Introduction to Physiology and Homeostasis PDF

Chapter 1 | Introduction to Physiology and Homeostasis  CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS Introduction to Physiology Take 1 minute to look at Fig. 1-1. The activities described are a samplin...

Chapter 1 | Introduction to Physiology and Homeostasis  CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS Introduction to Physiology Take 1 minute to look at Fig. 1-1. The activities described are a sampling of the body processes that occur all the time just to keep us alive. We usually take these life- sustaining activities for granted and do not really think about “what makes us live as a normal person,” but that’s what physiology is about. Physiology is the study of the functions of living things. Specifically, we will focus on how the human body works. Your eyes will convert the Your brain will receive and process visual image from this page into input, send output to your muscles to maintain electrical signals that will your posture, move your eyes across the page, transmit information to your and turn the page as needed. brain for processing. Your heart will beat 70 times, pumping 5 liters of blood to your lungs and another 5 liters to the rest of your body. Your lungs will breathe in and out about 12 times, exchanging 6 liters of air with the atmosphere. Your kidneys receive more than 1 L of blood, and will act on it to conserve the “wanted” materials and eliminate the “unwanted” materials producing about 1 mL of urine Your digestive system will act on the last meal, transfer the absorbed elements into the bloodstream and the non-absorbed to the large intestine to form stool. Your cells will consume 250 mL O2, produce 200 mL CO2, and use about 2 calories of energy derived from food to support your body’s metabolisme. Fig. 1-1 example of body processes that occur all the time just to keep us alive 2  CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS levels of Organization in the Body How the body is structurally organized into a total functional unit, from the chemical level to the whole body (Fig. 1-2). These levels of organization make possible life as we know it. I) The chemical level: Like all matter, living and nonliving, the human body is a combination of specific atoms, which are the smallest building blocks. The most common atoms in the body—oxygen, carbon, hydrogen, and nitrogen—make up approximately 96% of the total body chemistry. These common atoms and a few others combine to form the molecules of life, such as proteins, carbohydrates, fats, and nucleic acids (genetic material, such as DNA). These important atoms and molecules are the raw ingredients from which all living things arise. II) The cellular level: The non-living chemical components must be arranged and packaged in precise ways to form a cell which is the basic unit of life in an organism. III) The tissue level: Cells of similar structure and specialized function combine to form tissues, of which there are four primary types: muscle, nervous, epithelial, and connective. IV) The organ level: An organ is a unit made up of several tissue types. Organs consist of two or more types of primary tissue organized to perform particular functions. V) The body system level: A body system is a collection of related organs. Groups of organs are further organized into body systems. Each system is a collection of organs that perform related functions and interact to accomplish a common activity essential for survival of the whole body. For example, the digestive system consists of the mouth, pharynx (throat), esophagus, stomach, small intestine, large intestine, salivary glands, exocrine pancreas, liver, and gallbladder. These digestive organs cooperate to break food down into small nutrient molecules that can be absorbed into the blood for distribution to all cells. The human body has 11 systems: circulatory, digestive, respiratory, urinary, skeletal, muscular, integumentary, immune, nervous, endocrine, and reproductive VI) The organism level: The body systems are packaged into a functional whole body. Each body system depends on the proper functioning of other systems to carry out its specific responsibilities. The whole body of a multicellular organism—a single, 3 CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS independently living individual—consists of the various body systems structurally and functionally linked as an entity that is separate from its surrounding environment. Thus, the body is made up of living cells organized into life-sustaining systems. The different body systems do not act in isolation from one another. Many complex body processes depend on the interplay among multiple systems. For example, regulation of blood pressure depends on coordinated responses among the circulatory, urinary, nervous, and endocrine systems. We must focus on how the different body systems normally work together to maintain the internal conditions necessary for life. Fig. 1-2 levels of Organization in the Body 4  CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS Concept of Homeostasis The cells in a multicellular organism cannot live and function without contributions from the other body cells because most cells are not in direct contact with the external environment. The external environment is the surrounding environment in which an organism lives. A single-celled organism such as an amoeba obtains nutrients and O2 directly from its immediate external surroundings and eliminates wastes back into those surroundings. A muscle cell or any other cell in a multicellular organism has the same need for life- supporting nutrient and O2 uptake and waste elimination; yet the muscle cell is isolated from the external environment surrounding the body. How can it make vital exchanges with the external environment with which it has no contact? The key is