ECE 593- 728 Chapter One

What is physiology? Physiology is the study of how living organisms work. Physiologist are interested in function and integration- how parts of the body work together at various levels of organization and, most importantly, in the entire organism. The human body is organized into cells, tissues, organs, and organ systems.

What are cells? Cells are the simplest structural units into which a complex multicellular organism can be divided and still retain the functions characteristic of life. Cells can be classified broadly into four types namely muscle cells, nerve cells, epithelial cells, connective tissue cells. Muscle cells: these are specialized to generate the mechanical forces that produce movement. There are three types of muscle cells—skeletal, cardiac, and smooth. These muscles differ from each other in shape and in their location in the various organs of the body.

Cont… Nerve cells: these are specialized to initiate and conduct electrical signals, often over long distances. A signal may initiate new electrical signals in other nerve cells, or it may stimulate a gland cell to secrete or a muscle cell to contract. Thus, nerve cells provide a major means of controlling the activities of other cells. Epithelial cells: these are specialized for the selective secretion and absorption of ions and organic molecules, and for protection. Epithelial cells are located mainly at the surfaces that cover the body or line the walls of various tubular and hollow structures within the body. Connective tissue cells: these are cells that connect, anchor and support the structures of the body. Examples include; bone cells, fat storing cells, red blood cells and white blood cells.

What are Tissues, Organs and Organ systems? A tissue is a group of specialized cells that have similar structure and function together as a unit. There are four main tissue types in the body: muscle, nervous, epithelial and connective tissue. Each is designed for specific functions. An organ is a group of tissues that perform a specific function or group of functions. Examples of organs include; the heart, lungs, kidneys etc

Cont… An organ system is composed of two or more different organs that work together to provide a common function. There are 10 major organ systems in the body, they are the: skeletal, muscular, circulatory, nervous, respiratory, digestive, excretory, endocrine, reproductive and lymphatic system.

Homeostasis This is the property of living organisms; it is defined as a state of reasonably stable balance between physiological variables. What Characterizes the homeostatic control system Homeostatic control system is characterized by a feedback system; there are basically two types of feedback system— negative and positive feedback system.

Negative feedback system This is a system in which an increase or decrease in the variable being regulated brings about responses that tend to move the variable in the direction opposite the direction of the original change. A good illustration of the negative feedback mechanism can be seen on the next slide.

Negative feedback of Cortisol

Cont… Cortisol is one of the hormones in the human body that raises the level of glucose in the blood. When there is low glucose in the blood, the hypothalamus stimulates corticotrophin releasing hormone, which in turn stimulates the anterior pituitary gland. When this gland is stimulated it releases ACTH which then stimulates the adrenal cortex. The adrenal cortex then releases cortisol, cortisol raises the blood glucose level by stimulating the liver to produce glucose from stored non-carbohydrate sources such as proteins and lipids and to release it into the blood.

Cont… On the contrary, when there is excess glucose in the blood, cortisol can inhibit its production by either a short loop or a long loop negative feedback system. In the short loop, cortisol inhibits the anterior pituitary gland and prevents it from secreting ACTH, thus slowing the production of cortisol. In the long loop, cortisol can directly inhibit the hypothalamus and prevent it from secreting CRH also shutting down the production of cortisol and thus maintaining homeostasis.

Positive Feedback Mechanism This is a system in which an increase in the variable being regulated brings about responses that tend to move the variable in the direction same as that of the original change. The end result of a positive feedback is often amplifying and explosive.

Example of positive feedback A good illustration of positive feedback mechanism can be seen with parturition ( birth). As the uterine muscles contract and its walls are stretched during labor, signals from the uterus are relayed via nerves to a gland at the base of the brain called the posterior pituitary gland. This gland responds by secreting the hormone oxytocin, which is a potent stimulator of further uterine contractions.

Components of Homeostatic Control Systems Reflexes: A reflex is a specific involuntary, unpremeditated, unlearned response to a particular stimulus. Examples of such reflexes include pulling your hand away from a hot object or shutting your eyes as an object rapidly approaches your face. The pathway mediating a reflex is known as the reflex arc.

Components of Homeostatic Control Systems The reflex arc compose of several components namely; stimulus, receptor, integrating center, afferent pathway effector and efferent pathway. Stimulus: a stimulus is defined as a detectable change in the internal or external environment, such as change in temperature, or blood pressure. Receptor: a receptor detects the environmental change; it is usually acted upon by the stimulus to produce a signal that is sent to the integrating center.

Intercellular Chemical Messengers There are three categories of intercellular chemical messengers they are: hormones, neurotransmitters and paracrine and autocrine agents. Hormones: these are chemical messengers that carries signal from one hormone secreting cell or group of cells to another via the blood. Examples are epinephrine, dopamine, gastrin etc.

Cont… Neurotransmitters: these are chemical messengers that aid communication among nerve cells. Nerve cells are able to alter the activities of other nerve cells by releasing from its ending a neurotransmitter that diffuses through the extra cellular separating the two nerve cells and acts upon the second. Examples are acetylcholine, nitrogen monoxide etc. Paracrine agents: these are chemical messengers that exert its effect on cells near its secretion site. Autocrine agents: these are chemical messenger secreted into extra cellular fluid that acts upon the cell that secreted it.