Chapter 2: Homeostasis PDF

# Chapter (2) HOMEOSTASIS - Homoeostasis and homeostatic mechanisms ## ECF - Internal environment The ECF which constitutes the internal environment has a remarkably constant volume, hydrogen ion concentration and osmotic activity, as well as concentration of solutes like glucose, sodium, potassi...

# Chapter (2) HOMEOSTASIS - Homoeostasis and homeostatic mechanisms ## ECF - Internal environment The ECF which constitutes the internal environment has a remarkably constant volume, hydrogen ion concentration and osmotic activity, as well as concentration of solutes like glucose, sodium, potassium and gases (oxygen, carbon dioxide). Tissue perfusion pressure (which depends on the blood pressure) is also maintained within a narrow range. Mechanisms have evolved in the body to keep the internal environment relatively constant. These mechanisms are called homeostatic mechanisms and the state of relative constancy of the internal environment of the body, as a result of dynamic equilibrium, is called homeostasis. (homeo = same; stasis = constancy). ### Homeostasis ensures optimal cell function. Cell function depends upon the sum total of chemical reactions going on inside the cell, and the rates of chemical reactions are determined by enzymes and the availability of substrates. Without the chemical reactions that produce metabolic energy, the cell cannot survive. The activity of most of the enzymes is optimal at a temperature of 37° C and a pH (negative log. of H* concentration) of 7.4. Changes in the temperature and pH of the ECF eventually lead to similar changes in the intracellular fluid, adversely affecting enzyme activity and cell function. Thus, maintenance of ECF temperature at about 37° C ("temperature homeostasis") and ECF pH within a narrow range (7.4±0.05) ("H* homeostasis") ensures optimal cell function. Failure to maintain osmolality of the fluids surrounding the cells at 290 ± 5 mOsm/kg leads to osmotic movement of water out of or into the cells, resulting in shrinkage or swelling of the cells. In the case of blood cells, swollen cells may eventually burst. Failure of blood pressure homeostasis leads to hypertension and poor perfusion of tissues or to a disease state called hypotension. ECF volume is constant ### Homeostatic mechanisms operate on the principle of negative feedback. In its simplest form, when the direction of change in a system produced by a disturbance (called the stimulus) and the resulting response of the system are opposite, it is referred to as negative feedback. | Disturbance | Response | |---|---| | Increase | Decrease towards set value | Net effect of negative feedback: the change caused by the disturbance is minimized. The set value remains more or less the same (homeostasis). When the resulting response of the system is the same as that produced by the disturbance, it is referred to as positive feedback. | Disturbance | Response | |---|---| | Increase (A) | Further increase (B) | (A+B) then becomes the stimulus, evoking a greater response (C) in the same direction as shown below. | Stimulus (disturbance + initial response) | Response | |---|---| | Increase (A+B) | Further increase (C) | (A+B+C) then becomes the stimulus, evoking a still greater response in the same direction. As this self-generating process goes on, the change (an increase in this example) from the set value becomes progressively greater. Positive feedback mechanisms are utilized when sudden dramatic change within a very short period is required. Thus, ## Negative Feedback Mechanisms ### At the molecular level e.g. in a chemical reaction, y and z are enzymes and C is the end product | | | | |------------|----------|-------------| | y | z | C | | A | B | | If C can inhibit the enzymes responsible for its production, then the concentration of C will remain more or less constant because any rise in its concentration (the stimulus) will cause increasing inhibition of the enzymes resulting in a fall in concentration of C back towards the previous value. Similarly, any fall in its concentration (the stimulus) will cause decreasing inhibition of the enzymes resulting in a rise of concentration of C back towards the previous value (e.g. cholesterol formation). ## Negative Feedback Mechanisms ### At the organ level | | | | | |----------------|-----------------|--------------|-------------| | (examples) | | | | | 2.1 a rise in | stimulation of | insulin action | e.g maintain | | blood glucose | insulin secretion | (stimulates uptake | blood glucose | | | from pancreas | of glucose into the | | | | | cells) | | | 2.2 | | | | | A, B, C are endocrine organs producing respectively | | | | | hormones p, q, and z. | | | | | Say p stimulates B to secrete q, and q in turn