Chapter 1 : Introduction Dr. Haseeb Sattar Human Physiology Chapter 1 : Introduction Dr. Haseeb Sattar

Physiologia ( Greek ) – Natural Knowledge What is Physiology ? Physiologia ( Greek ) – Natural Knowledge Physiology is the study of the functions of the living organism as a whole or its constituent parts. Or Physiology is the discipline that deals with the bodily functions and their control.

Human Physiology Human physiology is the science of the mechanical, physical, and biochemical functions of human in good health, their organs, and the cells of which they are composed. The principal level of focus of physiology is at the level of organs and systems.

17.02.2019 Physiology Department in Sichuan Medical University

Levels of physiological research

HOMEOSTASIS (FROM THE GREEK WORDS FOR “SAME” AND “STEADY”) Maintenance of static or constant conditions in the internal environment

HOMEOSTASIS CONT… The term homeostasis was introduced by Walter Bradford Cannon of USA. Milieu Interne (internal environment)- French physiologist Claude Bernard Cells of our body are bathed in ECF. Also the interior of the cell contains ICF. Composition, volume, pH, temperature of the internal environment are almost always constant in health.

HOMEOSTASIS CONT… Our body contains numerous regulatory mechanisms that maintain the constancy of the internal environment in spite of the challenges that tend to destabilize. These regulatory mechanisms are called homeostatic mechanism and the whole process is called homeostasis. Ex- 1. The total amount of ECF should be constant. But after a severe diarrhea, the volume of ECF threatens to shrink, also the concentrations of sodium threatens to alter.

HOMEOSTASIS CONT… The homeostatic mechanism sees that despite the diarrhea, volume of ECF and sodium remain within reasonably safe limit. Ex 2. The BP value should remain within safe limits. If suddenly severe panic situation develops, the BP shoots up and there may be a cerebrovascular accident. fggggggh

HOMEOSTASIS-BP REGULATION The sinu aortic mechanism, a homeostatic mechanism, tries to bring back the rising value of BP.

How homeostasis is achieved? (Regulation of Physiological activities) A. Nervous regulation B. Humoral regulation C. Auto regulation

A. Nervous Regulation Our nervous system is made up of specialized nerve cells that transmit electrical impulses throughout our body. There is a special kind of nerve structure called receptor. It senses temperature, light, sound, pressure and chemical stimuli. Effector (muscles or glands) are special structure which respond to the commands of the nervous system. Once a stimulus is detected by the receptor nerve , impulses are created. Eventually the effector muscles or glands will respond to the change.

NERVOUS REGULATION CONT… Reflex Knee jerk reflex

NERVOUS REGULATION CONT…

Reflex arc Receptor Afferent (sensory) nerve Reflex centre (Brain or spinal cord) Efferent (Motor) nerve Effector

B. Humoral Regulation “Humoral” refers to the fluid of the body, in other words the extracellular fluid (ECF, which includes plasma). The endocrine cell responds to changes in the concentration of a substance in the ECF. The hormones act to stabilize and maintain the concentration of the substance within the appropriate physiological range.

Humoral Regulation cont.. Humoral regulation by hormone

HUMORAL REGULATION CONT… 1.Example of humoral regulation – PTH PTH (parathyroid horm.) is secreted by Parathyroid glands. The calcium receptor(sensor) in the parathyroid gland cell detects changes in the ECF calcium concentration. When ECF calcium is high, calcium binds to the receptor and this inhibits PTH secretion. When ECF calcium is low, the receptor is unbound and so there is no inhibition and PTH secretion occurs.

EXAMPLE OF HUMORAL REGULATION – PTH When ECF calcium is high, calcium binds to the receptor and this inhibits PTH secretion. When ECF calcium is low, the receptor is unbound and so there is no inhibition and PTH secretion occurs.

2.EXAMPLE OF HUMORAL REGULATION – INSULIN The most important stimulator of insulin secretion is the concentration of glucose in plasma. Metabolism of glucose leads to the generation of ATP which is the intracellular ligand for a ligand gated K+ channel. ATP binding closes K+ channels causing depolarization of the cell. Depolarization opens voltage-gated calcium channels, calcium enters the cell and triggers exocytosis of secretory vesicles containing insulin.

