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Basic Physiology: how the body functions

 

Introduction

The branches of science that will help you understand the body parts and and functions are anatomy and physiology. Anatomy deals with the study of the human body (the component parts, structure and position) and physiology the study how the body functions.

Systems

An understanding of the function and the benefits of exercise on the various body systems will be beneficial in an athlete's development. The following systems are reviewed here:

  • Skeletal
  • Muscular
  • Neurological
  • Endocrine
  • Cardiovascular
  • Respiratory
  • Digestive

Skeletal System

The skeletal system provides four basic functions:

  • Support for tissues and muscle
  • Protection for vital organs
  • Movement through bones and attached muscles
  • Storage for minerals and immature blood cells

Support

Bones and cartilage which make up the skeleton are the only rigid materials in the body. The 206 bones of the skeleton provide a framework and points of attachment for many of the soft tissues of the body. The five main classifications of bones are : Long (e.g. femur), Short (e.g. tarsal bones of the foot), Flat (e.g. frontal bone of the skull), Irregular (e.g. vertebrae) and Sesamoid (e.g. knee cap)

Protection

These structures protect some of the vital tissues and functional organs of the body. Typical examples are:

  • Skull - protects the brain
  • Vertebrae - protects the spinal cord
  • Thoracic cage - protects the heart and lungs

Movement

Bones act as levers during movement and provide solid structures to which muscles are attached. The joints allow movement between bones and these movements are directly related to the type of joint and range of motion. Joints fall into one of three categories: Fixed fibrous or Synarthroses (e.g. bones of the skull), Slightly moveable or Amphiarthroses (e.g. symphysis pubis) and Freely movable or Diarthroses.

Freely Movable joints comprises of four main groups: Ball and Socket (e.g. hip), Hinge (e.g. elbow), Pivot (e.g. radius and ulna) and Gliding (e.g. carpal joint of the wrist)

Storage

In some bones there is red marrow which produces red blood cells, some white cells and platelets. Minerals, especially calcium and phosphorous are also stored in bones and can be distributed to other parts of the body.

Effect of exercise on the skeletal system

The condition of bone may be improved by exercise as it responds to mechanical stresses. These mechanical stresses usually take the form of skeletal muscle pulling at their points of attachment being their origins and insertions. Where these mechanical stresses are applied most it has been shown that more mineral salts are deposited and more collagenous fibres are produced. Therefore both the density and size of bone in these areas may be increased and these changes in bone structure are stimulated by increased loads being placed on the skeleton. This has been borne out by greater bone mass being observed in weight lifters than in other lighter endurance athletes such as joggers. Other examples include racquet players who have been shown to have greater bone density in their playing arms. It has even been shown that if a leg is immobilised by being placed in plaster, due to a fracture, that even after a few weeks the bone becomes decalcified from lack of mechanical stress.

Whilst it may therefore be considered beneficial to utilise exercise to maintain healthy bones, great care must be taken with children whose bones and muscles are still developing. They should not be subjected to forms of sport involving high degrees of mechanical stress, partly because of the weaknesses which still exist within the bones, and also because of adverse effects on the development of these bones before maturity.

Muscular System

Muscle tissue has four main properties: Excitability (ability to respond to stimuli), Contractibility (ability to contract), Extensibility (ability of a muscle to be stretched without tearing) and Elasticity (ability to return to its normal shape).

Through contraction the muscular system performs three important functions:

  • Motion - walking, running etc.
  • Heat production - maintain normal body temperature
  • Maintenance of posture - standing, sitting etc.

Motion

To understand how the muscles combine with the skeleton in providing motion we must look at the basic mechanics of movement. The main framework of the body is covered by muscle, whose function is to permit movement. We know that to move or lift a load against another force, it is easier to use levers, and it is this principle which the musculoskeletal system adopts and which we must examine.

The component parts which are used in a lever are as follows:

  • Lever - nearly always the bone
  • Fulcrum - pivot point of the lever, which is usually the joint
  • Muscle Force - force that draws the opposite ends of the muscles together
  • Resistive Force - force generated by a factor external to the body (e.g. gravity, friction etc.) that acts against muscle force
  • Torque - the degree to which a force tends to rotate an object about a specified fulcrum

There are different types of levers dependent upon the position of fulcrum, effort and resistive force.

