Lecture for Year 2 Medical Students on 29 June 2001 by Professor Omar Hasan Kasule Sr.
A. CONCEPT OF FEED-BACK
THE QUR'ANIC CONCEPT OF THE GOLDEN MEAN
Four Qur’anic concepts relate to the golden mean: mizan, wasatiyah, tadhafu’u, and I’itidaal.
Concept of mizan: 42:17, 55:7, 57:25. Concept of wasatiyyat (2:143) and following the middle moderate course (MB # 2014 p
987). Concept of tadafu’u (2:251, ). The concept of I’itidal: 82:7.
The practical life and teachings of the prophet emphasise the concept of central tendency and avoiding either extreme. The
best of all matters is the middle one. There must be a balance in eating and drinking, dress, housing, sleep & wakeing
up, movement and rest, marriage, release (istifragh) and(ihtibaas)
HOMEOSTASIS AND EQUILIBRIUM
Scope of equilibrium: Between internal and external environment. Between
opposing actions and phenomena. Between change and constancy. The internal environment is controlled by: autonomic nervous
system, local messages from one cell to an adjacent one, and hormones. The autonomic control of viscera is independent of
Definition of homeostasis: WB Cannon coined the word homeostasis to
refer to the ‘various physiological arrangements which serve to restore the normal state once it has been disturbed.
Mammals are able to maintain their internal environment within a narrow range. Examples of homeostasis: Hunger & Satiety,
Thirst, Temperature, Cardiovascular, Ph control, catecholamine secretion, oxytocin, TSH, ACTH, GH, sexual behaviour, angiotensin,
insulin secretion, cholesterol synthesis, adrenal cortex secretions (glucocorticoids, mineralocorticoids), adrenal medulla
secretions (epinephrine and nor-epinephrine), Bone remodelling due to stresses & disuse atrophy, muscle hypertrophy and
atrophy, erythropoietin secretion.
Tauhid and homeostasis: Tauhid implies that the whole cosmos and what
it contains have one deliberate creator. Thus all the contents must relate to one another in some harmoneous way. It is unthinkable
that the one creator could create systems that are contradictory to one another. The harmony must however be looked at in
a dynanic way. Because there are constant changes, there must arise from time to time contradictions in the state of flux.
There must therefore exist mechanisms for restoring the status quo after the changes have been accomplished. Thus tauhid and
homeostasis are closely related.
STABILITY AMIDST CHANGE
Feed-back: Feed-back may be positive or negative. Negative feed-back
operates in homeostasis deviations from the norm are detected by specialised sensors and compensatory changes are triggerred
to restore the status quo. Many human diseases are actually manifestations of these compensatory mechanisms. Negative feed-back
also occurs in natural inanimate systems as well as human-made machines. On the other hand positive feed-back: a very dangerous
concept. If a change occurs, positive feed-back will reinforce it in a cyclic way that causes more of the same until all homeostasis
Concept of the baseline and change: The base-line is not constant.
It changes with the environment and time. Muscle hypertrophy is a new level of homeostasis. Types of variation: within the
range of normality & outside the range of normality. Temporary & permanent changes: Temporary: sleep (nawm): 30:23,
2:255, 25:47, 78:9, Physical fatigue, Psychological fatigue (irhaq nafsi/masahaqqa nafsiyat): 18:73, 72:6, 18:80. Permanent
changes: Lameness, Blindness:
Internal and external milieu. The human body is in harmony with its
internal and external milieu. The internal and milieu and the external milieu are themselves in harmony. Internal environment:
Hormones, fluids etc. External environment: water, air, elements, energy. Hormones are chemical messengers. Main areas of
hormone production: pituitary, thyroid, parathyroid, adrenal, pancreas, kidney. Human anatomy and physiology is ground of
interaction with both the internal and external
Central control: Cellular homeostasis is controlled by the nervous
system and the endocrine systems. The two systems are integrated in the hypothalamus.
