The body and its constituents Box 1.1 Examples of physiological variable Interstitial Temperature Water and electrolyte concentrations pH(acidity or alkalinity) of body fluids brane Blood glucose levels Blood and tissue oxygen and carbon dioxide levels ,a Small particles able to pass through pores in the cellmembrane A Large particles outside-cannot pass into the cell system.The thermostat(temperature detector) is sensitive a Large particles inside- cannot pass out of the cell to changes in room temperature(variable factor). The ther- Large particles inside and outside-cannot pass through cell membrane mostat is connected to the boiler control unit(control cen- tre), which controls the boiler (effector). The thermostat constantly compares the information from the detector with the preset ten ments are made to alter the room temperature. When the thermostat detects the room temperature is low it sends an Detector input to the boiler control unit, switching it on. The result hemostat the preset temperature is reached, the system is reversed Input The thermostat detects the higher room temperature and sends an input to the boiler control unit, turning it off.The Control centre output of heat from the boiler stops and the room slowly (boiler control unit cools as heat is lost. This series of events is a negative feed Turns off Output urns on back mechanism and it enables continuous self-regulation or control of a variable factor within a narrow range Effector Body temperature is a physiological variable con trolled by negative feedback(Fig. 1. 4). When body tem- perature falls below the preset level, this is detected by specialised temperature sensitive nerve endings. They .Room temperature transmit this information as an input to groups of cells in the hypothalamus of the brain which form the control centre. The output from the control centre activates Gradual heat loss from room mechanisms that raise body temperature (effectors) stimulation of skeletal muscles causing shivering Room temperature narrowing of the blood vessels in the skin reducing the blood flow to, and heat loss from, the peripheries Figure 1.3 Example of a negative feedback mechanism: control of behavioural changes, e.g. we put on more clothes or room temperature by a domestic boiler When body temperature rises to within the normal Negative feedback mechanisms range, the temperature sensitive nerve endings no longer stimulate the cells of the control centre and therefore the In systems controlled by negative feedback the effector output of this centre to the effectors ceases response decreases or negates the effect of the original Most of the homeostatic controls in the body use nega- stimulus, restoring homeostasis(thus the term negative tive feedback mechanisms to prevent sudden and serious feedback). Control of body temperature is similar to the changes in the internal environment. Many more of these non-physiological example of a domestic central heating are explained in the following chapters
The body and its constituents Box 1.1 Examples of physiological variables ^PlWiP _6 Figure 1.3 Example of a negative feedback mechanism: control of room temperature by a domestic boiler. Negative feedback mechanisms In systems controlled by negative feedback the effector response decreases or negates the effect of the original stimulus, restoring homeostasis (thus the term negative feedback). Control of body temperature is similar to the non-physiological example of a domestic central heating system. The thermostat (temperature detector) is sensitive to changes in room temperature (variable factor). The thermostat is connected to the boiler control unit (control centre), which controls the boiler (effector). The thermostat constantly compares the information from the detector with the preset temperature and, when necessary, adjustments are made to alter the room temperature. When the thermostat detects the room temperature is low it sends an input to the boiler control unit, switching it on. The result is output of heat by the boiler, warming the room. When the preset temperature is reached, the system is reversed. The thermostat detects the higher room temperature and sends an input to the boiler control unit, turning it off. The output of heat from the boiler stops and the room slowly cools as heat is lost. This series of events is a negative feedback mechanism and it enables continuous self-regulation or control of a variable factor within a narrow range. Body temperature is a physiological variable controlled by negative feedback (Fig. 1.4). When body temperature falls below the preset level, this is detected by specialised temperature sensitive nerve endings. They transmit this information as an input to groups of cells in the hypothalamus of the brain which form the control centre. The output from the control centre activates mechanisms that raise body temperature (effectors). These include: • stimulation of skeletal muscles causing shivering • narrowing of the blood vessels in the skin reducing the blood flow to, and heat loss from, the peripheries • behavioural changes, e.g. we put on more clothes or curl up. When body temperature rises to within the normal range, the temperature sensitive nerve endings no longer stimulate the cells of the control centre and therefore the output of this centre to the effectors ceases. Most of the homeostatic controls in the body use negative feedback mechanisms to prevent sudden and serious changes in the internal environment. Many more of these are explained in the following chapters. Temperature Water and electrolyte concentrations pH (acidity or alkalinity of body fluids Blood glucose levels Blood and tissue oxygen and carbon dioxide levels Blood pressure
Introduction to the human body SURVIVAL NEEDS OF THE BODY (specialised temperature sensitive nerve endings) Learning outcomes Control centre After studying this section you should be able to (groups of cells in the hypothalamus of the brain a describe the role of the body transport systems inhibition Stimulation a outline the roles of the nervous and endocrine skeletal muscles(shivering m outline how raw materials are absorbed by the state the waste materials eliminated from the bod nore clothes, curling up m outline activities undertaken by an individual for protection and surviva Loss of body heat By convention, the body systems are described sepa rately in the study of anatomy and physiology, but in reality they are all interdependent. This section provides r Body temperature an introduction to body activities linking them to sur- Figure 1. 4 Example of a physiological negative feedback framework, exploring human structure and functions in mechanism: control of body temperature health and illness using a systems approac Positive feedback mechanisms There are only a few of these amplifier or cascade systems in the body. In positive feedback mechanisms, the stimulus progressively increases the response, so that as long as Table 1.1 Survival needs and related body activities the stimulus is continued the response is progressively Survival need Body activities saing amplified. Examples include blood clotting and uterine contractions during labour Communication Transport systems: blood During labour, contractions of the uterus are stimu circulatory system, lymphatic lated by the hormone oxytocin. These force the babys system head into the cervix of the uterus stimulating stretch nternal communication: nervous receptors there. In response to this, more of the hormone system, endocrine system External communication: special oxytocin is released, further strengthening the contrac enses, verbal and non- verbal tions and maintaining labour. After the baby is born the stimulus (stretching of the cervix) is no longer present and the release of oxytocin stops(see Fig 9.5, P. 219) Intake of raw materials Intake of oxygen and elimination of waste Dietary intake Elimination of waste: carbon dioxide, urine, faeces Homeostatic imbalance Protection and survival Protection against the external This arises when the fine control of a factor in the internal environment:skin environment is inadequate and the level of the factor falls Resistance and immunity: outside the normal range. If control cannot achieve non-specific and specific defence homeostasis, an abnormal state develops that may mechanisms Body movement threaten health, or even life. Many of these situations are Reproduction explained in later chapters
Introduction to the human body 7_ SURVIVAL NEEDS OF THE BODY Figure 1.4 Example of a physiological negative feedback mechanism: control of body temperature. Learning outcomes After studying this section you should be able to: • describe the role of the body transport systems • outline the roles of the nervous and endocrine systems in internal communication • outline how raw materials are absorbed by the body • state the waste materials eliminated from the body • outline activities undertaken by an individual for protection and survival. By convention, the body systems are described separately in the study of anatomy and physiology, but in reality they are all interdependent. This section provides an introduction to body activities linking them to survival needs (Table 1.1). The later chapters build on this framework, exploring human structure and functions in health and illness using a systems approach. Positive feedback mechanisms There are only a few of these amplifier or cascade systems in the body. In positive feedback mechanisms, the stimulus progressively increases the response, so that as long as the stimulus is continued the response is progressively being amplified. Examples include blood clotting and uterine contractions during labour. During labour, contractions of the uterus are stimulated by the hormone oxytocin. These force the baby's head into the cervix of