First Class lever: Muscle force and resistive force are on different sides of the fulcrum e.g. the head resting on the vertebral column. As the head is raised, the facial portion of the skull is the resistance, the fulcrum is between the atlas and occipital bone, and the effort is the contraction of the muscles of the back.

Second Class lever: Muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through the level longer than that through which the resistive force acts - e.g. raising the body up onto the toes. The body is the resistance, the ball of the foot is the fulcrum, and the effort is the contraction of the calf muscle.

Third Class lever: Muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through the lever shorter than that through which the resistive force acts - e.g. adduction of the thigh. The weight of the thigh is the resistance, the hip joint is the fulcrum, and the contraction of the adductor muscle is the effort.

Most of the limbs of the human body are articulated by third class levers.

Each of the movements of the muscles for the various parts of the body are described by the following terms:

  • Abductor - Moves a limb away from the midline
  • Adductor - Moves a limb toward the midline
  • Extensor - Increases the angle at a joint (extends a limb)
  • Flexor - Decreases the angle at a joint (flexes a limb)
  • Pronator - Turns a limb to face downwards
  • Supinator - Turns a limb to face upwards
  • Rotator - Rotates a limb
  • Sphincter - Closes an orifice or opening

Heat production

Muscle contractions produce heat and as much as 70% of body heat is produced by energy produced in muscle tissue. Blood is an essential element in temperature control during exercise, taking heat from the body core and working muscles and redirecting it to the skin when the body is overheating. When the internal heat of the body reaches too low a level thermoreceptors in the skin relay a message to the hypothalamus in the brain. In response to this signal the skeletal muscles contract and relax in an involuntary manner (shivering) increasing muscle activity to generate heat. In turn, muscles are also responsive to exterior heat - cold air increases muscle tone, and hot conditions have a relaxing effect on muscles.

Maintenance of posture

As well as enabling movement, muscles also maintain posture and body position. Sensory receptors in the muscles monitor the tension and length of the muscles and provide the nervous system with crucial information about the position of the body parts, therefore enabling posture to be maintained. Muscles are never completely at rest, nor do they actually have to shorten in length when they contract. The tension or tone produced as a result of these contractions between various opposing groups of muscle helps us remain in a static position, even when we are asleep.

Neurological System

All of our body systems work in conjunction with each other and none are capable of working in isolation. The nervous system controls and coordinates the functioning of all other systems in response to our surroundings. Each stimulus or change in our environment is detected by our senses and messages are interpreted by the brain which in turn, sends directions to the various organs to respond and adapt according to the external conditions which affect our body.

The function of the Neurological System is to transmit and receive a constant series of messages via electrical impulses to and from the control centre situated in the brain. These messages are either those receiving "information" from various body tissues via the sensory nerves, or those initiating the function of other tissues such as organs, muscles, etc.

The nervous system is divided into the central nervous system (CNS) which includes the brain and spinal cord, and the peripheral nervous system (PNS) comprising cranial nerves and spinal nerves. The PNS includes nerves emerging from the brain (cranial nerves) and nerves emerging from the spinal cord (spinal nerves). These nerves are divided into sensory nerves which conduct messages from various parts of the body to the CNS, whilst motor nerves conduct impulses from the CNS to muscles and glands. The PNS is further divided into the Somatic System (SNS) and Autonomic System (ANS), depending on the area of the body these messages are transmitted to and from.

The SNS consists of sensory neurons from the head, body wall, and extremities, and motor neurons to skeletal muscle. The motor responses are under conscious control and therefore the SNS is voluntary. Certain peripheral nerves perform specialised functions and form the autonomic nervous system; they control various activities which occur automatically or involuntarily such as the contraction of smooth muscle in the walls of the digestive system. The autonomic system is further divided into the sympathetic and parasympathetic systems. These two systems provide nerve stimuli to the same organs throughout the body, but bring about different effects.

