Biological Systems PDF

C hapter (1) Biological Systems A- system: In science, any system consists of several building units interrelated in a certain way to give the proper action of that system. The complexity of any system is due to the inhomogeneity of its building un...

C hapter (1) Biological Systems A- system: In science, any system consists of several building units interrelated in a certain way to give the proper action of that system. The complexity of any system is due to the inhomogeneity of its building units. On this point of view, the physical (engineering) systems appear to be simpler than that of biology or biophysics. The reasons for that are due to the inhomogeneity and complexity of the later systems. In addition, in physical system, it is easy to control any unwanted surrounding parameters especially the environmental one, like temperature, pressure… etc. on the other hand, when studying a biological system, such as a certain organ in the body, we cannot isolate it from its surroundings and accordingly an accurate result could not be obtained. When a system is subjected to a stimulus, or force (this may be done simply by changing the environment of the system), the system shows a response to the force, and the response is related to the force in a manner described by the laws and properties of the system. Force (stimulus) System Response ----------->--------- (Laws, equations& properties) ------->-------- Comparison between Engineering and Biological systems Engineering System Biological system The building units are molecules of The building units are living cells solid, liquid, or gas and biological molecules. Simple and homogeneous Complex and inhomogeneous (heterogeneous) due to the inhomogenity of its building units Easy to control the surrounding Not easy to control and cannot be conditions (temperature, pressure, ) isolated from its surroundings. Can be described by laws, Most principle function and equations, and theories with definite phenomena are hypothetical and clear mechanisms final mechanisms are still unknown. Possibility of shaping and can take Trials for changing the cell shape any shape cause injury or death. The physical properties could be Study of the physical properties measured by the common physical needs a special design and instruments developed equipments and instruments The physical properties are constant The physical properties are under constant conditions (V,, continuously changed according to m,…) the function and biological demands B- Extracellular fluid - the internal environment About 60% of the adult human body is fluid. Although most of this fluid is inside the cells and is called intercellular fluid, about one third is in the spaces outside the cells and is called extracellular fluid. This extracellular fluid is in constant motion throughout the body; it is rapidly transported in the circulating blood then mixed between the blood and the tissue fluids by diffusion through the capillary walls. In the extracellular fluid are the ions and materials needed by the cells for maintenance of cellular life. Therefore, all cells live in essentially the same environment (the extracellular fluid) for which reason the extracellular fluid is called the internal environment of the body.  The term homeostasis is used by physiologists to mean maintenance of static or constant conditions of the internal environment. Homeostasis is the body's automatic tendency to maintain a relatively constant internal environment in terms of temperature, cardiac output, ion concentrations, blood pH, hydration, dissolved CO2 concentration in blood, blood glucose concentration, concentrations of wastes etc. This constancy of internal environment is maintained despite energy and molecules continuously entering and leaving the body. The values of these (and other) variables oscillate within a narrow range. The body can monitor these variables and uses negative feedback (almost exclusively) to raise the values if they get too low and to lower the values if they get too high. “Negative” feedback means that the body’s response opposes the stress. Thus, the body is in a dynamic state of equilibrium because its internal conditions change and vary (oscillate) within relatively narrow limits. Receptors monitor changes in these physiological variables, that is, they receive a stimulus. This stimulus is transmitted via an afferent pathway to an integrating center (e.g. the brain or a gland). The integrating center compares the stimulus to the normal level of the variable – the “set point”. If a response is required a message is sent via an efferent pathway to the effector organ. The effector produces a response that moves the value of the variable back towards the set point. The responses include altering the breathing or heart rate or blood pressure; vasoconstricting or vasodilating; eating, drinking and secreting. Essentially all of the organs and tissues of the body perform functions that help to maintain these constant conditions. C- Origin of nutrients in the extracellular fluid: i- Respiratory system Each time the blood passes through the body, it flows through the lungs. The blood picks up oxygen in the alveoli, thus acquiring the oxygen needed by the cells. ii- Gastrointestinal tract A large portion of the blood pumped by the heart also passes through the walls of the gastrointestinal organs. Here different resolved nutrients, including carbohydrates, fatty acids, and amino acids, are absorbed from the ingested food into the extracellular fluid. iii- Liver and other organs performing primary metabolic functions  The liver changes the chemical composition of substances absorbed from the gastrointestinal tract to more usable forms, and other tissues of the body (fat cells, kidneys, and endocrine glands) help to modify the absorbed substances or store them until they are needed. iv- Musculoskeletal system By the help of the