Unit 1 – Principles of Anatomy of Physiology – DF NOTES

The skeleton

The skeleton is the central structure of the body, providing a framework for all the soft tissues to attach to. It is made up of 206 bones of varying shape and size to suit their function, joints and cartilage. We are born with about 300 bones but as we develop a number of these fuse together.

Regions of the skeleton

These regions can in turn be arranged into two distinct parts known as the axial and appendicular skeleton, both of which have very different structures and functions within the body.

The axial skeleton forms the central axis of the body from which the limbs hang from. It consists of the 80 bones including the skull, sternum, ribs and vertebrae and its principle role is one of support for the body.

The appendicular skeleton is the parts hanging off the axial skeleton. It consists of the 126 bones that make up the pelvic and pectoral (shoulder) girdles, the arms and the legs. The principle role of the appendicular skeleton is one of movement, it provides a freely moveable frame which enables us to walk, run and jump.

Bones of the anterior skeleton

This diagram shows some of the main bones in the anterior skeleton.

Bones of the posterior skeleton

This diagram shows some of the main bones in the posterior skeleton.

Types of bone

There are five different types of bone, which are classified in relation to their shape, size and function. These are described below:

Type of Bone Function

Long Long bones are effective levers producing movement. They have greater length than width.

Short Short bones have good strength and weight bearing potential, but less movement. They are cube shaped nearly equal in length and width.

Flat Flat bones are generally thin offering protection and a large surface area for muscle attachment.

Irregular Irregular bones are complex shapes with various functions including protection and support for internal organs.

Sesamoid Sesamoid bones are small nodules of bone embedded in certain tendons. They modify pressure and diminish friction.

The main bones of the skeleton

This section describes the main bones which form the structure of the skeleton.

Skull

The skull is made from 28 bones which are joined together by fibres. 8 of these bones form the cranium (which protects the brain), 14 bones form the face (eye sockets, cheek, nose and jaw) and 6 bones form the middle ear.

Sternum

The sternum is the flat bone in the middle of the chest. It protects the heart and provides a place of attachment for the ribs and the clavicles.

Ribs

Adults have 12 pairs of ribs, which run between the sternum and the thoracic vertebrae, protecting the heart and lungs. 7 pairs of these are known as true ribs and attach to the sternum, 3 pairs attach to the 7th rib and the last 2 pairs, known as floating ribs, do not attach to anything other than a part of the vertebrae.

Vertebral column

The vertebral column is made up of 33 bones. 24 of these are moveable, separated from one another by intervertebral discs, and the other 9 are fused together. Apart from the first two cervical vertebrae (the atlas and axis) which allow us to nod and shake our head all the other vertebrae have a similar basic structure.

The basic structure of a vertebra

The vertebral column protects the spinal cord, supports the body and head, and provides a place for the ribs and muscles of the back to attach.

The vertebral column is divided into 5 main sections: cervical, thoracic, lumbar, sacral and the coccyx. These can be seen on the diagram below:

The main functions of the vertebral column are to enclose and protect the spinal cord, to support the body and head and to serve as a point of attachment for the ribs and muscles of the back.

Clavicle

The clavicle is attached to the top of the sternum and forms part of the shoulder girdle. It connects the upper arm to the trunk of body and keeps the scapula at the correct distance from the sternum.

Scapula

The scapula connects the humerus (upper arm) to the clavicle (collar bone) and provides a point of attachment for many of the muscles of the upper back and arms.

Humerus

The humerus is the long bone in the upper arm part of the arm.

Radius

The radius is one of the long bones in the lower part of the arm. It is positioned beside the ulna and runs to the thumb side.

Ulna

The ulna is the other long bone in the lower part of the arm. It is positioned beside the radius and runs to the little finger.

Carpals

The carpals are the 8 bones which make up the wrist. They are arranged in two rows of four.

Metacarpals

The metacarpals are the 5 bones between the wrist and the fingers.

Phalanges

The phalanges are the 14 bones of the fingers and thumb. There are 3 bones in each finger and two in the thumb.

Pelvis (illium, pubis and ishium)

The pelvis is also called the pelvic girdle or the hip girdle. It consists of 3 bones, the illium, pubis and ischium, which are fused together. These bones support the weight of the body from the vertebral column and protect and support the lower organs, including the urinary bladder and the reproductive organs.

Femur

The femur is the long bone in the upper part of the leg and forms the knee joint with the tibia.