The Sympathetic Nervous System helps prepare the body for "fight or flight" and create conditions in the tissues for physical activity. It is stimulated by strong emotions such as anger and excitement and will therefore speed up heart rate, increase the activity of sweat glands, adrenal glands, and decrease those of the digestive system. It also produces rapid redistribution of blood between the skin and skeletal muscles.

Conversely the Parasympathetic Nervous System slows down the body and helps prepare for a more relaxed state, ready for digestion and sleep. It will therefore increase peristalsis of the alimentary canal, slow down the heart rate, and constrict the bronchioles in the lungs. The balance between these two systems is controlled to create a state of homeostasis which is where the internal stability of the bodily systems are maintained in response to the external environment.

Effect of exercise on the neurological system

Good coordination and balance is vital to the sports person. The quicker the reactions, the more chance the individual has of having "the edge" over the opposition, or of improving his or her personal best. As with most of the Systems, the neurological system can be trained and improved with repetitive exercises. New and extended movements may be progressively attempted, practised and perfected involving action through the use of the musculoskeletal systems and, as importantly, the voluntary nervous system to initiate those actions.

It is also important to realise that to some extent nerves serving muscles under our conscious control work in pairs and therefore when rehabilitating an injured part it is often beneficial to exercise and stimulate the symmetrically uninjured part as well.

We have already noted how other systems may benefit from exercise and these improvements take place with the help of the neurological system. Like other systems, the neurological system may also be adversely affected by smoking, lack of sleep, poor diet and stress.

Conversely the nervous system is adversely affected by age. As we grow older neurons are lost and not replaced. There is also a decreased capacity for transmitting impulses to and from the brain. In addition, both voluntary and reflex actions become slower.

Endocrine System

The endocrine system affects bodily activities by releasing chemical messages, called hormones, into the bloodstream from exocrine and endocrine glands. The function of hormones are to:

  • Control the internal environment by regulating its chemical composition and volume
  • Respond to environmental changes to help the body cope with emergencies - infection, stress etc.
  • Help regulate organic metabolism and energy balance
  • Contribute to the management of growth and development

Hormones are chemicals which cause certain changes in particular parts of the body. Their effects are slower and more general than nerve action. They can control long-term changes such as rate of growth, rate of activity and sexual maturity.

The endocrine or ductless glands secrete their hormones directly into the blood stream. The hormones are circulated all over the body and reach their target organ via the blood stream. When hormones pass through the liver, they are converted by the kidneys. Tests on such hormonal end products in urine can be used to detect pregnancy.

The endocrine system consists of a series of glands which secrete hormones; they are found throughout the body and include the pituitary, thyroid, parathyroids, thymus, supra-renal or adrenal glands, part of the pancreas and parts of the ovaries and testes. Although these glands are separate it is certain that they are functionally closely related because the health of the body is dependent upon the correctly balanced output from the various glands that form this system.

The Pituitary Gland (Hypophysis)

This gland has been described as the leader of the endocrine orchestra. It consists of two lobes, anterior and posterior. The anterior lobe secretes many hormones, including the growth-promoting somatotropic hormone which controls the bones and muscles and in this way determines the overall size of the individual. Over secretion of the hormone in children produces gigantism and under secretion produces dwarfism. The anterior lobe also produces gonadotropic hormones for both male and female gonad activity. Thyrotropic hormones regulate the thyroid and adrenocorticotropic hormones regulate the adrenal cortex. It also produces metabolic hormones.

The posterior lobe produces two hormones - oxytocin and vasopressin. Oxytocin causes the uterine muscles to contract; it also causes the ducts of the mammary glands to contract and, in this way, helps to express the milk which the gland has secreted into the ducts. Vasopressin is an antidiuretic hormone which has a direct effect on the tubules of the kidneys and increases the amount of fluid they absorb so that less urine is excreted. It also contracts blood vessels in the heart and lungs and so raises the blood pressure. It is not certain whether these two hormones are actually manufactured in the posterior lobe or whether they are produced in the hypothalamus and passed down the stalk of the pituitary gland to be stored in the posterior lobe and liberated from there into the circulation.