stimulates C to secrete z. | | | | | (A) | Ⓑ | P | १ | | | | | Z | If z inhibits secretion of hormones p and q by A and B, then the concentration of z will remain more or less constant because any rise in its concentration (the stimulus) will cause increasing inhibition of the hormones p and q resulting in a fall of concentration of z back towards the previous value. Similarly, any fall in its concentration (the stimulus) will cause decreasing inhibition of the hormones p and q resulting in a rise of concentration of z back towards the previous value. e.g. Thyroid hormones maintain its own plasma level (thyroid hormone homeostasis) by negative feedback effect on the anterior pituitary. ### Reflex arc & components (3) Generally, homeostasis is achieved by means of a system consisting of: (a) a detector, which detects the deviation from normal (i.e. change in the environment, which is also known as stimulus). (b) a regulator, eg baroreceptor receives information from the detector and integrates it with other information received from other detectors; sends out impulses to the effector organs. (c) an effector, gives the response to reduce the deviation i.e. counteracts the stimulus (negative feedback). (If the effect is to cause greater deviation, then it is a positive feedback mechanism). The new state is continuously assessed by the detector and the regulator is given fresh information. The activity of the regulating device is therefore constantly modified on the basis of the information fed to it from the detector. Such control systems are called "FEEDBACK" systems. | | | | |------------|-------------------------------------------|----------------------| | INPUT | REGULATOR | OUTPUT (a) Open loop system | | INPUT | REGULATOR | OUTPUT (b) Feedback (or) Closed loop system | | | DETECTOR | | Most functions in the body operate in a negative feedback fashion. e.g. a rise in blood pressure is counteracted by a fall in blood pressure so that blood pressure is maintained in a narrow range optimal for tissue perfusion (blood pressure homeostasis). | STIMULUS | DETECTOR | INTEGRATING CENTRE (REGULATOR) | |---------------------------------|------------------------------|------------------------------| | Increase in blood pressure (BP) | baroreceptors: | inhibition of | | | stimulation of sensory nerves | in brain stem | | | | decreased sympathetic discharge | | A fall in BP towards normal | Decreased cardiovascular activity | | ## Positive Feedback Mechanisms ### At the molecular level 1.1 Self-regeneration of digestive enzyme trypsin from trypsinogen since trypsin, once formed, hydrolyses trypsinogen. Trypsin on comes in 46. digestion come enterokinase Trypsinogen trypsin protein digestion comE 1.2 Thrombin, once formed, activates the factors (V and VIII) that help in its formation, thus making the blood coagulation much faster. coagulationy Jon enzymes ### At the cellular level | (cell membrane only) | (.: cellular level) | |--------------------|---------------------| | Voltage change across | Nat entry | | cell membrane | | | | V-gated Nat channel | | | (+) | | | more voltage change | | | More Nat channels open | | | more Nat entry | ### At the organ level | | | | |---------------------------------------|-----------------------------|-------------------------------------------------------------| | Contraction of uterus | Descent of fetus in birth canal | Nerve com a que discharge ??? | | | | Discharge of strech receptros in birth canal | | | | Further contraction of uterus-release of oxytocin from posterior pituitary | Because of the positive feedback, uterine contractions progressively become powerful enough to expel the baby out of the uterus ("parturition"). In the above examples of positive feedback, it can be seen that they contribute towards the long term goal of homeostasis e.g. rapid formation of trypsin accelerates protein digestion and restoration of plasma amino acid levels; rapid changes in membrane potentials ( or voltages) ensures generation of electrical signals coction potential) essential for working of the nervous system as a homeostatic mechanism mechanism, thrombin accelerates blood coagulation so that a homeostatic state called haemostasis (see blood physiology) is achieved; parturition ensures perpetuation of species, thereby maintaining homeostasis through generations. When, however, the negative feedback mechanisms fail, the positive feedback mechanisms cause the body functions to deteriorate rapidly. e.g. a severe fall in blood pressure decreases blood supply to the heart, and this causes a fall in pumping activity of the heart leading to a further fall in blood pressure. This vicious cycle continues until heart can no longer pump. (-)ve FB apply (+) FB OEMALE ဆို ခွာ