerve endings. a. Noncapsulated receptors Free nerve endings, Merkel's corpuscles and hair follicle receptors are noncapsulated receptors of the skin. Free nerve endings are involved in pain, light, touch, pressure and probably temperature sensation. Merkel's corpuscles are involved in reception of pressure and light touch. They are present in deep epidermal layers of the palms and soles. Hair follicle receptors are involved in the reception of touch in the region of the nerve net around the hair follicle. b. Capsulated receptors Meissner corpuscles, Paccinian corpuscles, Krause corpuscles and Ruffini corpuscles are the capsulated receptors of the skin. Meissner corpuscles are involved in reception of touch in the palm, sole and lips. Paccinian corpuscles are involved in reception of mechanical stimuli. They are pressure receptors located deep in the dermis Krause corpuscles are involved in reception of cold and pressure. Ruffini corpuscles are involved in reception of heat, touch and pressure. CIRCULATORY SYSTEM Organisms require transportation systems to bring in supplies (nutrients and oxygen for energy) and remove garbage (metabolic wastes) without disturbing their internal environments. Simple organisms can solve this problem without using any complex systems. By means of diffusion through their body surfaces, needed substances can be taken into their cells or bodies, and wastes can be removed by the same method. Animals more complex than flatworms don't have a great enough surface-to-body ratio to solve their problems. They have to use special, more complex sytems. The circulatory system in large active animals carries vital materials to cells and removes wastes. A circulatory system includes three main structures: a pump, fluid and vessels. Circulatory systems transport fluid in one direction, powered by a pump that forces the fluid through vessels to all parts of the body. Among animals, one of two types of circulatory systems can be used. Open Circulatory Sytem (OCS) In an OCS, the body fluid is not contained in vessels. Arthropods, gastropods and bivalves (types of mollusks) have open circulatory systems. This system includes a heart, and blood vessels that lead to spaces where the fluid, hemolymph, directly bathes the cells. Material exchange occurs between the cells and fluid before the fluid returns to the heart. In such a system there are only two types of blood vessels: arteries, which carry fluid from the heart to the cells; and veins, which carry hemolymph back to the heart. Closed Circulatory System (CCS) In a CCS, blood remains within vessels. Arteries, leading from the heart, conduct blood from the heart and branch into smaller vessels, called arterioles, which then diverge into a network of very tiny, thin vessels called capillaries. Material exchange occurs between the capillaries and cells. Blood then collects into veins, which carry blood back to the heart. Annelids are the simplest animals with a closed circulatory system. THE HUMAN CIRCULATORY SYSTEM The human circulatory system is composed of the heart, arteries, capillaries and veins. All these structures are filled with blood, a fluid connective tissue composed of water, solutes, blood cells and platelets. Together they form an internal transport system within the body for substances to and from the cells. This system is also known as the cardiovascular system (cardio- means heart, while vascular means vessels). 1. Heart The heart is located within the chest (thoracic cavity), between the lungs and under the sternum or breastbone. In adult males, the heart weighs approximately 280--340 grams, and in females, it weighs approximately 230--280 grams. Each day the human heart sends 7000 liters of blood through the body, and it contracts more than 2.5 billion times in a lifetime. The heart is divided into two halves, forming the basis of the two cardiovascular pathways: the pulmonary circuit and the systemic circuit. Pulmonary circulation takes blood to the lungs and returns it to the heart. In pulmonary circulation, blood that is low in oxygen but high in carbon dioxide is pumped from the right side of the heart to the lungs where gas exchange takes place. The blood is oxygenated from inhaled air, and carbon dioxide diffuses out and is released by the lungs. From there the blood flows to the left side of the heart where the freshly oxygenated blood is distributed to the body by the systemic circulation. Oxygen is used by all parts of the body, and carbon dioxide is released as a waste product. The oxygen-depleted blood travels back to the right side of the heart where the process is repeated. The