Tibia

The tibia is one of the long bones in the lower part of the leg. It is the weight-bearing bone of the lower leg and positioned towards the front of the leg.

Fibula

The fibula is the other long bone in the lower part of the leg. It is the non weight-bearing bone of the lower leg and positioned towards the back of the leg.

Patella

The patella bone lies within the patella tendon and floats over the knee. It is one of the three bones, along with the tibia and femur, which form the knee joint.

Tarsals

The tarsals are the 7 bones which make up the ankle.

Metatarsals

The metatarsals are the 5 bones which travel from the ankle to the toes.

Phalanges

The phalanges are the 14 bones of the toes. There are 3 bones in each toe and two in the big toe.

Classification of joints

A joint is a site in the body where two or more bones meet. They can be classified according to their function and structure. Functional classification is based on the amount of movement there is between the articulating surfaces, where as structural classification is based on the presence or absence of a joint cavity.

There are three types of joint in the body:

Fibrous or fixed Cartilaginous or slightly movable Synovial or freely movable

Fibrous or fixed joints

A fibrous joint or a fixed joint allows no movement at all. The bones are joined together by tough fibrous tissue and there is no joint cavity. Examples in the body include the sutures in the skull.

Cartilaginous or slightly movable joints

A cartilaginous joint allows slight movement. The ends of bones, which are covered in articular or hyaline cartilage, are separated by pads of fibrocartilage. Examples include the vertebrae and the pubic bones.

Synovial or freely movable joints

A synovial joint is a freely moving joint and is the most commonly occurring type of joint in the body. Synovial joints such as the knee, elbow and hip have a number of common features that are outlined below.

Descriptions of joint movements

The following table shows the different types of movements that are possible. The possible range of movement in synovial joints varies according to their shape, surface and joint type.

Movement type Description ExampleFlexion Decreasing the angle of a joint, or

bending a limbBending the knee

Extension Increasing the angle of a joint, or straightening a limb

Straightening the knee

Abduction Taking a limb away from the mid-line of the body

Lifting the arms from the side of body

Adduction Taking a limb towards the mid-line of the body

Lowering the arms towards the side of the body

Rotation When a limb rotates about it's own axis Looking over your shoulderCircumduction When one end of the limb describes a

circleDoing the butterfly stroke in swimming

Supination Rotation of the forearm causing the palm of the hand to face up

Turning your hands from facing down, to turning up

Pronation Rotation of the forearm causing the palm of the hand to face down

Turning your hands from facing up to facing down

Eversion At the ankle when the sole of the foot is turned outwards

Kicking a football with the instep

Inversion At the ankle when the sole of the foot is turned inwards

When you twist your ankle it is excessive inversion

Dorsi flexion At the ankle joint when the toes are pulled upwards towards the shin

When you do a calf stretch

Plantar flexion At the ankle when the toes are pointed downwards

When you stand on tip toes

Depression Downward movement of the shoulder girdle

Pushing the shoulder blades down

Elevation Upward movement of the shoulder girdle

Shrugging the shoulders

Horizontal flexion & extension

Occurs when the arm (at shoulder height) moves across the body, and back out to the side.

Bringing the arm across and then away from the body

Hyper extension Excessive extension beyond straight A crab position in gymnasticsLateral flexion Bending to the side Tilting of the headProtraction The shoulders are drawn forwards Rounding of the shouldersRetraction The shoulders are drawn backwards Opening out the chest

The muscular system

There are over 600 muscles in the human body, accounting for approximately 40% of the average person's weight and these are responsible for all voluntary and involuntary movement that the body makes.

Function of the muscular system

The primary function of the muscular system is movement. Muscles allow us to move from place to place as well as performing involuntary functions such as pumping/moving blood around the body, pushing nutrients and faeces through the digestive system, and

aiding breathing. Muscles also provide the body with stability and posture, and help to regulate body temperature.

Types of muscle

There are three types of muscles within the human body, which vary in their structure, location and type of control.

Skeletal muscle (voluntary)

Smooth muscle (involuntary)

Cardiac muscle (involuntary)

Muscles and movement

Muscles act in groups rather than singly to generate movement, with most arranged in opposing pairs, for example flexors and extensors such as the biceps and triceps, and the quadriceps and hamstrings.

Muscles can only pull on their bony attachment (exerting a contracting force), they cannot push. The muscle responsible for a movement is called the prime mover or agonist, for example the biceps flexing the arm at the elbow.