the uterus stimulating stretch receptors there. In response to this, more of the hormone oxytocin is released, further strengthening the contractions and maintaining labour. After the baby is born the stimulus (stretching of the cervix) is no longer present and the release of oxytocin stops (see Fig. 9.5, p. 219). Homeostatic imbalance This arises when the fine control of a factor in the internal environment is inadequate and the level of the factor falls outside the normal range. If control cannot achieve homeostasis, an abnormal state develops that may threaten health, or even life. Many of these situations are explained in later chapters. Table 1.1 Survival needs and related body activities Survival need Body activities Communication Intake of raw materials and elimination of waste Protection and survival Transport systems: blood, circulatory system, lymphatic system Internal communication: nervous system, endocrine system External communication: special senses, verbal and non-verbal communication Intake of oxygen Dietary intake Elimination of waste: carbon dioxide, urine, faeces Protection against the external environment: skin Resistance and immunity: non-specific and specific defence mechanisms Body movement Reproduction
The body and its constituents Communication a chemical substances synthesised by body cells, In this section, transport and communication are consid- waste materials produced by body cells to be ered. Transport systems ensure that all cells have access eliminated from the body by excretion to the internal and external environments; the blood the circulatory system and lymphatic system are involved. Blood cells. There are three distinct groups, classified All communication systems involve receiving, collating according to their functions(Fig. 1.5) and responding to appropriate information Erythrocytes(red blood cells) are concerned with the There are different systems for communicating with transport of oxygen and, to a lesser extent, carbon dioxide the internal and external environments. Internal commu- between the lungs and all body cells nication involves mainly the nervous and endocrine sys- Leukocytes(white blood cells) are mainly concerned tems, these are important in the maintenance of with protection of the body against microbes and other homeostasis and regulation of vital body functions. potentially damaging substances that gain entry to the Communication with the external environment involves body. There are several types of leukocytes which carry the special senses, and verbal and non-verbal activities, out their protective functions in different ways. These and all of these also depend on the nervous system cells are larger than erythrocytes and are less numerous Thrombocytes(platelets)are tiny cell fragments which Transport systems play an essential part in the very complex process of blood clotting The blood transports substances around the body Circulatory system( Ch 5) hrough a large network of blood vessels. In adults the This consists of a network of blood vessels and the heart body contains 5 to 61 of blood (Ch. 4). It consists of two (Fig. 1.6) uspendedin fluid called plasma and cells which are plasma Blood vessels. There are three types arteries, which carry blood away from the heart Plasma. This is mainly water with a wide range of sub- veins, which return blood to the heart stances dissolved or suspended in it. These include capillaries, which link the arteries and veins n nutrients absorbed from the alimentary cana Capillaries are tiny blood vessels with very thi in wa consisting of only one layer of cells. They are the site of Thrombocytes Erythrocytes red blood cells) Heart Blood vessels Lymphocyte Neutrophil subtype of white blood cells Figure 1.5 Blood cells after staining in the laboratory viewe Figure 1.6 The circulatory system
The body and its constituents _8 Communication In this section, transport and communication are considered. Transport systems ensure that all cells have access to the internal and external environments; the blood, the circulatory system and lymphatic system are involved. All communication systems involve receiving, collating and responding to appropriate information. There are different systems for communicating with the internal and external environments. Internal communication involves mainly the nervous and endocrine systems; these are important in the maintenance of homeostasis and regulation of vital body functions. Communication with the external environment involves the special senses, and verbal and non-verbal activities, and all of these also depend on the nervous system. Transport systems Blood The blood transports substances around the body through a large network of blood vessels. In adults the body contains 5 to 6 1 of blood (Ch. 4). It consists of two parts —a sticky fluid called plasma and cells which are suspended in the plasma. Plasma. This is mainly water with a