The Thyroid

The right and left lobes of this gland lie on either side of the trachea united by the isthmus. Average size of each lobe is 4cm long and 2cm across but these sizes may vary considerably. The secretion of this gland is thyroxine and tri-iodothyronine. Thyroxine controls the general metabolism. Both hormones contain iodine but thyronine is more active than thyroxin. Under-secretion of this hormone in children produces cretinism; the children show stunted growth (dwarfism) and fail to develop mentally. Under secretion in adults results in a low metabolic rate. Over secretion in adults gives rise to exophthalmic goitre and the metabolic rate is higher than usual. Such persons may eat well but bum up so much fuel that they remain thin. This is usually accompanied by a rapid pulse rate. This gland, therefore, has a profound influence on both mental and physical activity.

The Parathyroid Glands

There are four of these glands, two on either side lying behind the thyroid. Their secretion is parathormone - the function of which is to raise the blood calcium as well as maintain the balance of calcium and phosphorus in both the blood and bone structures. Under secretion gives rise to a condition known as tetany in which the muscles go into spasm, and over secretion causes calcium to be lost to the blood from the bones giving rise to softened bones, raised blood calcium and a marked depression of the nervous system.

The Thymus Gland

This gland lies in the lower part of the neck and attains a maximum length of about 6cm. After puberty the thymus begins to atrophy so that in the adult only fibrous remnants are found. Its secretion is thought to act as a brake on the development of sex organs so that as the thymus atrophies, the sex organs develop. Recent research into the activity of this gland reveals that it plays an important part in the body's immune system by producing T lymphocy - the T standing for thymus derived.

The Suprarenal or Adrenal Glands

These are two in number, triangular in shape and yellow in colour. They lie one over each kidney. They are divided like the kidney into two parts -the cortex and the medulla. The cortex is the outer part of the gland and produces a number of hormones called cortico-steroids. Their function is to control sodium and potassium balance, stimulate the storage of glucose and affect or supplement the production of sex hormones. The medulla or inner layer produces adrenaline, a powerful vasoconstrictor. Adrenaline raises vessels and raised the blood sugar by increasing the output of sugar from the liver. The amount of adrenaline secreted is increased considerably by excitement, fear, or anger, which has caused the adrenals sometimes to be referred to as the glands of fright and fight.

The Gonads or Sex Glands

These glands are naturally different in men and women because they serve different, though, in many respects, complementary functions. In the female the gonads are the ovaries and in the male the testes. Female sex hormones are oestrogen and progesterone. The male sex hormone is testosterone, though each sex produces a small quantity of the opposite hormone. The female hormones are responsible for developing the rounded, feminine figure, breast growth, pubic and axillary hair and all the normal manifestations of femininity and reproduction. Male hormone is responsible for voice changes, increased muscle mass, development of hair on the body and face and the usual development of manliness.

Pancreas

The endocrine part of the pancreas consists of clumps of cells called islets of Langerhans which secrete insulin. Insulin regulates the sugar level in the blood and the conversion of sugar into heat and energy. Too little insulin results in a disease known as diabetes mellitus. This disease is divided into one form, juvenile onset, which occurs before the age of 25, and another form which begins in maturity. It is a very common disease. It is known that some half million people in the United Kingdom suffer from it sufficiently badly to need treatment but is has been estimated that there are many more people in whom the disease exists at a sub-treatment level.

Cardiovascular System

The cardiovascular system comprises of the heart, blood, blood vessels and lymphatic system.

Heart

The function of the heart is to pump blood around the body. The heart is a hollow, muscular organ divided by a vertical wall called the septum. These two chambers are further divided into the thin walled atrium above, and a thick walled ventricle below, making a total of four chambers. Between each pair of chambers are valves preventing any back flow of blood. Blood vessels leaving the heart generally carry oxygenated blood through vessels known as arteries. These are large, hollow elastic tubes with thick muscular walls which are designed to withstand the high pressure with the blood leaving the heart. Their size gradually diminishes as they spread throughout the body, ultimately reaching fine, hair-like vessels known as capillaries. Blood vessels which return blood to the heart are known as veins which generally carry de-oxygenated blood to the heart. They are elastic tubes containing valves to help prevent back flow of blood. Blood is forced through arteries by the pressure from the heart whereas venous flow is aided by muscular contraction.