musculoskeletal system, the body can move to the appropriate place at the appropriate time to obtain foods required for nutrition. The musculoskeletal system also provides motility for protection against adverse surroundings. D- Removal of metabolic end products i- Removal of CO2 by the lungs At the same time that blood picks up oxygen in the lungs, CO2 is released from the blood into the alveoli, and the respiratory movement of the air into and out of the alveoli carries CO2 to the atmosphere. CO2 is the most abundant of all the end products of metabolism. ii- kidneys Passage of the blood through the kidneys removes most of the other substances besides CO2 from the plasma that are not needed by the cells. These substances include especially different end products of cellular metabolism, such as urea and uric acid which take their way through the renal tubules into the urine. E- Regulation of body functions i- Nervous system The nervous system is composed of three major parts: the sensory portion (sensory receptors detect the state of the body or surroundings), the central nervous system, and the motor output portion. - Receptors (sensory portion) present everywhere (skin, eyes, ears…) The central nervous system is composed of the brain and spinal cord. The brain can store information, generate thoughts, create ambition, and determine reactions the body performs in response to the sensation. Appropriate signals are then transmitted through the motor output portion of the nervous system to carry out one’s desires. A large segment of the nervous system is called the autonomic system. It controls many functions of the internal organs, including the level of pumping activity by the heart, movement of gastrointestinal tract, and glandular secretion. ii- Hormonal system of regulation (endocrine system) Eight major endocrine glands secrete chemical substances called hormones. Hormones are transported in the extracellular fluid to all parts of the body to help regulate cellular functions. Hormonal system regulates mainly metabolic functions. iii- Reproduction It does help to maintain static conditions by generating new beings to take the place of those that are dying. F- Control systems of the body  These include most of the body systems such as kidneys, lungs, spleen, liver, etc., in addition to the genetic control system at the cellular level.  They control the body functions through two ways: a- Open – Loop Control: in which the parameters are adjusted only by manual controls. Examples are draft of drugs against health hazards, blood supply against bleeding and accidents. b- Closed Control: in which the parameters are automatically controlled by the body control systems. Under normal operating conditions the body parameters (temperature, blood pressure, blood amount, blood sugar, ionic concentrations…) would often drift out of the proper range if there are no means of controlling them. Accordingly, the control systems in the body operate to control the function within the cell or individual parts of the organs. Examples of control systems: i-The genetic system; that operates in all cells to control intracellular functions as well as all extracellular functions. ii- The respiratory system; operating in association with the nervous system to regulate the concentration of CO2 in the extracellular fluid. iii- The liver and pancreas; regulate the concentration of glucose in the extracellular fluid. iv-The kidneys; regulate concentrations of hydrogen, sodium, potassium, phosphate, and other ions in the extracellular fluid. V-Heart; controls the blood supply to the different parts of the body through circulation. VI-Musculoskeletal; provides motility for protection. VII-Gastrointestinal system; maintains nutrients. Characteristics of control systems Biological systems operate on a mechanism of inputs and outputs, each caused by and causing a certain event. A feedback loop is a biological occurrence wherein the output of a system amplifies the system (positive feedback) or inhibits the system (negative feedback). Feedback loops are important because they allow living organisms to maintain homeostasis. Control systems are characterized by negative feedback and positive feedback. i- Negative feedback If some factor becomes excessive or deficient, a control system initiates negative feedback, which consists of a series of changes that return the factor toward a certain mean value, thus maintaining hemostasis. Most of the control processes inside the living body such as the blood pressure value, sugar and hormones level… etc. act by the negative feedback. Negative feedback systems generally contain four essential parts:  Stimulus  Sensor  Controller  Effector The stimulus is the trigger for the activation of the system. The sensor then identifies changes, which reports these changes back to the controller. The controller compares this to a set point and, if the difference is sufficient, activates an effector, which brings about changes in the stimulus. Examples of negative feedback: 1-Blood pressure homeostasis The following figure shows a schematic diagram of how negative feedback can control the blood pressure in the body. This needs the control system to overcome the change by modulating the heart beats and vasoconstriction or vasodilatation of blood vessels. The regulation of blood pressure is highly complex, involving multiple mechanisms that act in both the short term and the long term. Vasodilation and vasoconstriction refer respectively to the expansion or narrowing of the diameter of the arterioles. The physiological condition of the organism determines the set point for blood pressure. The following figure indicates that in a system controlled by negative feedback the level is never maintained perfectly, but constantly oscillates about the set point. An efficient homeostatic system minimizes the size of the oscillations. Changes in blood pressure act as the stimulus