EXAMPLE OF HUMORAL REGULATION – INSULIN Sulfonylureas and meglitinides are drugs that enhance insulin secretion by binding to and closing the potassium channels for which ATP is the normal ligand.

C. Autoregulation Intrinsic ability of an organ to maintain constant normal environment independent of neural and hormonal influences. It mainly deals with a specific form of homeostasis associated with autoregulation of blood flow despite of changes in perfusion pressure. (By metabolic and myogenic mechanisms) Most clearly seen in kidney, the heart and the brain. Perfusion of these organs (especially the later two) is essential for life. Through autoregulation the body can divert blood (and thus oxygen) where it is most needed.

Renal Autoregulation In the kidneys, autoregulation is primarily concerned with maintaining renal blood flow and glomerular filtration rate. 2 main techniques 1. Myogenic mechanism-as blood flow increases, the afferent arterioles become stretched, they contract and subsequently reduce blood flow. 2. Tubuloglomerular feedback- as blood flow decrease the macula densa senses low BP and causes vasoconstriction of efferent arteriole and dilatation of afferent arteriole , thus increasing GFR.

Cardiovascular Autoregulation In the case of heart tissue that is in a state of high metabolic activity, autoregulation occurs due to increased metabolic bi-products such as lactic acid, CO2, and decreased O2 and causes vasodilation. The autoregulation and coronary vessels is mediated by the equilibrium of ATP, ADP, AMP and adenosine in the myocardial cell.

Cardiovascular Autoregulation When there is a high amount of oxygen, it is shifted towards ATP. When there is lack of oxygen, it is shifted towards adenosine. Adenosine causes vasodilation and therefore increases the supply of oxygen.

Cerebral Autoregulation Cerebral autoregulation is defined as the relationship between cerebral blood flow and cerebral perfusion pressure. Complex neurohumoral processes are involved in myogenic and metabolic mechanism to maintain cerebral blood flow at a constant level.

CEREBRAL AUTOREGULATION CONT… Cerebral perfusion pressure must be maintained within narrow limits. Too little pressure could cause brain tissue to become ischemic and too much could cause increased intracranial pressure (ICP)

FUNDAMENTAL CHARACTERISTICS OF LIVING ORGANISM 1. Metabolism : It is the ability to use energy to perform vital functions such as growth, movement, and reproduction 2. Excitability and Responsiveness : An organism is responsive if it can sense (excitability) changes in the environment and make adjustments that help maintain its life. 3. Reproduction : The formation of new cells or new organisms.

Feedback control systems Feedback is a process in which some proportion of the output signal of a system is fed (passed) back to input. Whenever any changes occurs, system receives and reacts to two types of feedback : 1. Negative feedback mechanism 2. Positive feedback mechanism

1. Negative Feedback Most homeostatic mechanisms involve Negative Feedback , i.e. a corrective mechanism involving an action that directly opposes a variation from normal limits. Here the stimulus produces a response that depresses the stimulus i.e. the stimulus and response are opposite to each other. Ex- Secretion of thyroxin Maintenance of water balance by osmoreceptors &ADH Arterial pressure regulating mechanism by baroreceptors. Regulation of majority of the hormone release

Negative Feedback- example TSH (Thyrotropin) is released from pituitary . TSH stimulates thyroid gland. Thyroid gland secretes thyroxin. When thyroxin level increases it inhibits the secretion of TSH from pituitary. Secretion of thyroxin from thyroid gland decreases.

2. Positive Feedback “Positive” implies that, when a deviation from a normal value occurs, the response of the system is to make the deviation even greater. ( positive feedback also called “vicious cycle”) The initial stimulus produces response that reinforces (exaggerates) the original stimulus. Positive feedback is less common than negative feedback. Eg : Marked fall in BP (stimulus)--- decreased blood supply to heart-----decreased myocardial contraction- --- further fall in BP (response )

Other examples of Positive feedback Parturition- Contraction of uterus---release of oxytocin---- more contraction. Milk ejection reflex. Both of the above involve oxytocin secretion.