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Before proceeding to the various anatomical levels that can be found in the human body, it would be useful to have some simple definitions. The definition of anatomy is the study of structures that make up the human body and how they relate to each other, for example, how does the skeletal structure relate to the muscular structure, or how does the cardiovascular structure relate to the respiratory structure? The definition for physiology is the study of the function of body structures, for example, how do the neural impulses transmit down a nerve and affect the structuring at the end of the nerve.
In understanding these interactions, the application of electronics to monitor these systems will be more readily understood.
To describe the location of particular parts of the body, anatomists have defined the anatomical position. This is shown in FIG. 1.
2. Anatomical Terminology
There is standardized terminology to describe positions of various parts of the body from the midline. These are shown in FIG. 2. When the body is in the 'anatomical position', it can be further described with relation to body regions. The main regions of the body are the axial, consisting of the head and neck, chest, abdomen and pelvis; the appendicular, which includes the upper extremities--shoulders, upper arms, forearms, wrists and hands; and the lower extremities--hips, thighs, lower legs, ankles and feet. These are shown in FIG. 3. Further subdivision in order to identify specific areas of the body can be carried out by considering various planes. These are shown in FIG. 4. The midsagital plane divides the left and right sides of the body lengthwise along the midline. If the symmetrical plane is placed off centre and separates the body into asymmetrical left and right sections it is called the sagital plane. If you face the side of the body and make a lengthwise cut at right angles to the midsagital plane you would make a frontal (coronal) plane, which divides the body into asymmetrical anterior and posterior sections. A transverse plane divides the body horizontally into upper (superior) and lower (inferior) sections. An understanding of these terminologies is important, as it is the common language for locating parts in the human body. Without these definitions, confusion would arise in describing the relationship between one body part and another.
The cell has to communicate with its environment. This is done via the plasma membrane, which lines the whole cell. Messengers in the form of molecules can be transmitted across this membrane, as it is permeable to specific molecules of various shapes and sizes. Movement of these messengers across the membrane is achieved by two mechanisms.
1. Simple diffusion: molecules pass through the membrane from high to low concentrations.
2. Active diffusion: basic fuel for the human body is adenosine triphosphate (ATP). This fuel acts on a pump that pushes molecules from a low concentration to a high concentration.
When many similar cells combine to perform a specific function, they are called tissues.
Examples of human tissue are epithelial, connective, muscle and nervous. It is important to stress that the difference between tissues is that the cells combine to perform a specific function associated with each tissue.
Epithelial tissues line all body surfaces, cavities and tubes. Their function is to act as an interface between various body compartments. They are involved with a wide range of activities, such as absorption, secretion and protection. For example, the epithelial lining of the small intestine is primarily involved in the absorption of products of digestion, but the epithelium also protects it from noxious intestinal contents by secreting a surface coating.
Connective tissue is the term applied to the basic type of tissue which provides structural support for other tissue. Connective tissue can be thought of as a spider's web that holds together other body tissues. Within this connective tissue web, various cells that fight the bacteria which invade the body can be found. Similarly, fat is also stored in connective tissue.
An organ is an amalgamation of two or more kinds of tissue that work together to perform a specific function. An example is found in the stomach; epithelial tissue lines its cavity and helps to protect it. Smooth muscle churns up food, breaks it down into smaller pieces and mixes it with digestive juices. Nervous tissue transmits nerve impulses that initiate the muscle contractions, whilst connective tissue holds all the tissues together.
The next structural level of the body is called systems. The system is a group of organs that work together to perform a certain function. All body systems work together in order that the whole body is in harmony with itself. Listed in Table 1 are the body systems and their major functions. Systems that are often monitored in order to analyze the well-being of the body include those associated with respiratory, skeletal, nervous and cardiovascular.
Table 1--Body Systems
The structures of each system are closely related to their functions.
