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Control – Physiology

The Common Vein copyright 2007

Principles

We have previously discussed how the nervous system and the endocrine system control the body functions with the whole biological unit (in this case the person) in mind.  We will start to advance the complexity of these control systems in this module.  There are other control systems that need elucidation.  For example the destiny of the individual is predetermined to some extent by the genes with which the person is born.  As science and biology progress our abilities to read and modify the genetic instructions are being slowly realised.  In addition, there are systems that are local to the cell, tissue and organ that are also functioning to optimise the efficiency of the organism, that enable it to adapt to the changing environmemt, and direct the system toward a more orderly state.  Within these systems there are operations such as the negative feedback mechanisms that control supply, need, and demand.  These operate at both the local as well as systemic levels.  Chemical and mechanical stimuli are sensed locally and the reactive mechanisms are genenerated through mechanical and chemical effects.  At our basic abilities we really are only able to do two things – secrete and contract.  Secretion is our chemical efferent and contraction our mechanical efferent..

Genetic Control

The Helical Double Stranded Structure of the Gene
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Genetic control not only carries information of evolution spanning thousands of years but also information that is important in the day to day lives of the cell.  The genes determine the daily schedule of activity by controlling the synthesis of proteins and enzymes.  The proteins for example may be lipoproteins that form the infrastructure of the organelles in the cell and the enzymes have extensive involvement in innumerable functions in the body including the syntheis of more complex structures and molecules such as glycogen, lipid and ATP.  Without these raw materials the factories of the body cannot produce and so the genetic component in controlling the production oand refining of raw materials control the overall functioning of the organism as well –  both in the timespan of a single life but also in the day to day activity and health of the organism or biological unit.

 The genes are nucleic acids and are made from a double strand of  DNA (deoxyribonucleic acid). The DNA controls the formation of RNA (ribonucleic acid) which is able to leave the inner sanctum of the nucleus and transmit the work list and the recipes to the workers in the cytoplasm.  In so doing both strutural elements and functional elements result, and hence both structure and function are controlled indirectly and directly by the genes.

Power of the Genes
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Nervous System

The nervous system has a slightly different picture in mind than the genes do.  The most basic and primitive function is survivability of an individual.  The reflex action of a person after touching a hot stove is an almost instantaneous reflex withdrawal of the hand without a concious thought necessarily being involved.  This particular reflex is initiated from a peripheral sensory nerve and executed through the spinal cord without getting the brain involved.  The autonomic nervous system also functions automatically.  It controls and integrates bodily functions. Some of these functions are vital at a second to second time interval including the maintenance of  breathing and heart beat, while others such as digestion are important at an hour to hour time interval.  The processing component of the autonomic nervous system though is within the lower levels of the brain.  At a higher level the control arises from decision making from what clothes to wear,  to where one will live and with whom we will live, and what work or profession we will be best suited for.  This aspect currently goes beyond the boundaries of this module but the perspective of control and decision making is certainly an important facet to touch upon.

We thus can recognise three levels of the nervous system that have functional control.  The spinal cord, the lower brain level and the higher brain or cortical level.  The lower brain consists of the medulla, pons, mesencephalon, hypothalamus, thalamus cerebellum and basal ganglia each of which has its own specialised function in the system. The cortical level contains 75% of all the cell bodies and appears to be an ougrowth of the thalamus.

The Autonomic Nervous System

The autonomic nervous system receives sensory input from the organs at three major centres; spinal cord; brain stem; hypothalamus. From these centres the processing and coordiantion of bodily needs takes place and the efferent system is executed via sympathetic and parasympathetic nerves.

Sympathetic Efferent System
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Parasympathetic Efferents
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Endocrine System

Pituitary Gland – Master Endocrine Organ
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The endocrine system together with the brain are the major controlling systems in the body.  They have both discrete and unique function but also collaborate at some levels.  The pituitary as the master endocrine organ is in fact intimately related to the nervous system.  It lies within the cranium at the base of the brain and it is directly connected to the brain via the hypothalmus .  The nervous systema nd endocrine system also share some ogans.  For example the adrenal gland has a medullary portion that is under the control of the sympathetic nervous system and a cortical component that is under the control of the pituitary.