As muscles work in opposing pairs when the agonist contracts the opposing muscle has to relax to allow the movement to occur. This muscle is called the antagonist, so when the biceps flex the elbow, the triceps are the antagonist and relax to allow the movement to occur.

Muscles known as fixators or stabilisers hold or fix joints in a stable position. These tend to be large postural muscle groups which work the trunk and legs (core muscles). They fix the position of the body when a specific movement or action takes place.

Other muscles known as synergists (which tend to be smaller muscles) assist the prime mover.

For example:

Movement Agonist Antagonist Fixator SynergistElbow Flexion Biceps Triceps Deltoids Brachialis

Types of muscle contraction

When a muscle contracts it can either shorten, lengthen or stay the same length. When a muscle either shortens or lengthens the contraction is known as an isotonic contraction, if the muscle does not alter in length it is referred to as an isometric contraction.

Isotonic contraction

An isotonic contraction is where muscles change their length during an exercise. When the muscle shortens it is known as a concentric contraction and when it lengthens it is known as an eccentric contraction. These types of contraction are the standard way in which muscles work to produce movement for example walking, running and kicking.

For example

During the upward phase of the bicep curl the biceps are the agonist and are contracting concentrically. During the downward phase the biceps are still the agonist but this time they are contracting eccentrically. They are in effect acting as a brake, resisting gravity to control the downward phase of the exercise.

This is sometimes a difficult concept to understand and you may find it useful to try the exercises and feel how the muscles are working.

Concentric Eccentric

Muscle length Shortens under tension Lengthens under tensionEffect on movement Causes movement Controls movementExample 1 Bicep curl upward phase Bicep curl lowering phaseExample 2 Leg extension Upward phase of a lateral

pull down the biceps and latissimus dorsi are controlling the movement

Isometric contractions

An isometric contraction is where the muscle is working, but does not change its length. The muscles are working but in a static manner. Examples include holding a bar bell at arms length, arm wrestling, or a rugby scrum. The muscles are still working, but are not changing length.

Isokinetic contractions

An isokinetic contraction occurs when the muscle shortens at a constant speed, and exerts maximum tension over the full range of movement. This is only fully achieved with specially designed weight training equipment, which automatically adjusts the resistance over the full range of movement.

Muscle fibre types

All skeletal muscle fibres are not alike in structure or function. Based on various structural and functional characteristics, skeletal muscle fibres can be sub divided into three different types:

Type 1 fibres (slow twitch) Type 2a fibres (fast twitch) Type 2b fibres (fast twitch)

Type 1 fibres

These fibres are also known as slow twitch or slow oxidative fibres. They are red in colour, contract slowly but can keep going for long periods of time and withstand the onset of fatigue. These fibres are smaller than fast twitch fibres, develop less force and are best suited to aerobic endurance type activities.

Type 2 fibres

Type 2 fibres are also known as fast twitch fibres. They are white in colour, contract rapidly but are quickly tired. These fibres are fast and strong and are suited to anaerobic type work.

Type 2 fibres can be divided into 2a and 2b. Type's 2a fibres are called intermediate fast-twitch fibres and they can use both aerobic and anaerobic pathways almost equally to create energy. In this way they are a combination of Type 1 and Type 2 muscle fibres.

Type 2b fibres use anaerobic metabolism to create energy and are the 'classic' fast twitch muscle fibres that excel at producing quick, powerful bursts of speed. This muscle fibre has the highest rate of contraction (rapid firing) of all the muscle fibre types, but they also have a much faster rate of fatigue and can't last as long before they need rest.

All individuals have a combination of all types of muscle fibres in their muscles. However, the exact composition of types will vary from individual to individual and is genetically determined. Different parts of the body will also have different combinations of muscle fibre types.

Our genetic make up may well influence what sports we are naturally good at. Top class athletes tend to fall into sports that match their genetic predispositions. For example world class sprinters possess about 80% fast twitch fibres whilst marathon runners tend to possess about 80% slow twitch fibres.

Fibres cannot change their type during life, however, they can be trained, so type 1 fibres can have some properties of type 2, or vice versa. 2a fibres can easily be trained to either favour aerobic exercise (long distance running), or anaerobic exercise (sprinting), or a combination of both (football player).

Introduction to the cardiovascular system

The cardiovascular system is composed of the heart, the blood vessels and the blood. Its function is to deliver oxygen and nutrients to all the cells of the body and to excrete waste products.