Body system | Major functions
[ CARDIOVASCULAR (heart, blood, blood vessels)
DIGESTIVE (stomach, intestines, other digestive structures)
ENDOCRINE (ductless glands)
INTEGUMENTARY (skin, hair, nails, sweat LYMPHATIC (glands, lymph nodes, lymph, lymphatic vessels) and oil glands)
MUSCULAR (skeletal, smooth cardiac muscle)
NERVOUS (brain, spinal cord; peripheral nerves; sensory organs)
REPRODUCTIVE (ovaries, testes, reproductive cells, accessory glands, ducts)
RESPIRATORY (airways, lungs)
SKELETAL (bones, cartilage)
URINARY (kidneys, ureters, bladder, urethra)
[ Heart pumps blood through vessels; blood carries materials to tissues; transports tissue wastes for excretion.
Breaks down large molecules into small molecules that can be absorbed into blood, removes solid wastes.
Endocrine glands secrete hormones, which regulate many chemical actions within the body.
Covers and protects internal organs; helps regulate body temperature.
Returns excess fluid to blood; part of immune system.
Allows for body movement; produces body heat.
Regulates most bodily activities; receives and interprets information from sensory organs; initiates actions by muscles.
Provides mechanism for breathing, exchange of gases between air and blood.
Supports body, protects organs; provides lever mechanism for movement; produces red blood cells.
Eliminates metabolic wastes; helps regulate blood pressure, acid-base and water-salt balance.
4. Muscular System
The function of muscle is to allow movement and to produce body heat. In order to achieve this, muscle tissue must be able to contract and stretch. Contraction occurs via a stimulus from the nervous system. There are three types of muscle tissue; smooth, cardiac and skeletal.
Skeletal muscle by definition is muscle which is involved in the movement of the skeleton. It is also called striated muscle as the fibers, which are made up of many cells, are composed of alternating light and dark stripes, or striations. Skeletal muscle can be contracted without conscious control, for example in sudden involuntary movement.
Most muscle is in a partially contracted state (tonus). This enables some parts of the body to be kept in a semi-rigid position, i.e. to keep the head erect and to aid the return of blood to the heart. Skeletal muscle is composed of cells that have specialized functions. They are called muscle fibers, due to their appearance as a long cylindrical shape plus numerous nuclei. Their lengths range from 0.1 cm to 30 cm with a diameter from 0.01 cm to 0.001 cm. Within these muscle fibers are even smaller fibers called myofibrils. These myofibrils are made up of thick ax? thin threads called myofilaments, The thick myofilaments are called myocin and the thin myofilaments are called actin. FIG. 6 shows a progression from the gross to the molecular structure of muscle.
Control of muscle is achieved via the nervous system. Nerves are attached to muscle via a junction called the motor end plate. Shown in FIG. 7 is a diagrammatic representation of a motor end plate.
4.1. Mechanism of Contraction of Muscle
Muscle has an all or none phenomenon. In order for it to contract it has to receive a stimulus of a certain threshold. Below this threshold muscle will not contract; above this threshold muscle will contract but the intensity of contraction will not be greater than that produced by the threshold stimulus.
The mechanism of contraction can be explained with reference to FIG. 8. A nerve impulse travels down the nerve to the motor end plate. Calcium diffuses into the end of the nerve. This releases a neurotransmitter called acetylcholine, a neural transmitter. Acetylcholine travels across the small gap between the end of the nerve and the muscle membrane. Once the acetylcholine reaches the membrane, the permeability of the muscle to sodium (Na') and potassium (K') ions increases. Both ions are positively charged. However, there is a difference between permeabilities for the two ions. Na' enters the fiber at a faster rate than the K+ ions leave the fiber. This results in a positive charge inside the fiber. This change in charge initiates the contraction of the muscle fiber.