Specialized Functions of the Endocrine System – Metabolic Control
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The pituitary gland is the central organ of the endocrine system and in general helps to control the metabolic functions of the body – perhaps the equivalent of the chief operating officer for maintenance of system operations.  The endocrine system in general controls transport of substances into the cells, the rate of processing, and the rate at which the products of the cells are exported.  Some hormones such as gastrin, are secreted locally in response to a local event, are absorbed into the blood and have distant effects.  Otheres are secreted directly by an endocrine organ into the blood and have far reaching systemic effects.  Insulin from the pancreas is such an example being produced by the beta cells of the Islets of Langerhans and causing muscle fibres to take up glucose, convert it to glycogen, and take up amino acids from the blood and convert them to protein.  It also acts on the liver cells causing them to take up glucose and convert it to glycogen, inhibiting gluconeogenesis, as well as acting on adipose tissue.  Lastly it acts on the hypothalamus to reduce appetite.

Negative Feedback – Mechanisms in Self Control

The concentration and therefore the activity of the hormones is controlled by the needs of the tissues  through a  negative feedback mechanism.  This means that once the concentration of hormone or chemical to produce a certain level of function is achieved then a message inhibiting the continued production is sent to the organ of origin. This principles of supply, need, and demand are universal in many of the body’s controlling systems, and not surpisingly in fact, a hallmark economic principle as well.  So for example after a large meal the glucose level in the serum will rise stimulating the pancreas to prooduce insulin, which will distribute the glucose to the muscle , liver and fat cells, causing the glucose to transfer from the serum to the cells resulting in a relative fall in the glucose level. At a defined lower level of glucose the production of insulin will be turned off by negative feedback.  This mechanism is applied to many production systems in the body.

Cardiovascular System

The systemic integrative function of the cardiovascular system is controlled mostly by the autonomic nervous system and to lesser extent by the endocrine system, but there are local mechanisms at the arteriolar level that control the flow of blood to a particular organ.. The coronary artery for example will respond to local metabolic needs by vasodilating in the presence of increasing levels of carbon dioxide and decreasing levels of oxygen.  In 1998, three American scientists were awarded the Nobel prize for their independant work on the effects of nitric oxide on the smooth muscle cells of the blood vessels. manifesting  Nitric oxide  is generated by many cells in the body, but its production by vascular endothelium is particularly important in the regulation of  blood flow.

Respiratory System

The systemic integrative function of the respiratory system is controlled mostly by the autonomic nervous system and to lesser extent by the endocrine system, but there are local mechanisms at the bronchovascular  level that control air and blood flow to the alveoli..  A concept relating to the ventilation perfusion ratio is a local mechanism that reflects the integrative function of the blood flow and ventilation of the pulmonary units.  If for example there is no ventilation to a particular portion of the lung as seen when there is air trapping or a large air space called a bulla is present, the perfusion or blood flow to that region diminishes.

Within the Right Middle Lobe – Air Trapping – Vasoconstriction of the Vessels

This series of images shows some subtle changes that reflect the local control of blood flow to a small segment of the right midlle lobe.  Note that in image a, there is a small area of increased lucency (blacker) in the right lung just lateral to the vesels of the right hilum.  This regoin is highlighted in b.  Note also that in b, the rapid diminution of the size of the blood vessel to that subsegment when compared to the size change of the vessels in the image in c.  The lucent appearance of the lung suggests air trapping and the vasoconstriction reflects decreased perfusion – ie with decreased ventilation there is an associated consequent associated decerease in perfusion.

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Gastrointestinal System

The systemic integrative function of the gastrointestinal system is controlled mostly by the endocrine system and to lesser extent by the autonomic nervous system, but there are local mechanisms that are initiated and executed at the mucosal level that control the way the food is handled.  The transport, digestion, processing and absorbtion of  food, is a time consuming and time sensitive  process and integrated function of the gastrointestinal tract with the accessory organs of digestion requires complex integrative mechanisms.  For example at a very simple level, when one thinks about, sees or smells food, the brain will send messages of preparation through the autonomic nervous system to start to prepare the gastrointestinal tract for the meal.  As the food bolus passes throgh the esophagus it causes distension of the esophagus at the intake level which induces a peristaltic wave to propel the food.  In addition it will cause the receiving segment to relax to enable the food to be received.  This pattern is repeated many times as the food is pushed down the esophagus.  The stretch receptors in the esophagus also send a message to the gastroesophageal junction to relax at the appropriate time so that the food can pass into the somach without resulting in reflux of gastric contents.  Once the food has passed into the stomach, the GE junction needs to close once again.  In the stomach there are mechanical and chemical sensors that determine the bulk as well as the chemical makeup of the food.  The mechanical sensors will induce a churning effect, while the chemical sensors will send messages downstream via gastrin which is released by the antral cells and subsequently absorbed into the circulation.  The pylorus as a result will open, the GE junction will close and even further downstream the ileocecal valve will open. Gastrin is also particulalrly sensitive to the prresence of meat in the food and will cause the fundic cells to produce acid.