heart is divided into left and right hemispheres separated by a muscular wall, the septum. Each half of the heart has two chambers: an atrium and a ventricle. The tricuspid, or three--flapped, valve connects the right atrium to the right ventricle and a bicuspid, or two--flapped, valve connects the left atrium to the left ventricle. Each half of the heart also has a valve known as the semilunar valve located between the ventricle and the arteries leading away from the heart. The function of all the valves is to prevent the backflow of blood and to keep the blood moving in one direction. The valves are unidirectional: they only allow blood flow into, and not out of, the ventricles. Any defect in these valves can result in heart malfunction. Oxygen-depleted blood is transported from the right ventricle to the lungs by the pulmonary artery. In the lungs the blood is oxygenated and returned to the heart by way of the pulmonary veins. The blood enters the left side of the heart and flows into the left ventricle where it is then pumped out to all regions of the body. a. Structure of the Heart The heart is composed of three main layers: ™ Endocardium ™ Myocardium ™ Pericardium The endocardium, the innermost layer of the heart, is composed of a single layer of epithelial cells. It also contains connective tissue, connecting the endocardium to the myocardium. The endocardium contains no blood vessels. Additionally, its gelanitous structure prevents the erosion of the heart during contraction and relaxation. The myocardium is the middle layer of the heart and is composed of cardiac muscle. It is the main layer of the heart, since the main function of the heart is that of a pump. The thickness of the myocardium varies. It is thin in the artia but thicker in the ventricles. The left ventricle however, has a thicker layer of myocardium than the right ventricle. The cells of heart muscle do not obtain their nutrients from the blood within the heart chambers directly. The heart, as a hard-working organ, must be fed perfectly. Its nutrition is effected by a special branch of the systemic circulation, the cardiac circulation. The Pericardium: This forms the outermost layer of the heart and is composed of fibrous tissue. The space between its two surfaces is filled with fluid. The colloidal structure of the pericardium facilitates heart function and protects it from external hazards. Blood Blood is a tissue and originates from the mesoderm of the embryo. It consists of 45% cells and 55% plasma. There are approximately 15 liters of fluid in an adult human body. Blood comprises only 5 liters of the total volume of liquid. It can be easily separated by centrifugation due to a difference in density between plasma and its other components. Blood plasma has a density of 1.03 g/cm3, while the other components of blood have a density of approximately 1.09 g/cm3. Paraffin is smeared on the centrifugation tubes to prevent coagulation, and heparin or citrate is added to precipitate the calcium. b. Functions of the Blood 1\. Nutrient Transport: Nutrients, such as glucose, amino acids, vitamins, minerals and oxygen, are transported to cells by the blood. Metabolic wastes, such as carbon dioxide, urea, and water, are removed from cells and excreted. 2\. Hormone Transport: Hormones secreted by the endocrine glands enter the blood and are transported to target cells or tissues. 3\. Homeostasis: Blood helps maintain homeostasis by regulating pH at 7.4. It also regulates water and temperature levels. 4\. Immune response: Invading viruses, bacteria and other foreign substances are phagocytosed by antibodies and leucocytes in the blood. 5\. Clotting: During injury, blood loss is prevented due to the clotting capability of the blood. As previously mentioned, blood consists mainly of cells and plasma and has a homogeneous appearance. If it is centrifuged without clotting, its homogeneous appearance is lost. Dense cells remain at the base of the tube and straw-colored plasma remains at the top. Serum may be obtained by the removal of fibrinogen from the plasma. The ratio of red blood cells to the total volume after centrifugation is termed the hematocrit. Erythrocytes comprise the lowest band (44%), leucocytes the middle band (0.05%), and thrombocytes form a very thin layer above the others (2.4%). a. Plasma Plasma constitutes 55% of the blood, of which 90-92 % is water, 7-9% plasma proteins, and the remaining 1% is amino acids, carbohydrates, lipids, hormones, urea, uric acid, lactic acid, enzymes, alcohol, antibodies, sodium, potassium, calcium, chloride, phosphate, magnesium, copper, iron, bicarbonate, iodine