The mechanism of contraction involves the actin and myocin filaments which, in a relaxed muscle, are held together by small cross bridges. The introduction of calcium breaks these cross bridges and allows the actin to move using ATP as a fuel. Relaxation of muscle occurs via the opposite mechanism. The calcium breaks free from the actin and myocin and enables the cross bridges to reform. Recently there has been a new theory of muscle contraction. This suggests that the myocin filaments rotate and interact with the actin filaments, similar to a corkscrew action, with contacts via the cross bridges. The rotation causes the contraction of the muscle.
4.2. Types of Muscle Contraction
Muscle has several types of contraction. These include twitch, isotonic and isometric and tetanus.
Twitch: This is a momentary contraction of muscle in response to a single stimulus. It is the simplest type of recordable muscle contraction.
Isotonic Asometric: In this case a muscle contracts, becoming shorter. This results in the force or tension remaining constant as the muscle moves. For example, when you lift a weight, your muscles contract and move your arm, which pulls the weight. In contrast an isometric contraction occurs when muscle develops tension but the muscle fibers remain the same length. This is illustrated by pulling against an immovable object.
Tetanus: This results when muscle receives a stimulus at a rapid rate. It does not have time to relax before each contraction. An example of this type of contraction is seen in lock-jaw, where the muscle cannot relax due to the rate of nervous stimulus it is receiving.
Myograms: During contraction the electrical potential generated within the fibers can be recorded via external electrodes. The resulting electrical activity can be plotted on a chart.
These myograms can be used to analyze various muscle contractions, both normal and abnormal.
4.3. Smooth Muscle
Smooth muscle tissue is so called because it does not have striations and therefore appears smooth under a microscope. It is also called involuntary because it is controlled I2.y the autonomic nervous system. Unlike skeletal muscle, it is not attached to bone. It is found within various systems within the human body, for example the circulatory, the digestive and respiratory. Its main difference from skeletal muscle is that its contraction and relaxation are slower. Also, it has a rhythmic action which makes it ideal for the gastro-intestinal system.
The rhythmic action pushes food along the stomach and intestines.
4.4. Cardiac Muscle
Cardiac muscle, as the name implies, is found only in the heart. Under a microscope the fibers have a similar appearance to skeletal muscle. However, the fibers are attached to each other via a specialized junction called an 'intercalated disc'. The main difference between skeletal and cardiac muscle is that cardiac muscle has the ability to contract rhythmically on its own without the need for external stimulation. This of course is of high priority in order that the heart may pump for 24 hour day. When cardiac muscle is stimulated via a motor end plate calcium ions influx into the muscle fibers. This results in contraction of the cardiac muscle.
The intercalated discs help synchronize the contraction of the fibers. Without this synchronization the heart fibers may contract independently, thus greatly reducing the effectiveness of the muscle in pumping the blood around the body.
4.5. Muscle Mechanics
Movement of the skeletal structure is achieved via muscle. Skeletal muscles are classified according to the types of movement that they can perform. For simplicity, there are basically two types of muscle action --flexion and extension. Examples of flexion and extension are seen in FIG. 9. The overall muscular system of the human body can be seen in Figures 2.10 and 2.11.
Most body movement, even to perform such simple functions as extension or flexion, involves complex interactions of several muscles or muscle groups. This may involve one muscle antagonizing another in order to achieve a specific function. The production of movement of the skeletal system involves four mechanisms --agonist, antagonist, synogists and fixators.
Agonist is a muscle that is primarily responsible for producing a movement. An antagonist opposes the movement of the prime mover. The specific contraction or relaxation of the antagonist working in co-operation with the agonist helps to produce smooth movements.
The synogist groups of muscles complement the action of the prime mover. The fixator muscles provide a stable base for the action of a prime mover--for example muscles that steady the proximal end of an arm, while the actual movement takes place in the hand.
All four of these muscle groups work together with an overall objective of producing smooth movement of the skeletal structure.
Muscle is usually attached to a bone by a tendon--this is a thick cord of connective tissue comprising collagen fibers. When muscle contracts, one bone remains stationary, whilst the bone at the other end of the muscle moves. The end of the muscle that is attached to the bone that remains stationary is commonly called 'the origin', whilst the other attachment to the moving bone is called 'the insertion'.