the presence of communications between cells together (Intercellular communication) and between cells and the surrounding fluid (internal environment) through which cell make life-sustaining exchanges (Transport across cell membranes) Intercellular communication Direct: through physical contact between the interacting cells e.g. gap junctions Indirect: through extracellular chemical messengers, of which there are four types, on being released into the ECF by appropriate stimulation, these extracellular chemical messengers act on other particular cells (target cells), to exert its effect (Fig. 1-3). Paracrine: local chemical messengers whose effect is exerted only on neighboring cells in the immediate environment of their site of secretion. Neurotransmitters: short-range chemical messengers secreted by nerve terminals in response to electrical signals (action potentials). act locally on an adjoining target cell, which may be another neuron, a muscle, or a gland. Hormones: long-range chemical messengers secreted into the blood by endocrine glands in response to an appropriate signal. The blood carries the messengers to other sites in the body, where they exert their effects on their target cells away from site of their release. Neurohormones: hormones released by neurosecretory neurons into the blood when an action potential reaches the axon terminals. The neurohormone is then distributed through the blood to distant target cells. 5  CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS Fig. 1-3 Methods of intercellular communication Transport through Cell Membrane All the cells in the body must be supplied with essential substances like nutrients, water, electrolytes, etc. Cells also must get rid of many unwanted substances like waste materials, carbon dioxide, etc. The cells achieve these by means of transport mechanisms across the cell membrane. The structure of the cell membrane is well suited for the transport of substances in and out of the cell. Lipids and proteins of cell membrane play an important role in the transport of various substances between extracellular fluid (ECF) and intracellular fluid (ICF). Two types of basic mechanisms are involved in the transport of substances across the cell membrane (Fig. 1-4): Fig. 1-4 mechanisms are involved in the transport of substances across the cell membrane 6  CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS 1. Passive transport Passive transport is the transport of substances along the concentration gradient or electrical gradient or both (electrochemical gradient). It is also known as diffusion or downhill movement. It does not need energy. Passive transport is like swimming in the direction of water flow in a river. Here, the substances move from region of higher concentration to the region of lower concentration. Diffusion is of two types, namely simple diffusion and facilitated diffusion. Simple diffusion of substances occurs either through lipid bilayer or protein channels of the cell membrane. Facilitated diffusion occurs with the help of the carrier proteins of the cell membrane. Thus, the diffusion can be discussed under three headings: I. Simple diffusion through lipid bilayer Lipid bilayer of the cell membrane is permeable only to lipid-soluble substances like oxygen, carbon dioxide and alcohol. The diffusion through the lipid bilayer is directly proportional to the solubility of the substances in lipids. II. Simple diffusion through protein channels Throughout the central lipid bilayer of the cell membrane, there are some pores. Integral protein molecules of the protein layer invaginate into these pores from either surface of the cell membrane. Thus, the pores present in the central lipid bilayer are entirely lined up by the integral protein molecules and form the protein channels for the diffusion of water, electrolytes and other water-soluble substances, which cannot pass through the lipid layer. each channel can usually permit one type of ion to pass through it (selective permeability). Types of Protein Channels Some of the protein channels are always opened (ungated or leak channels) while most of the channels are closed by gates (gated channels). The gated channels are divided into three categories (Fig. 1-5): i. Voltage-gated channels Channels which open whenever there is a change in the electrical potential. For example, in the neuromuscular junction, when action potential reaches axon terminal, the calcium channels are opened, and calcium ions diffuse into the interior of the axon terminal from ECF. ii. Ligand-gated channels Channels which open in the presence of some hormonal substances. The hormonal substances are called ligands, and the channels are called ligand- 7 CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS gated channels. During the transmission of impulse through the neuromuscular junction, acetylcholine is released from the vesicles to the synaptic cleft, and open sodium channels in the postsynaptic membrane. Thus, sodium ions diffuse into the neuromuscular junction from ECF. iii. Mechanically gated channels Mechanically gated channels are the channels which are opened by some mechanical factors, e.g. Pacinian corpuscles (when subjected to pressure, deformation of its core fiber causes opening of sodium channel and development of receptor potential. And hair cells of organ of Corti (movements of the cilia cause opening of potassium channels and development of receptor potential). Fig. 1-5 different protein channels involved in passive transport across the cell membrane 8  CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS III. Facilitated or carrier-mediated diffusion. Facilitated or carrier-mediated diffusion is the type of diffusion by which the water-soluble substances having larger molecules are transported through the cell membrane with the help of a carrier protein. By this process, the substances are transported across the cell membrane faster than the transport by simple diffusion e.g. glucose and amino acids bind with carrier protein. Then, some conformational change occurs in the carrier protein. Thus, the molecule reaches the other side of the cell membrane (Fig. 1-5). Special types of passive transport In addition to diffusion, there are some special types of passive transport e.g. osmosis Osmosis Osmosis is a special type of diffusion. It is the net diffusion of water down its own concentration gradient (movement of water or any other solvent from an area of lower concentration to area of higher concentration of a solute, through a semipermeable membrane that permits passage of only water or other solvents but not the solutes). It occurs also either through lipid bilayer or through specific protein channels named aquaporins (Fig. 1-5). 