The duodenum has chemical sensors that are sensitive to fat in the diet, acidic PH of the chyme and also the presence of protein.  If the PH is too low (acidic), and or the fat or protein content high, then the enterogastric reflex causes emptying of the stomach to slow down. The presence of fat in the small bowel will cause the release of another hormone called cholecystokinin (CCK), which like gastrin is secreted by mucosa and absorbed into the circulation.  The circulating CCK finds appropriate receptors in the smooth muscle of the galbladder and causes the gallbladder to contract, and shincter of Oddi to relax.  Bile is thus released into the duodenum via the biliary ductal system which aids in the digestion of fats and fatty acids.

As we have already seen in the discussion of the endocrine system the accessory organs of digestion including the liver and pancreas are controlled by factors such as chemical levels of glucose for example in the bloo and in the tissues.

Genitourinary System

Urinary System

The systemic integrative function of the urinary system is controlled equally by the endocrine system and by the autonomic nervous system, but there are also local mechanisms that are initiated and executed at the mucosal level that control the way the water, electrolytes and urine is handled.  The filtering, transport, refiltering, and voiding mechanisms have some general as well as unique features.  The countercurrent mechanism in the tubes of Henle – controlled and motivated by chemial and osmotic gradients is particulalrly ingeneous and unique to the kidney.

Blood flow is a key factor to kidney function and is controlled both by the autonomic nervous system, localised arteriolar mechanisms.  Neurohormonal control includes antidiuretic hormone (ADH) secreted by the hypothalamic posterior pituitary system and renin angiotensin mechanisms.

Genital System

Female

The systemic integrative function of the female generative organs is controlled mostly by the endocrine system with some input by the nervous system as well. The monthly cycle is almost entirlely controlled by an hormonal system that originates in the anterior pituitary gland which secretes follicular stimulating hormone (FSH) and luteinizing hormone. (LH).  These two hormones in the female specifically target the ovary and their cyclical interrelated function is quite unique and under the evolutionary control incorporated in the genetic makeup of the species. The FSH and LH act in concert performing similar but different functions causing a progressive chain reaction of events that will evolve in one of two ways depending on whether the female becomes pregnant in the cycle.  The FSH and LH stimulate the production of adenyl cyclase which in turn increases the cyclic AMP in the cells causing growth a primitive follicle in the ovary.  The follicle starts to produce estrogen and progesterone under the influence of FSH and LH.  Just prior to ovulation and rupture of the follicle the concentration of LH surges by about 10 fold and the FSH increases by about 2 fold.  If the woman does not fall pregnant – the FSH and LH as well as estrogen and progesterone levels fall and the cycle is repeated in the new cycle.  If on the other hand the woman falls pregnant then the evolving placenta will secrete human gonadotrophic hormone which will sustain the function of the corpus luteum.  The progesterone and estrogen A primary follicle will mature under the control of FSH, will rupture and release an ovum, and then under the influence of LH develop into a corpus lutem

Follicles in a Reproductive Female – Cyclical Phases -Size and Time
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Evolving Dominant Follicle
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Ovulation – Mid Cycle
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but there are also local mechanisms that are initiated and executed at the mucosal level that control the way the water, electrolytes and urine is handled.  The filtering, transport, refiltering, and voiding mechanisms have some general as well as unique features.  The countercurrent mechanism in the tubes of Henle – controlled and motivated by chemial and osmotic gradients is particulalrly ingeneous and unique to the kidney.

MSK Skeletal System

Muscular System

Relaxion and Contraction of the Biceps
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Acute Hemorrhage in the Left Cerebellar Vermis
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A Single Disastrous Event
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References

Nitric Oxide – Cardiovascular Physiology by Richard Klabunde PhD

Physiology on Line Autoregulation of Glucocose Production  Tappy et al