5. Skeletal System
The adult skeleton consists of 206 different bones. However, it is common to find an individual with an extra rib or an additional bone in the hands or feet. Shown in FIG. 12 is the adult human skeleton. Bone is a composite material consisting of different substances interconnected in such a way as to produce a material with outstanding mechanical properties.
It consists of a matrix of an organic material, collagen, and a crystalline salts, called hydroxyapetite.
There are two types of bone--cortical (or compact) and cancellous (trabecullar). Cortical bone is a hard dense material visible on the bone's surface. Due to its appearance it is often called compact bone. Cancellous bone exists within the shell of the cortical bone (FIG. 13). Cancellous bone is often referred to as spongy bone, as it consists of widely spaced interconnecting fiber columns called trabecullar. The centre of a long bone is filled with marrow, and this area is called the medullary cavity. It has an important role in producing blood cells during childhood. The two ends of a human long bone are called the 'epiphysis', while the mid region is referred to as the 'diaphysis'.
Articulation of the skeletal systems occurs via joints. These joints are classified according to their movement. In hinge joints, as the name implies, movement occurs similar to that on hinges of the lid of a box. For pivot joints, the best example is the skull rotating on a peg, attached to the vertebra. Finally there are ball and socket joints, a typical example of which is found in the hip, in which the head of the femur articulates with the socket of the assetablum.
Most major joints are encapsulated and lubricated by synovial fluid. A typical example is the hip joint shown in FIG. 14.
6. The Nervous System
The human body reacts to a number of stimuli, both internally and externally. For example, if the hand touches a flame from a cooker, the response would be to pull the hand away as quickly as possible. The mechanism to achieve this response is controlled via the nervous system. Impulses travel from the tips of the fingers along nerves to the brain. The information is processed and the response organized. This results in the hand being pulled away from the flame using the muscular system.
The nervous system is also responsible in regulating the internal organs of the body. This is in order that homeostasis can be achieved with minimal disturbance to body function. The signals that travel along the nervous system result from electrical impulses and neurotransmitters that communicate with another body tissue, for example muscle.
For convenience, the nervous system is split into two sections, but it is important to stress that both these networks communicate with each other in order to achieve an overall steady state for the body. The two systems are termed Central and Peripheral.
The central nervous system consists of the brain and the spinal cord and can be thought of as a central processing component of the overall nervous system.
The peripheral nervous system consists of nerve cells and their fibers that emerge from the brain and spinal cord and communicate with the rest of the body. There are two types of nerve cells within the peripheral system--the afferent, or sensory nerves, which carry nerve impulses from the sensory receptors in the body to the central nervous system; and the efferent, or motor nerve cells which convey information away from the central nervous system to the effectors. These include muscles and body organs.
The highest centre of the nervous system is the brain. It has four major sub-divisions; the brain stem, the cerebellum, cerebrum and the diencephalon. The location in the brain of these various divisions is seen in FIG. 15. Each is concerned with a specific function of the human body. The brain stem relays messages between the spinal cord and the brain. It helps control the heart rate, respiratory rate, blood pressure and is involved with hearing, taste and other senses. The cerebellum is concerned with co-ordination for skeletal muscle movement.
The cerebrum concentrates on voluntary movements, and co-ordinates mental activity. The diencephalon connects the mid brain with the cerebral hemispheres. Within its area it has the control of all sensory information, except smell, and relays this information to the cerebrum.
Other areas within the diencephalon control the autonomic nervous system, regulate body heat, water balance, sleep/wake patterns, food intake and behavioral responses associated with emotions.
The human brain is mostly water; about 75% in the adult. It has a consistency similar to that of set jelly. The brain is protected by the skull. It floats in a solution called the cerebrospinal fluid and is encased in three layers of tissue called the cranial meninges--the inflammation of which is termed meningitis. The brain is very well protected from the injury that could be caused by chemical compounds. Substances can only enter the brain via the blood brain barrier. The capillaries within the brain have walls that are highly impermeable and therefore prevent toxic substances causing damage to the brain. Without this protection the delicate neurons could easily be damaged.