2. Active transport: Active transport is the movement of substances against the concentration or electrical or electrochemical gradient. It is like swimming against the water tide in a river. It is also called uphill transport. Active transport requires energy, which is obtained mainly by breakdown of high energy compounds like adenosine triphosphate (ATP), and carrier proteins which either transport only one substance in a single direction (uniport carrier), or transport two different substances at a time, in the same direction (symport carrier) or in opposite (antiport carrier). There are two types of active transport (Fig. 1-6): i. Primary active transport Energy is liberated directly from the breakdown of ATP e.g. Sodium/Potassium Pump (transports sodium to outside the cell and potassium to inside the cell). ii. Secondary active transport. Energy for movement of sodium is obtained by breakdown of ATP. And the energy released by the movement of sodium is utilized for movement of another substance. Thus, the transport of sodium is coupled with transport of another substance by means of a common carrier protein, either in the same direction 9  CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS (Cotransport e.g. Sodium/glucose transporter, SGLT) or in the opposite direction (Counter transport e.g. Sodium/calcium exchanger, NCX). Special types of active transport In addition to primary and secondary active transport systems, there are some special categories of active transport, generally called vesicular transport and includes (Fig. 1-6): 1. Endocytosis: internalization of extracellular material within a cell through pinocytosis (cell drinking) or phagocytosis (cell eating) 2. Exocytosis: release of substances originating within the cell to the exterior (cell vomiting) These different methods of transport through the cell membrane allow cells to communicate with the surrounding fluid, keeping the internal environment of the living organism stable and balanced regarding concentrations of nutrients, waste products, O2, CO2, water, electrolytes, pH, blood pressure, and temperature for optimal function of cells. This is simply the definition of Homeostasis. Which is also necessary for normal cell function and survival of the organism. Thus, there is an Interdependent relationship between cells, body systems, and homeostasis. And the aim of studying human physiology is to study the functions of different body systems and how they work together to maintain homeostasis. Fig. 1-6 Examples of different types of active transport across the cell 10 membrane  CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS Mechanism of maintaining homeostasis To maintain homeostasis, the nervous and endocrine systems orchestrate a range of adjustments that help the body maintain homeostasis in response to any stress through Detection of deviations of the internal environmental factor from normal value, integration of this information with any other relevant information, and making appropriate adjustments responsible for restoring this factor to its desired value. Components of homeostatic system The homeostatic system in the body acts through a self-regulating mechanism, which operates in a cyclic manner (Fig. 1-7). This cycle includes 3 components: 1. Sensors or detectors: recognize the deviation of the internal environmental factor from the normal set point (a stable level or range of a physiological parameter that the body actively regulates to maintain homeostasis) 2. Control center: receives information from sensors and send orders to effectors 3. Effectors: making appropriate adjustments responsible for correcting the deviation Transmission of information may be an electrical process in the form of impulses through nerves or a chemical process mainly in the form of hormones through blood and body fluids Mechanism of action of homeostatic system The homeostatic control system acts through Negative or Positive feedback mechanism, depending upon requirement of the situation: Negative feedback mechanism: the response opposes the stimulus (inhibit and reverse the initial change). Positive feedback mechanism: the response amplifies the stimulus (support and accelerate the initial change). Fig. 1-7 Mechanism of action of homeostatic system 11  CHAPTER 1 INTRODUCTION TO PHYSIOLOGY AND HOMEOSTASIS Examples of negative feedback mechanisms: Regulation of arterial blood pressure Regulation of body temperature Regulation of blood glucose Examples of useful positive feedback mechanisms: Blood coagulation Labor During excitation of membranes Example of homeostasis disturbance: Fever (pyrexia): Pyrogens secreted by bacteria or released from degenerating tissues are phagocytosed by macrophages which release interleukin-1 (endogenous pyrogen), that stimulate formation of prostaglandins (mainly PGE2), rise of the set point of the hypothalamic thermostat above 37°C, produces fever within 8-10 minutes through activating the heat production mechanisms. 12