B. CARDIO-VASCULAR HOMEOSTATASIS:
The control centers are in the spine, medulla oblongata, the cortex, or the chemoreceptors. The input is from:
baro-receptors (arterial, cardiac, and pulmonary), chemo-receptors (oxygen, carbon dioxide, and metabolites), cerebellum (muscle
activity and posture), and cerebrum (sleep, emotions, and reflexes). The output is through the autonomic nervous systemto the heart and blood vessels. The hormonal factors involved are: autocoids, adrenal
medulla catecholamines, adrenal cortical hormiones, vasopressin.Oxygen and carbon
dioxide have a direct effect on the smooth muscles of the pre-capillary sphincterds of arterioles. Kidney auto-regulation
also keeps blood flow at a constant 1 liter/minute.The integrated control of cardiac output depends: sympathetics, catecholamines,
and increased venous return.
Cardiac output is increased: anxiety and excitement 50-100%, eating 30%, exercise 70%, high environmental temperature,
pregnancy, epinephrine. Cardiac output is decreased by sitting, rapid arythmia, heart disease. Sleep and moderate change in
environmental temperature have no impact.
Control of blood circulation: depends oncardiac output, resistance
of arterioles (vasoconstriction & vasodilation), volume of blood in veins (venoconstriction & vasodilation). Both
the nervous system and hormones are involved. Nervous control of blood pressure is of two types: vasomotor nerves and baro-receptor
reflexes. Vasomotor nerves are either sympathetic which are adrenergic and cause vasoconstriction or are parasympathetic which
are cholinergic and cause vaso-dilation. Baro-receptor refelexes are mediated via carotid and aortic sinuses. The renin-angiotensin
system plays a role in BP control. Increased circulating volume increases cardiac output and blood pressure. Emotional factors
such as fear, anger and anxiety increase heart rate and blood pressure. BP decreases on sleep and rises on wal\king up due
to mobilisation of accumulated fluid.
C. RENAL EXCRETORY HOMEOSTATASIS:
D. RESPIRATORY HOMEOSTASIS
Neural and chemical mediators are involved in respiratory control. The respiratory rate is controlled by: the respiratory
center, stretch receptors in the lungs, peripheral chemoreceptors, and central chemo-receptors. The diaphragm and the intercostals
are responsible for respiratory movements. The respiratory rate is 6-10/minute at rest and can rise to 70/minute in exercise.
Sinus arythmia (change of heart rate with respiration).
Glucose is the final pathway ofcarbohydrate metabolism. The level
of glucose in the nblood is finely controlled. Three basic mechanisms are used: (a) glucose production in the liver (b) uptake
of glucose from peripheral tissues (c) insulin secretion. Insulin is a major anabolic hormone whose functions are: (a) trans-membrane
transport of glucose and amino acids (b) glycogen formation in liver and muscles (c) glucose conversion to triglycerides (d)
nucleic acid synthesis and (e) protein synthesis. Both obesity and diabetes mellitus are examples of breakdown of this control.
Glucose balance depends on dietary intake, rate of entry into cells, and the homeostatic activity of the liver. There is a
balance among glu\ycolysis (breakdown of glucose into pyruvate for entry into the Embden-Myerhoff glycolytic pathway, the
Krebs cycle), gluconeogenesis (synthesis of glucose from amino acids, galactose, and fructose), and glycogenolysis (breakdown
of glycogen to form glucose). There are glycogen stores in the liver and the muscle. Liver glyvogen is usable by all parts
of the body whereas muscle glycogen is used only for muscle anerobic processes.
The level of free plasma fatty acids is finely controlled. It is increased by: lipolysis of adipose tissue, as
a result of digection. It is decreased by incorporation into lipoproteins, binding to albumin, and deposit as triglycerides.
Plasma cholesterol is also kept in balance. It is increased by: dietary intake, liver synthesis through a negative feed-back
control, and breakdown of lipid stores. It is removed through feces and bile salts. Many compounds are synthethised from cholesterol.
Hypercholesterolemia and hyper lipidemia are states of imbalance that are associated with coronary heart disease. High dietary
intake decreased body cholesterol synthesis.
Intake of nitrogen is dietary. The output is by excretion is in the form of excretion as
urea and ammonia; about 80% of the ingested nitrogen is excreted as urea.