The brain is connected to the spinal cord via the brain stem. The spinal cord extends from the skull to the lumbar region of the human back. Presented in FIG. 16 is the distribution of the nerves from the spinal cord. Similar to the brain, the spinal cord is bathed in cerebrospinal fluid. The cord and the cerebrospinal fluid is contained within a ringed sheath called the duramatter. All these structures are contained within the vertebral column.
The vertebral column is made up of individual vertebra that are separated from each other by annular intervertebral discs. These discs have similar consistency to rubber and act as shock absorbers for the vertebral column. Each vertebra has a canal from which the spinal nerve can leave the spinal column and become a peripheral nerve. FIG. 17 illustrates the function of a peripheral nerve. It transmits sensory information to the spinal cord, from which information can either be transmitted to the higher nervous system, the brain, for interpretation and action, or can be acted on directly within the spinal cord and the information sent back down the ventral route to initiate the response. This latter action is best illustrated by the simple reflex arc, illustrated in FIG. 18.
If the spinal cord is injured, the resulting disability is related to the level of the injury. Injuries of the spinal cord nearer the brain result in larger loss of function compared to injuries lower down the cord. Illustrated in FIG. 19 are two types of paralysis that can occur due to transection of the cord.
Paraplegia is the loss of motor and sensory functions in the legs. This results if the cord is injured in the thoracic or upper lumbar region. Quadriplegia involves paralysis of all four limbs and occurs from injury at the cervical region. Hemiplegia results in the paralysis of the upper and lower limbs on one side of the body, This occurs due to the rupture of an artery within the brain. Due to the architecture of the connections between the right and left hand side of the brain, damage to the right hand side of the brain would result in hemiplegia in the opposite side.
The nervous system contains over one hundred billion nerve cells, or Neurons. They are specialized cells which enable the transmission of impulses from one part of the body to another via the central nervous system.
Neurons have two properties; excitability, or the ability to respond to stimuli; and conductivity, the ability to conduct a signal. A neuron is shown diagrammatically in FIG. 20.
Dendrites conduct information towards the cell body. The axon transmits the information away from the cell body to another nerve body tissue. Some axons have a sheath which is called myelin. The myelin sheath is segmented and interrupted at regular intervals by gaps called neurofibral nodes. The gaps have an important function in the transmission of impulses along the axon. This is achieved via neurotransmitters. Unmyelinated nerve fibers can be found in the peripheral nervous system. Unlike the myelinated fibers they tend to conduct at a slower speed.
6.3. Physiology of Neurons
Neurons transmit information via electrical pulses. Similar to all other body cells, transmission depends upon the difference in potential across the membrane of the cell wall. With reference to FIG. 21, a resting neuron, is said to be polarized, meaning that the inside of the axon is negatively charged with relation to its outside environment. The difference in the electrical charge is called the potential difference. Normally the resting membrane potential is -70 mV. This is due to the unequal distribution of potassium ions within the axon and sodium ions outside the axon membrane. There are more positively charged ions outside compared to within the axon.
FIG. 22 shows the sodium/potassium pump that is found in the axon membrane. This pump is powered by ATP and transports three sodium ions out of the cell for every two potassium ions that enter the cell.
In addition to the pump the axon membrane is selectively permeable to sodium/potassium through voltage gates, known as open ion channels. These come into operation when the concentration of sodium or potassium becomes so high on either side that the channels open up to re-establish the distribution of the ions in the neuron at its resting state (-70 mV).
6.4. The Mechanism of Nerve Impulses
The process of conduction differs slightly between unmyelinated and myelinated fibers. For unmyelinated fibers the stimulus has to be strong enough to initiate conduction. The opening of ion channels starts the process called depolarization.