wide range of substances dissolved or suspended in it. These include: • nutrients absorbed from the alimentary canal • oxygen absorbed from the lungs • chemical substances synthesised by body cells, e.g. hormones • waste materials produced by body cells to be eliminated from the body by excretion. Blood cells. There are three distinct groups, classified according to their functions (Fig. 1.5). Erythrocytes (red blood cells) are concerned with the transport of oxygen and, to a lesser extent, carbon dioxide between the lungs and all body cells. Leukocytes (white blood cells) are mainly concerned with protection of the body against microbes and other potentially damaging substances that gain entry to the body. There are several types of leukocytes which carry out their protective functions in different ways. These cells are larger than erythrocytes and are less numerous. Thrombocytes (platelets) are tiny cell fragments which play an essential part in the very complex process of blood clotting. Circulatory system (Ch. 5) This consists of a network of blood vessels and the heart (Fig. 1.6). Blood vessels. There are three types: • arteries, which carry blood away from the heart • veins, which return blood to the heart • capillaries, which link the arteries and veins. Capillaries are tiny blood vessels with very thin walls consisting of only one layer of cells. They are the site of Figure 1.5 Blood cells after staining in the laboratory viewed through a microscope. Figure 1.6 The circulatory system
introduction to the human body exchange of substances between the blood and body tis- larger than those of the blood capillaries. Lymph is tissue sues,e.g. nutrients, oxygen and cellular waste products. fluid containing large molecules, e.g. proteins, fragments Blood vessels form a network that transports blood to: of damaged tissue cells and microbes. It is transported s the lungs(pulmonary circulation) where oxygen is along lymph vessels and is returned to the bloodstream absorbed from the air in the lungs and at the same There are collections of lymph nodes situated at various time carbon dioxide is excreted from the blood inte points along the length of the lymph vessels. Lymph is the air filtered as it passes through the lymph nodes, and m cells in all parts of the body(general or systemic microbes, noxious substances and some waste materials The lymphat stem provides the sites for formation Heart. The heart is a muscular sac. It pumps the blood and maturation of lymphocytes, the white blood cells round the body and maintains the blood pressure in the involved in immunity lungs and general circulation. This is essential for life The heart muscle is not under conscious(voluntary) Internal communication control. At rest, the heart contracts between 65 and 75 times per minute. The rate may be greatly increased dur Communication and the nervous system The nervous system is a rapid communication syst needs of the muscles moving the limbs are increased, and (Ch 7). The main components are shown in Figure in some emotional states The rate at which the heart beats can be counted by The central nervous system consists of taking the pulse. The pulse can be felt most easily where the brain, situated inside the skull an artery lies close to the surface of the body and can be the spinal cord, which extends from the base of the pressed gently against a bone. The wrist is the site most skull to the lumbar region and is protected from Lymphatic system The peripheral nervous system is a network of nerve The vessels, which begin as blind-ended tubes in the spaces between the blood capillaries and tissue cells(Fig. 1. 7). sensory or afferent, providing the brain with'input' Structurally they are similar to veins and blood capilar- motor or efferent, which convey nerve impulses ies but the pores in the walls of the lymph capillaries are carrying'output from the brain to effector organs the muscles and glands Spinal cord -Lymph nodes Peripheral nerves Central nervous system Peripheral nervous system Figure 1.7 The lymphatic system: lymph nodes and vessels Figure 1. 8 The nervous system
Introduction to the human body 9 exchange of substances between the blood and body tissues, e.g. nutrients, oxygen and cellular waste products. Blood vessels form a network that transports blood to: • the lungs (pulmonary circulation) where oxygen is absorbed from the air in the lungs and at the same time carbon dioxide is excreted from the blood into the air • cells in all parts of the body (general or systemic circulation). Heart. The heart is a muscular sac. It pumps the blood round the body and maintains the blood pressure in the lungs and general circulation. This is essential for life. The heart muscle is not under conscious (voluntary) control. At rest, the heart contracts between 65 and 75 times per minute. The rate may be greatly increased during physical exercise, when the oxygen and nutritional