The heart

The heart is about the size of a closed fist and is located in the thorax between the two lungs (slightly to the left of the centre of the chest). It consists of four chambers, two upper atria and two lower ventricles. The walls of the four chambers are made of cardiac muscle, called the myocardium, and each chamber has a one-way valve at its exit that

prevents blood from flowing backwards. The arteries carry blood away from the heart and the veins carry blood back towards the heart.

Structure of the heart

The heart valves

Blood is pumped through the chambers of the heart aided by four heart valves which prevent the backflow of blood, ensuring a one way flow. They consist of two or three flaps (cusps) of tissue that open to let blood through then seal shut to prevent any backflow.

Atrioventricular valves (tricuspid and bicuspid valves)

The tricuspid valve is located between the right atrium and right ventricle. Deoxygenated blood is returned from the body via the vena cava into the right atrium. From the right atrium the blood passes through the tricuspid valve into the right ventricle.

The bicuspid (also referred to as the mitral valve) is located between the left atrium and left ventricle. After receiving a fresh supply of oxygen the blood is returned from the lungs via the pulmonary vein into the left atrium. From the left atrium the blood passes through the bicuspid valve into the left ventricle.

Semilunar valves

The pulmonary semilunar valve is located between the right ventricle and the pulmonary artery. The right ventricle pumps blood through the pulmonary semi-lunar valve into the pulmonary artery which carries the blood to the right and left lungs.

The aortic semilunar valve is located between the left ventricle and the aorta. The left ventricle pumps blood leaves through the aortic semilunar valve into the aorta which ultimately carries the oxygenated blood out to all parts of the body.

Circulatory system

The vascular system has two pathways of circulation, the pulmonary circulation (to the lungs) and the systemic circulation (to the body).

Pulmonary circulation

This is the flow of blood from the right side of the heart to the lungs and then back to the left side of the heart. The right atrium collects deoxygenated blood that is returning from the body via two large veins the inferior and superior vena cava. The blood then moves from the right atrium into the right ventricle. From the right ventricle the deoxygenated blood is pumped to the lungs through the pulmonary artery. Within the lungs pulmonary diffusion takes place, the blood is enriched with oxygen and carbon dioxide is removed before the blood returns to the heart via the pulmonary vein.

Systemic circulation

This is the flow of blood from the left side of the heart to all parts of the body and then back into the right side of the heart. The left atrium receives oxygenated blood from the lungs via the pulmonary vein. The blood then moves into the left ventricle. This oxygenated blood is then pumped through the aorta (the largest artery in the body) to the rest of the body. It transports the oxygen and nutrients to the working muscles, internal organs and all other living tissues within the body.

The muscular walls of the left ventricle are three times thicker than those of the right; this is because the left ventricle is responsible for pumping blood all round the body, whereas the right ventricle is only responsible for pumping blood to the lungs.

Blood vessels

The circulatory system sometimes referred to as the vascular system consists of the arteries, veins and capillaries through which the heart pumps blood around the body. It ensures that every cell in the body receives an adequate supply of oxygen and nutrients and removes any waste products such as carbon dioxide and water.

Arteries

The arteries always carry blood away from the heart. They have thick muscular walls and contain elastic cartilage and smooth muscle, allowing the vessels to contract and expand to cope with the high pressure of blood within them. The blood within arteries is generally bright red as it is oxygenated; the one exception to this is the pulmonary artery as this is carrying de-oxygenated blood from the heart to the lungs. Most arteries lie deep in the body, however, some such as the carotid artery are nearer the surface and it is in these places that a pulse can be felt.

By branching and re-branching the arteries progressively become smaller and are known as arterioles. These arterioles then join the smallest blood vessels known as capillaries and it is here that tissue diffusion occurs.

Capillaries

Capillaries are the essential link between arterioles and venules. They are found in all parts of the body and allow the exchange of materials between the blood and tissues take place, this process is known as tissue diffusion. Diffusion of gases (oxygen and carbon dioxide) and nutrients can occur easily in the capillaries as they are only one cell thick and have a semi permeable membrane.

Veins

After diffusion the blood moves from the capillaries into the tiniest of veins know as venules which then join to form larger and larger veins. The veins always carry blood back towards the heart. They have thin walls that are less muscular than the arteries as they do not have to withstand the same pressure as arteries. The blood within the veins is of a

bluish colour as the majority of it is de-oxygenated. The one exception to this is the pulmonary vein as this is carrying oxygenated blood from the lungs back to the heart.