The only two exceptions to the above are the pulmonary artery, which carries de-oxygenated blood from the heart to the lungs, and the pulmonary vein, which carries oxygenated blood from the lungs to the heart. The circulation is divided into two principle systems known as the general or systemic circulation which is the circulation around the body, and the pulmonary circulation to and from the lungs.

Blood

The fluid which surrounds tissue cells throughout the body is called interstitial fluid and is serviced by blood transporting oxygen and nutrients to it whilst lymph removes toxins and waste products. The blood has three main functions, transport, regulation, and protection.

Transports

  • oxygen from the lungs to the cells
  • carbon dioxide from the cells to the lungs
  • nutrients from the intestines to the cells
  • waste material from the cells
  • hormones from the endocrine glands to the cells
  • heat from various cells

Regulates

  • pH (concentration of hydrogen ions)
  • body temperature
  • salts
  • water content in the cells

Protection

  • Blood prevents loss by clotting and combats toxins

Lymphatic

As blood is the main transport system to the body, so it may also bring bacteria to the tissues. The lymphatic system is the protective system which picks up materials, cleanses them of waste products and toxins, and returns them to the blood. Although it is described as a separate system it is really part of the vascular system, being intertwined with the blood circulation.

Effect of exercise on the cardiovascular system

The effects of regular exercise on the vascular system:

  • The supply of blood vessels to the heart will increase thereby lowering blood pressure and improving the functioning of the heart
  • Lowers the cholesterol levels in the blood helping to reduce the risk of arteries "furring up" and possible heart disease
  • The period needed for the heart rate to return to normal after exercise is reduced
  • The network of capillaries in a muscle will increase thereby increasing the supply of blood, oxygen and nutrients to the working muscle

Respiratory System

The respiratory system comprises of the nose, mouth, throat, larynx, trachea, bronchi and lungs. The function of the respiratory system is to facilitate gaseous exchange to take place in the lungs and tissue cells of the body.

Oxygen is required by cells in the body to allow various metabolic reactions to take place and to produce energy and is therefore essential to life. The respiratory system may be defined as the organs and tissues through which air is passed into and out of the body to allow the necessary gaseous exchanges to take place.

External respiration is the means by which oxygen from the air passes into the blood stream for transportation to the tissue cells and carbon dioxide is collected and transferred back to the lungs and expelled from the body (see Vascular System).

Internal respiration involves the vital chemical activities which take place in every living cell requiring oxygen and glycogen to combine and release energy, water and carbon dioxide.

The normal rate of inspiration and expiration, the respiration rate, is about 16 times a minute in an adult.

Effect of exercise on the respiratory system

In the Cardiovascular section the benefits of exercise were discussed in relation to the improved functioning of the heart and the lowering of blood pressure. Combined with increased maximum oxygen consumption (VO2 max), or lung capacity, these are all vital contributors to being fit and healthy.

An athlete who has not properly trained their cardiovascular system is likely to incur other injuries more easily by the rapid onset of fatigue and the consequent lowering of motivation and mental awareness. For anyone competing at varying altitudes, they must allow themselves a considerable period to acclimatise before an event. Even climbing to a moderate altitude decreases the maximum uptake by 7% to 8% due to the change in atmospheric pressure. This decrease in oxygen being supplied to the muscles may decrease performance by 4 to 8% depending on the duration of competition, a considerable disadvantage at the finish line.

Even the athlete who prepares and acclimatises well may still not match natives of high altitude areas such as the Andes, who have a larger chest capacity, more alveoli, larger capillary beds and higher red blood cell count. Since people may suffer from altitude sickness when moving from low to high altitudes, sufficient time must also allowed for these symptoms to disappear before starting intensive training.

used with permission, The Sports Coach


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