needs of the muscles moving the limbs are increased, and in some emotional states. The rate at which the heart beats can be counted by taking the pulse. The pulse can be felt most easily where an artery lies close to the surface of the body and can be pressed gently against a bone. The wrist is the site most commonly used for this purpose. Lymphatic system The lymphatic system (Ch. 6) consists of a series of lymph vessels, which begin as blind-ended tubes in the spaces between the blood capillaries and tissue cells (Fig. 1.7). Structurally they are similar to veins and blood capillaries but the pores in the walls of the lymph capillaries are larger than those of the blood capillaries. Lymph is tissue fluid containing large molecules, e.g. proteins, fragments of damaged tissue cells and microbes. It is transported along lymph vessels and is returned to the bloodstream. There are collections of lymph nodes situated at various points along the length of the lymph vessels. Lymph is filtered as it passes through the lymph nodes, and microbes, noxious substances and some waste materials are removed. The lymphatic system provides the sites for formation and maturation of lymphocytes, the white blood cells involved in immunity. Internal communication Communication and the nervous system The nervous system is a rapid communication system (Ch. 7). The main components are shown in Figure 1.8. The central nervous system consists of: • the brain, situated inside the skull • the spinal cord, which extends from the base of the skull to the lumbar region and is protected from injury by the bones of the spinal column. The peripheral nervous system is a network of nerve fibres, which are: • sensory or afferent, providing the brain with 'input' from organs and tissues, or • motor or efferent, which convey nerve impulses carrying 'output' from the brain to effector organs: the muscles and glands. Figure 1.7 The lymphatic system: lymph nodes and vessels. Figure 1.8 The nervous system
The body and its constituents The somatic(common)senses are pain, touch, heat and cold, levels of particular substances in the blood, including and they arise following stimulation of specialised sen- specific hormones. Changes in blood hormone levels are sory receptors at nerve endings found throughout the controlled by negative feedback mechanisms(Fig. 1.3) skin. There are different receptors in muscles and joints The endocrine system provides slower and more preci that respond to changes in the position and orientation of control of body functions than the nervous system the body, maintaining posture and balance. Yet other receptors are activated by stimuli in internal organs and Communication with the external ntain control of vital body functions, e.g. hea rat respiratory rate and blood pressure. Stimulation of any of environment these receptors sets up impulses that are conducted to the Special senses brain in sensory(afferent)nerves. Communication along These senses arise following stimulation of specialised nerve fibres(cells) is by electrical impulses that are gener- sensory receptor cells located in sensory organs or tissues ated when nerve endings are stimulated in the head. The senses and the special organs involved Communication between nerve cells is also required, are shown in Box 1.2 since more than one nerve is involved in the chain of Although thes se senses are usually considered separate events occurring between the initial stimulus and the and different from each other, one sense is rarely used physiological reaction to it. Nerves communicate with alone(Fig. 1.9). For example, when the smell of smoke is each other by releasing a chemical (the neurotransmitter) perceived then other senses such as sight and sound are into tiny gaps between them. The neurotransmitter used to try and locate the source of a fire. Similarly, taste quickly travels across the gap and either stimulates and smell are closely associated in the enjoyment, or oth- inhibits the next nerve cell, thus ensuring the message is erwise, of food. The brain collates incoming informatic transmitted with information from the memory and initiates a Sensory nerves and chemical substances circulating in response by setting up electrical impulses in motor(effer- the blood provide information to appropriate parts of the ent)nerves to effector organs, muscles and glands.Such brain, which collates it and then responds via motor respo able the individual to escape from the fire, or herves to effector organs, often through a negative feed- to prepare the digestive system for eating back mechanism(Fig. 1.3). Some of these activities are understood and perceived, e.g. pain, whereas others take verbal communication ce subconsciou are. Sound is a means of communication and is produced in Nerve impulses travel at great speed along nerve fibres the larynx as a result of blowing air through the space leading to rapid responses; adjustments