Since the blood is at low pressure in the veins they are supplied with valves at regular intervals to prevent back flow and to keep the blood flowing in one direction.

Function of the cardiovascular system

The function of the cardiovascular system is to circulate blood through the network of vessels that run through the body. The purpose of this is to provide each cell with oxygen and nutrients needed to work, and to help dispose of waste products.

The oxygen rich blood leaves the heart via the arteries and delivers oxygen and nutrients to the body cells. The oxygen is used by the cells to produce energy, and in exchange the cells release waste products, including carbon dioxide and water. These waste products are carried in the oxygen poor blood back to the heart via the veins. The heart then

pumps this blood to the lungs where carbon dioxide is 'unloaded' and exhaled and fresh oxygen is inhaled and 'taken up'. The now oxygen rich blood is carried back to the heart where the cycle can begins again.

As well as circulating oxygen around the body, and removing waste products, the cardiovascular system regulates body temperature. The process of ensuring the body temperature remains constant is called thermoregulation. Body temperature can be affected by the environment including the weather, through illness and by taking part in physical activity. It is important that body temperature is kept relatively constant, as extreme changes in temperature can affect the body's ability to function normally. To help keep the temperature relatively constant the blood vessels widen (vasodilation) to promote heat loss if the body gets too hot, and narrow (vasoconstriction) to conserve heat loss if the body gets too cold.

Function of the blood

Blood is the transport system within the body. It transports oxygen, carbon dioxide, nutrients, hormones, enzymes and electrolytes. There are about 9 pints of blood in the human body. However, small increases to this can be achieved through training. Blood is made up of four different components:

Red blood cells White blood cells Plasma Platelets

Red blood cells

Red blood cells are sometimes referred to as erythrocytes and are manufactured within the bone marrow. They make up around 99% of the blood cells in the body and are red in colour as they contain a red-coloured protein known as haemoglobin. Haemoglobin combines readily with oxygen to form a compound called oxyhaemoglobin through which oxygen is transported to the tissue cells.

White blood cells

White blood cells are sometimes referred to as leucocytes. They are colourless and much fewer in number. The white blood cells are manufactured within the bone marrow, the lymph nodes and the spleen. There are several kinds of white blood cells, some fight and

destroy bacteria, some are involved in tissue repair, whilst some actually produce chemicals to protect against infection.

Plasma

Plasma is a straw coloured liquid in which dissolved salts, hormones, fats and sugar are carried. It accounts for approximately 55% of the total blood volume.

Platelets

Platelets are parts of cells that help to produce clotting when a blood vessel is damaged. They become sticky when in contact with the air to form the initial stage of repair to damaged tissue.

Introduction to the respiratory system

The respiratory system is made up of a system of tubes and muscles that deliver air into our two lungs. It responsible for transporting the oxygen from the air we breathe into our blood stream. This oxygen can then be delivered to the cells of the body where it is used in combination with the food we have eaten to produce the energy needed for all bodily functions. The average person takes around 26,000 breaths a day to deliver the amount of oxygen required by the body cells.

Structure of the respiratory system

Read the information on the structures of the respiratory system by clicking on the link below.

Structures of the respiratory system

The lungs

The lungs are two spongy, cone-shaped organs in the thoracic cavity and it is here that gaseous exchange takes place between the air and the blood. A pleural membrane surrounds each lung. This contains a fluid which prevents any friction of the lungs against the rib cage when they expand and contract during breathing.

The capacity of the lungs varies with the size and age of the person. Taller individuals tend to have larger lungs. As we get older our lungs also begin to lose their elasticity thus reducing their capacity.

The oxygen used by the body comes from the air that we breathe in. Pure dry air contains 20.93% oxygen and 0.03% carbon dioxide along with other gases and water vapour. Expired air contains much less oxygen and more carbon dioxide (16% oxygen and 4.5% carbon dioxide).

Respiratory muscles

The two muscles that form part of the respiratory system are the diaphragm and the intercostal muscles.

The diaphragm

The diaphragm is a large dome-shaped muscle situated at the bottom of the ribcage. When it contracts it flattens and increase the size of the thoracic cavity.