and other trace elements. Some Important Proteins in Plasma: There are more than 70 different types of plasma proteins. Three of them are well-known and have important functions. ™ Fibrinogen is involved in blood clotting. ™ Albumin regulates osmotic pressure of the blood and interstitial fluid. ™ Globulins participate in the structure of antibodies. Most blood proteins are produced by the liver. The glucose level of blood is approximately 80-120 mg per 100 ml. If the amount of glucose decreases to 40 mg or below, hyper stimulation, fainting, shivering of the muscles and death, preceded by coma, occurs. A diabetic is unable to control the level of glucose in the blood. If the glucose level decreases to a critical level, sugar must be ingested immediately. If the glucose level rises above a certain level, an injection of insulin is necessary to restore it to normal. It is clear that glucose is used as a main energy source by the brain, so the amount of glucose must be kept constant by hormones in the blood. Besides all this, dissolved oxygen, nitrogen and carbon dioxide are also found in blood. Blood cells are classified as erythrocytes (red blood cells), leucocytes (white blood cells) and thrombocytes (platelets). 1. Erythrocytes Erythrocytes are 8 mm in length and 2 mm thick. There are approximately 5 to 5.5 million per mm3 in the average male, and 4 to 4.5 million per mm3 in the average female. Moreover, the number of erythrocytes varies considerably between humans living at different altitudes. At high altitudes, greater amounts of erythrocytes are expected due to a decrease in the partial pressure of oxygen in the atmosphere. Mammalian erythrocytes are unique since they have no nucleus. However, the absence of a nucleus reduces their life span to between 80 and 120 days. a\. The Structure of Erythrocytes Mature erythrocytes in mammals lack a nucleus, mitochondria, Golgi apparatus and endoplasmic reticulum. The lack of these organelles decreases the metabolism of the cell and increases their surface area. Half of the erythrocyte mass is available for oxygen loading. Hemoglobin consists of mainly two parts ™ A heme group ™ A globin group. The heme group is an iron containing complex, whereas the globin group is composed of globular proteins (four polypeptide chains). Oxygen molecules bind weakly to the iron of the heme group. The globin group differs in each species of animal. Hemoglobin resembles chlorophyll in structure. However, iron is substituted for the magnesium found in chlorophyll. An erythrocyte contains approximately 265,000 hemoglobin molecules. This structure increases the oxygen carrying capacity of blood. If a specialized physiological fluid were unavailable for oxygen transport, the amount of blood would need to be 75 times the normal volume, and the rate of blood transport would also have to be increased 75 fold. Vertebrate hemoglobin is confined largely to the blood cells, while invertebrate hemoglobin is found in the plasma. Production of Erythrocytes In the fetus, erythrocytes are produced by the liver and the spleen. They are also produced in the red bone marrow of the skeletal system, in the ribs and sternum. From the fifth month of development until the end of life, production occurs mostly in the marrow of the long bones. Erythrocytes proliferate from erythroblasts of red bone marrow. They lack hemoglobin, have a nucleus and divide rapidly, losing their nucleus, Golgi apparatus, mitochondria and other organelles after hemoglobin has been synthesized. They become characteristically concave and disk-shaped on both surfaces and are termed erythrocytes. This concavity facilitates their passage through the capillaries and increases their capacity to bind with oxygen due to the increase of their surface area. Another advantage of this concavity is the ability of the surface membrane to increase when needed. The erythrocyte membrane can swell under tension or in order to carry more O2 and CO2. Mature erythrocytes cannot respire aerobically since they lack mitochondria. The most important vitamin in erythrocyte production is vitamin B12. This vitamin stimulates cells to produce blood. Vitamin B6 is responsible for recycling of erythrocytes, and vitamin B12 affects erythrocyte production in the bone marrow. Human erythrocytes live for approximately 80-120 days in the circulatory system. This short life span can be explained by their lack of a nucleus and other organelles necessary for protein synthesis. Since erythrocytes cannot renew and regenerate themselves, old, worn-out erythrocytes are removed from the circulation by the spleen, liver and lymphoid