to body between the vocal cords during expiration. Speech is the functions occur within a few seconds manipulation of sound by contraction of the muscles of Communication and the endocrine system the throat and cheeks, and movements of the tongue and lower jaw The endocrine system consists of a number of endocrine glands situated in different parts of the body. They syn- Non-verbal communication thesis and secrete chemical messengers called hormones Posture and movements are associated with non-verbal that circulate round the body in the blood. Hormones communication, e.g. nodding the head and shrugging the stimulate target glands or tissues, infl enci g metabolic and other cellular activities and regulating body growth and maturation. Endocrine glands detect and respond to mmm Box 1.2 The senses and related sense organs Hearing-ears Balance-ears Taste -tongue Figure 1.9 Combined use of the special senses: vision, hearing smell and taste
The body and its constituents 10 The somatic (common) senses are pain, touch, heat and cold, and they arise following stimulation of specialised sensory receptors at nerve endings found throughout the skin. There are different receptors in muscles and joints that respond to changes in the position and orientation of the body, maintaining posture and balance. Yet other receptors are activated by stimuli in internal organs and maintain control of vital body functions, e.g. heart rate, respiratory rate and blood pressure. Stimulation of any of these receptors sets up impulses that are conducted to the brain in sensory (afferent) nerves. Communication along nerve fibres (cells) is by electrical impulses that are generated when nerve endings are stimulated. Communication between nerve cells is also required, since more than one nerve is involved in the chain of events occurring between the initial stimulus and the physiological reaction to it. Nerves communicate with each other by releasing a chemical (the neurotransmitter) into tiny gaps between them. The neurotransmitter quickly travels across the gap and either stimulates or inhibits the next nerve cell, thus ensuring the message is transmitted. Sensory nerves and chemical substances circulating in the blood provide information to appropriate parts of the brain, which collates it and then responds via motor nerves to effector organs, often through a negative feedback mechanism (Fig. 1.3). Some of these activities are understood and perceived, e.g. pain, whereas others take place subconsciously, e.g. changes in blood pressure. Nerve impulses travel at great speed along nerve fibres leading to rapid responses; adjustments to many body functions occur within a few seconds. Communication and the endocrine system The endocrine system consists of a number of endocrine glands situated in different parts of the body. They synthesise and secrete chemical messengers called hormones that circulate round the body in the blood. Hormones stimulate target glands or tissues, influencing metabolic and other cellular activities and regulating body growth and maturation. Endocrine glands detect and respond to levels of particular substances in the blood, including specific hormones. Changes in blood hormone levels are controlled by negative feedback mechanisms (Fig. 1.3). The endocrine system provides slower and more precise control of body functions than the nervous system. Communication with the external environment Special senses These senses arise following stimulation of specialised sensory receptor cells located in sensory organs or tissues in the head. The senses and the special organs involved are shown in Box 1.2. Although these senses are usually considered separate and different from each other, one sense is rarely used alone (Fig. 1.9). For example, when the smell of smoke is perceived then other senses such as sight and sound are used to try and locate the source of a fire. Similarly, taste and smell are closely associated in the enjoyment, or otherwise, of food. The brain collates incoming information with information from the memory and initiates a response by setting up electrical impulses in motor (efferent) nerves to effector organs, muscles and glands. Such responses enable the individual to escape from the fire, or to prepare the digestive system for eating. Verbal communication Sound is a means of communication and is produced in the larynx as a result of blowing air through the space between the vocal cords during expiration. Speech is the manipulation of sound by contraction of the muscles of the throat and cheeks, and movements of the tongue and lower jhaw. Non-verbal communication Posture and movements are associated with non-verbal communication, e.g. nodding the head and shrugging the Figure 1.9 Combined use of the special senses: vision, hearing, smell and taste. Box 1.2 The senses and related sense organs Sight-eyes Hearing-ears Balance-ears Smell-nose Taste-tonguef