The intercostal muscles

There are two types of intercostal muscles; the external intercostals and the internal intercostals. The internal intercostal muscles are located inside of the ribcage and extend from the front of the ribs, around back and past the bend in the ribs. The external intercostal muscles are located outside the ribcage and wrap around from the back of the rib almost to the end of the bony part of the rib in front. The internal intercostals are responsible for pulling the ribs down, decreasing the thoracic cavity.

The external intercostals are responsible for the lifting the ribs up and out, expanding the thoracic cavity.

Function of the respiratory system

The main function of the respiratory system is to supply the blood with oxygen in order for the blood to then deliver the oxygen to the working tissues. It does this through breathing. The respiratory system is also responsible for expelling waste products produced by our cells like carbon dioxide.

Gaseous exchange

Oxygen passes into the body and carbon dioxide leaves the body through the process called gaseous exchange. For this process to work we must control the amount of each gas on each side of a semi-permeable membrane (a wall one cell thick that lets some things through but not others).

A gas will always move from an area of high concentration (or pressure) to an area of low concentration (or pressure). When we breathe in there will be a high concentration of oxygen in the lungs and a low concentration in the blood. Therefore the oxygen will pass from the lungs into the blood capillaries surrounding the alveoli.

This highly oxygenated blood now transports the oxygen to the muscle capillaries, which have a low concentration of oxygen, thus allowing the oxygen to pass into the muscle where it will be used to release energy. Carbon dioxide is diffused from the muscles into the bloodstream where it is transported to the lungs and diffused out into the alveoli.

Breathing

For much of the time breathing goes on unconsciously. It is controlled by the respiratory centre of the brain, which responds very quickly to any changes in carbon dioxide levels in the blood. A small increase in the carbon dioxide level will increase the volume of air breathed in and out. At rest breathing rate is normally about 12-14 breaths per minute.

For air to be drawn into the lungs, the pressure of the air within the lungs must be lower than that in the atmosphere. The greater the difference in pressure, the faster air can be drawn into the lungs. The pressure is created by altering the size of the thoracic cavity. The main respiratory muscles are the diaphragm and the muscles between the ribs (the intercostals).

Inspiration and expiration

Inspiration

To breathe in (inspiration) we reduce the pressure inside the lungs by increasing the size of the thoracic cavity. The external intercostal muscles pull the ribs up and out, and at the same time the diaphragm contracts, which causes the dome shaped muscle to descend by approximately 1.5cm. The combined effect of the intercostal muscles and the diaphragm is to increase the volume occupied by the lungs. Air is then drawn into the lungs until the pressure inside the lungs is equal to the atmospheric pressure.

Expiration

To breathe out (expiration) the diaphragm and the external intercostal muscles relax, thus reducing the size of the thoracic cavity. This is aided by the internal intercostal muscles which help to pull the rib cage down. The lungs return to their original size thus increasing the pressure inside. This greater pressure forces the air out of the body until the pressure is equal to that of the atmosphere.

Lung volumes

The volume of air inspired and expired varies greatly between quiet breathing and forced breathing (as when exercising) but no matter how hard we try we can never totally empty our lungs of air; if we did they would collapse and stick together. We therefore have reserves, which are part of our total lung volume. Lung volumes are an important aspect of pulmonary function testing because they can provide information about the physical condition of the lungs. More information on lung volumes can be found by clicking on the link below the graph.

The device commonly used to measure lung volumes is the spirometer; this works by measuring the amount of air that we breathe in and out over a period of time. The results of this are displayed in a graph as shown below:

Control of breathing

Our breathing or respiration is controlled by the respiratory centre. This is an area located in the brain stem, found between the spinal cord and the upper brain. The respiratory centre controls both the depth and rate of breathing and does this through neural and chemical control.

During normal, quiet breathing the respiratory centre sends nerve impulses to the diaphragm and external intercostal muscles causing them to contract, and air to be inspired. After approximately 2 seconds the respiratory centre ceases sending impulses and the intercostals and the diaphragm relax, causing air to be expired. When breathing is controlled in this way it is said to be under neural control.

Chemical control of breathing relies on the sensory organs in the brain, the aorta and the carotid arteries. These monitor the blood and sense oxygen and carbon dioxide levels and the brain adjusts the depth and rate of breathing accordingly. Normally, an increased concentration of carbon dioxide is the strongest stimulus to breathe more deeply and more frequently. This would occur for example when exercising. Conversely, when the carbon dioxide concentration in the blood is low, the brain decreases the depth and frequency of breaths. During breathing at rest, the average adult inhales and exhales about 12-15 times a minute.