Once an area of the axon is depolarized it stimulates the adjacent area and the action potential travels down the axon. After depolarization the original balance of sodium on the outside of the axon and potassium inside is re-stored by the action of the sodium/potassium pumps. The membrane is now re-polarized.
There is a finite period whereby it is impossible to stimulate the axon in order to generate an action potential. This is called the refractory period and can last anything from 0.5 to 1 ms. A minimum stimulus is necessary to initiate an action potential. An increase in the intensity of the stimulus does not increase the strength of the impulse. This is called an all or none principle. In myelinated fibers the passage of the impulse is speeded up. This is because the myelin sheath around the axon acts as an insulator and the impulses jump from one neurofibral node to another. The speed of conduction in unmyelinated fibers ranged from 0.7 to 2.3 meters/second, compared with 120 meters/second in myelinated fibers.
6.5. The Autonomic Nervous System
A continuation of the nervous system is the Autonomic nervous system, which is responsible in maintaining the body's homeostasis without conscious effort. The autonomic nervous system is divided into sympathetic and para-sympathetic. The responsibility of each of these divisions is shown in Tables 2 and 3. The best example involving the autonomic nervous system is the 'Flight or fight' reaction. Most people have experienced this in the form of fear.
The body automatically sets itself up for two responses--either to 'confront' the stimuli, or run away, The decision on which to do is analyzed on a conscious level. It is obvious from looking at the roles of these divisions that the homeostasis of the body would be extremely difficult, if not impossible, to achieve without this important system. Failure of any of these effects would be a life threatening condition.
Table 2 Sympathetic System--Neurotransmitter Noradrenaline
Table 3 Parasympathetic System--Neurotransmitter Acetylcholine
7. The Cardio-Vascular System
The centre of the cardio-vascular system is the heart. The heart can be considered as a four chambered pump. It receives oxygen deficient blood from the body; sends it to get a fresh supply of oxygen from the lungs; then pumps this oxygen rich blood back round the body. It has approximately 70 beats per minute and 100,000 per day. Over 70 years the human heart pumps 2.5 billion times. Its size is that approximately of the clenched fist of its owner and it weighs anything between 200 and 400 grams, depending upon the sex of the individual. It is located in the centre of the chest, with two thirds of its body to the left of the mid line.
Heart muscle is of a special variety, termed cardiac. Due to the inter-collated discs, the cells act together in order to beat synchronously to achieve the aim of pumping the blood around the body. The physiology of the action potential within the cells is similar to that of the nerves.
The anatomical structure of the heart is shown in FIG. 23. De-oxygenated blood returns from the body via the veins into the right atrium. The right atrium contracts, sending the blood into the right ventricle. The one-way valve enables the blood, on the contraction of the right ventricle, to be expelled to the lungs, where it is oxygenated (pulmonary system). The returning oxygenated blood is fed into the left atrium, and then into the left ventricle. On contraction of the left ventricle, again via a one-way valve, the blood is sent to the various parts of the body via blood vessels (FIG. 24). The systemic/pulmonary cardiac cycle is shown in FIG. 25. The whole cycle is repeated 70 times per minute.
The contraction of the cardiac muscle is initiated by a built-in pacemaker that is independent of the central nervous system. With reference to FIG. 26, the specialized nervous tissue in the right atrium is called the sino atrial node; it is responsible for initiating contraction. The signals are passed down various nervous pathways to the atrio-ventricular node. This causes the two atria to contract. The nervous signal then travels down the atrio-ventricular bundles to initiate the contraction of the ventricles. The transmission of the various impulses along these pathways gives off an electrical signal. It is the measurement of these signals that produce the electro-cardiograph (ECG)(FIG. 27). The P region of the electro-cardiograph represents atrial contraction. The ventricular contractions are represented by the QRS wave, whilst the T waveform is ventricular relaxation.