nodules. Phagocytotic cells found in these organs digest erythrocytes and separate iron to be reused in new erythrocyte production in the red bone marrow. The porphyrine component of the heme group is converted to bilirubin and transported to the liver to form bile. This product is used by bacteria in the large intestine and is responsible for the color of the feces. When certain cells in the kidney do not receive enough oxygen, they produce a substance that combines with a plasma protein to form the hormone erythropoietin. This hormone stimulates red blood cell production in red bone marrow. Anemia is a reduction in red blood cells or in the amount of hemoglobin. These conditions decrease the amount of oxygen delivered to cells and causes fatigue and lack of tolerance to cold. The most common cause of anemia is iron deficiency, treated by eating more iron-rich food. Leucocytes (White Blood Cells) Leucocytes are nucleated, spherical, white cells. They are also referred to as white blood cells due to their color. The number of leucocytes in the blood of a healthy person is approximately 6000/mm3. Leucocytes may be found in both blood and interstitial fluid. They can attach themselves to the internal surface of the endothelium or vessel wall and move against the flow of blood. They can also cross the capillary walls in interstitial fluid. There are three main types of leucocytes and all of them are produced both in red bone marrow and in lymph nodes. a. Types of Leucocytes (WBC) 1. Granulocytes 2. Agranulocytes The nucleus of an agranulocyte lacks lobes and is partly spherical in shape. It also differs from a granulocyte by its ability to divide. Most agranulocytes, lymphocytes and monocytes are produced in the lymph nodes, the spleen and thymus. 1. The Human Respiratory System a. Lungs At birth, the lungs of infants are slightly hard, and occupy all the unexpanded thorax. The intercostal muscles are free and uncontracted. As the first breath is taken, the chest cavity expands to a greater extent than the lungs, thus producing a vacuum. Both structures continue to grow rapidly thereafter. Respiratory Movements Gas exchange in the lungs is carried out through changes in chest volume, which result in alterations in pressure. The movement of the chest cavity parallels that of the movement of the lungs. The chest consists of the ribcage, the ribs and the intercostal muscles between the ribs. The ribs are connected to the vertebrae at an angle. The contraction of the intercostal muscles results in upward movement of the ribs and expansion of the chest. The diaphragm, a dome shaped membrane separating the chest cavity from the coelom (abdominal body cavity), flattens during breathing. Thus, the volume of the chest cavity increases. When the volume increases, the pressure within the chest cavity decreases, becoming less than atmospheric pressure. To equalize pressure inside and outside the body, air rushes in (inhalation). The volume of the chest cavity is reduced by relaxation of the diaphragm and the intercostal muscles. At that time, pressure inside the chest cavity becomes greater than atmospheric air pressure, and air is sent out (exhalation). The movements of the chest cavity during its expansion and reduction result in changes in the volume of the lungs, since the outer membrane of the lungs is closely connected to the ribcage, aided by fluid in the pleural cavity When we breathe, the amount of air moved in and out with each breath is called the tidal volume. Normally, the tidal volume is about 500 mL, but we can increase the amount inhaled and exhaled by deep breathing. The maximum volume of air that can be moved in and out during a single breath (or the air that can be exhaled after a deep inhalation) is called the vital capacity. Some air, about 1200 mL, stays inside the lungs after exhaling (residual air). Inspiration volume can be increased up to 3100 mL; tidal volume is 500 mL; expiratory reserve volume (the air that can be exhaled with force, using the abdominal muscles) is about 1400 mL. Vital capacity is the sum of all these volumes, or about 6 L. An individual cannot respire if the chest cavity is perforated, as air entering through the perforation destroys the vacuum of the lungs. If the chest cavity of some mammals, like dogs, is perforated, both lungs collapse, shrinking as the membrane surrounding them is punctured. Under the same circumstances, only one lung of humans collapses, since each lung is housed separately within a pleural membrane. b. Oxygen and Carbon Dioxide Transport 1. Oxygen Transport 2. Carbon Dioxide Transport c. Regulation of Respiration