and the sensors are pressure receptors located within blood vessel walls, mainly of the aorta and carotid. These receptors send signals to the nervous system which act as the controller. The effectors include the heart and blood vessels. Increases in blood pressure stretch the walls of the aorta and carotid. This activates the pressure receptors, which then send signals to the effector organs. In response, the heart rate decreases, and blood vessels undergo vasodilation. Combined, this lowers blood pressure. On the flip side, decreases in blood pressure have the opposite effect. The decrease is still detected by pressure receptors but instead of the blood vessels being stretched further than normal, they are less stretched than normal. This triggers an increase in heart rate and vasoconstriction, which work to increase the blood pressure back to baseline. When pressure in blood vessels is “too high”, vessels stretched and stretching more and more for minutes or hours, resulting in decrease blood pressure in vessels toward normal.  Blood pressure variation around a set point 2- Blood glucose homeostasis The control of blood sugar (glucose) by insulin is another good example of a negative feedback mechanism. When blood sugar rises, receptors in the body sense a change. In turn, the control center (pancreas) secretes insulin into the blood effectively lowering blood sugar levels. Once blood sugar levels reach homeostasis, the pancreas stops releasing insulin. 3-Temperature Homeostasis One of the most important examples of negative feedback control is the regulation of body temperature. Not all animals can do this; animals that maintain a constant body temperature are called homeotherms (birds, mammals…) while those that have a variable body temperature are called poikilotherms (snakes, reptiles...). The homeotherms maintain their body temperatures at around 37°C, so are sometimes called warm-blooded animals, but in fact poikilothermic animals can also have very warm blood during the day by basking in the sun. In human, temperature homeostasis is controlled by the thermoregulatory centre in the hypothalamus. It receives input from two sets of thermoreceptors: receptors in the hypothalamus itself monitor the temperature of the blood as it passes through the brain (the core temperature), and receptors in the skin monitor the external temperature. Both pieces of information are needed so that the body can make appropriate adjustments. The thermoregulatory centre sends impulses to several different effectors to adjust body temperature. The thermoregulatory centre is part of the autonomic nervous system, so the various responses are all involuntary. The body has a range of responses available, depending on the internal and external temperatures. The thermoregulatory centre normally maintains a set point of 37 ± 0.5 °C in most mammals. The degree of effectiveness with which a control system maintains constant conditions is determined by the gain of the negative feedback. The gain of the system is calculated by the ratio between the correction of the disturbed variable and the error from normal. 𝒄𝒐𝒓𝒓𝒆𝒄𝒕𝒊𝒐𝒏 Gain (G) = 𝐞𝐫𝐫𝐨𝐫 Example: A person is subjected to a certain accidental situation; as a result, his blood pressure falls from the normal average value of 100 mmHg to 90 mmHg. Due to his internal control negative feedback system, the blood pressure increases to only 95 mmHg. Calculate the gain for this control system. Answer:  error 100 mmHg 95 mmHg (-ve feedback) 90 mmHg 𝐜𝐨𝐫𝐫𝐞𝐜𝐭𝐢𝐨𝐧 𝟗𝟓 𝟗𝟎 𝟓 G= = = -1 𝐞𝐫𝐫𝐨𝐫 𝟗𝟓 𝟏𝟎𝟎 𝟓 ii- positive feedback Positive feedback usually does not lead to the stability of the system because it derives it in the same direction of error. This sometimes causes vicious circles and death. Hemorrhage is a good example for that because it leads to death unless any external restorative process takes place as a blood transfusion. In some cases, the body has learned to use positive feedback to its advantage. Examples of a positive feedback 1-Hemorrhage: Hemorrhage leads to a decrease in blood pressure, which, in turn, leads to a decrease in blood flow in coronary arteries. The consequences of the decrease in such flow are:  Increased lactic acid and hydrogen ion accumulation, which lead to further decrease in coronary blood flow  Increased vasodilator metabolites, which lead to further decreased blood pressure  Decreased contraction of the ventricles of the heart, which leads to decreased cardiac output and further decreased blood pressure. Clearly, none of these consequences is good. Several passages through this system will lead to excessive decrease in blood pressure and death. This is a positive feedback system because all the consequences tend to increase the effect of the hemorrhage in lowering blood pressure. Another example of positive feedback is the nerve impulse propagation and the case of child birth, but in these cases, the feedback is useful. 2-The nerve impulse propagation Positive feedback in nerve impulse propagation The nerve impulse propagation is a positive feedback loop in which an initial membrane depolarization leads to a change in the membrane potential. The initial depolarization must reach or surpass threshold to activate voltage-gated Na+ channels. Opening of Na+ channels allow Na+ inflow which, in turn, further depolarizes the membrane. Additional depolarization activates additional Na+ channels and so on. 3-Childbirth It is an example of a valuable use of positive feedback. when uterine contractions become strong enough for the baby’s head to begin pushing through the cervix, stretch of the cervix sends signals to the uterine muscles back to the body of the uterus causing even more powerful contractions. Thus, the uterine contractions stretch the cervix, and the cervical stretch causes more contractions and so on until this process becomes by time powerful enough, the baby is born. Childbirth and positive feedback