Typical times for the duration of the various complexes are shown in Table 4. Recording of these signals is obtained by placing electrodes on various parts of the body. These are shown in FIG. 28. Other than their own specialized cells to conduct the nerve impulses, the heart receives other nerve signals. These come mainly from the sympathetic and para-sympathetic autonomic nervous system. The sympathetic system, when stimulated, tends to speed up the heart, while the parasympathetic system tends to slow the heart rate down. If for some reason the mechanism for transmitting the nervous signals from the atrium to the ventricles is disrupted, then the heart must be paced externally. This can be achieved by an electronic device called the pacemaker. This device feeds an electrical current via a wire into the right ventricle. This passes an impulse at a rate of approximately seventy per minute.
7.1. Measurement of Blood Pressure
When the heart contracts, it circulates blood throughout the body. The pressure f the blood against the wall is defined as the blood pressure. Its unit of measurement is millimeters of mercury (mmHg). When the ventricles contract, the pressure of the blood entering the arterial system is termed systolic. The diastolic pressure corresponds to the relaxation of the ventricle.
The difference between these two pressures is termed the blood pressure (systolic/diastolic). A normal young adult's blood pressure is 120/80 mmHg. If the blood pressure is considerably higher then the patient is termed to be hypertensive. Blood pressure varies with age. The systolic pressure of a new-born baby may only be 40, but for a 60 year old man it could be 140 mmHg. Causes of abnormal rises in blood pressure are numerous. Blood pressure rises temporarily during exercise or stressful conditions and a systolic reading of 200 mmHg would not be considered abnormal under these circumstances.
8. Respiratory System
The body requires a constant supply of oxygen in order to live. The respiratory system delivers oxygen to various tissues and removes metabolic waste from these tissues via the blood. The respiratory tract is shown in FIG. 29.
Breathing requires the continual work of the muscles in the chest wall. Contraction of the diaphragm and external intercostal muscles expands the lungs' volume and air enters the lungs. For expiration, the external intercostal muscles and the diaphragm relax, allowing the lung volume to contract. This is accompanied by the contraction of abdominal muscles and the elasticity of the lungs.
We return to a discussion of measurement of cardio-vascular function and the control of certain of its disorders in Section 4.
8.1. Volumes of Air in the Lung
With reference to FIG. 30, pulmonary ventilation can be broken down into various volumes and capacities. These measurements are obtained using a respirometer. During normal breathing at rest, both men and women inhale and exhale about 0.5 liter with each breath--this is termed the tidal volume.
The composition of respiratory gases entering and leaving the lungs is shown in Table 5.
Table 5 Composition of main respiratory gases entering and leaving lungs (standard atmospheric pressure, young adult male at rest)
Percentages do not add up to 100 because water is also a component of air.
8.2. Diffusion of Gases
The terminal branches in the lung are called the alveoli. Next to the alveoli are small capillaries. Oxygen and carbon dioxide are transported across the alveoli membrane wall.
Various factors affect the diffusion of oxygen and carbon dioxide across the alveoli capillary membrane. These include the partial pressure from either side of the membrane, the surface area, the thickness of the membrane, and solubility and size of the molecules.
The inspired oxygen transfers across the alveoli membrane to the red blood cells in the capillaries. Oxygen attaches itself to the hemoglobin, whilst carbon dioxide is released from the hemoglobin and travels in the reverse direction to the alveoli. The carbon dioxide is then expired as waste through the respiratory system. Similarly, at the tissue, the oxygen is released from the red blood cells and is transported across the tissue membrane to the tissue. Carbon dioxide travels in the opposite direction.
The transportation of oxygen and carbon dioxide in the red blood cells depends upon the concentration of a protein called hemoglobin. Hemoglobin has a high affinity for oxygen and therefore is a necessary component in the transfer of oxygen around the human body.
8.3. The Control of Breathing
The rate and depth of breathing can be controlled consciously but generally it is regulated via involuntary nerve impulses. This involuntary process is mediated via the medullary area of the central nervous system.