Energy introduction

Every cell within our body requires energy to function and sustain life even when we are asleep. This energy is provided by the food that we eat, and the more physically active we are the more energy we require. When we start exercising the body is required to produce energy at a faster rate than it does at rest. The energy used for muscle contraction (movement) comes from carbohydrates, fats and protein although proteins are only used when the others are not available and as a last resort.

Carbohydrate

Carbohydrates are the body's preferred energy source; examples include bread, pasta, rice, potatoes and honey. It is stored in our muscles and liver cells in the form of glycogen. Glycogen is broken down into glucose, which can be used by all tissues in the body. I gram of carbohydrate provides 4kcals of energy.

Fat

Fat is stored beneath the skin (adipose tissue) and around other internal organs in the body where it serves not only as a fuel store but also as insulation and protection of vital

organs. Fats are broken down into fatty acids and triglycerides. Fatty acids are stored as adipose (fat) tissue and triglycerides are stored in the muscles. Common food sources of fat include butter, cheese, corn oil, oily fish and fat in meat. 1 gram of fat provides 9kcals of energy.

Protein

Protein is used for the growth and repair of body tissues. It is made up of amino acids, often referred to as the building blocks of all body cells. It is not stored in the body in the same way as fat and carbohydrate and where there is an excess of protein some will be converted to fat and stored and the rest excreted. Protein can be used to provide energy, however, this only occurs under conditions of prolonged starvation or exercise such as long distance running or cycling events. Common sources of protein include eggs, meat, and fish. 1 gram of protein provides 4kcals of energy.

Adenosine triphosphate (ATP)

Energy is needed for muscular activity to take place. The only useable source of energy in the body is a compound found in muscle cells called Adenosine Triphosphate (ATP).

All forms of biological work (digestion, production of hormones, transmission of nerve impulses, manufacture and repair of tissue) within the body require energy in the form of ATP.

All sources of energy found in the food that we eat have to be converted into ATP before the potential energy in them can be used. ATP is a high energy compound made up of one molecule of adenosine and three phosphates.

Special high-energy bonds exist between the phosphate groups and breaking one of the phosphate bonds releases the energy. ATP is broken down into adenosine diphosphate (ADP) and free phosphate (Pi) releasing the stored energy.

The quantity of ATP in the muscles is very limited so once this breakdown has occurred the ADP needs to be resynthesised back into ATP to ensure the constant generation of energy within the body. Note that energy is required to resynthesise ATP and it is the energy we get from food that provides this.

The process by which ATP is resynthesised is called the ATP cycle and it is the body's energy systems that are responsible for this.

Energy systems

The body has three ways in which it can regenerate ATP:

The phosphocreatine system The lactic acid system (anaerobic glycolysis) The aerobic system

Phosphocreatine system

The phosphocreatine (PC) system uses a substance called creatine phosphate to 'glue' or resynthesise the third phosphate molecule back on to ADP to make ATP. It is capable of reforming ATP very quickly, however, because there is only a very small amount of

creatine phosphate stored in the muscles the energy supply is very limited. No oxygen is required for this system and no by products such as lactic acid are produced.

This system can only be used for immediate or very short bursts of activity such as throwing a ball or running for a bus. It is used at the start of exercise for activities which require a brief maximal effort, for example, explosive events such as the long jump, javelin or 100m and is exhausted after about 10 seconds of maximal activity.

As energy demands continue after the initial burst of activity the body has to utilise other energy systems such as the lactic acid system to continue to make more ATP.

Step 1:

[D]

Step 2:

The energy released in step 1, is then used to resynthesise the ATP (as per earlier)

Lactic acid system (anaerobic glycolysis)

When the ATP and PC stores have run out the energy the body needs is provided by the lactic acid system. This system relies on the breakdown of glucose (from carbohydrates) which has been stored in the muscles as glycogen. The process by which glucose is broken down to release energy is called glycolysis. As the energy is needed quickly, and

the body does not have time to deliver oxygen to the muscles, the glucose is broken down without oxygen. It is therefore referred to as anaerobic glycolysis and provides 2 ATP per glucose molecule.

It is the breakdown of glucose that provides the energy to rebuild ADP into ATP. Glycolysis is far more complex than the ATP-PC system since it requires many reactions to occur. Since there is no oxygen present pyruvic acid is formed during glycolysis. It is then converted by the enzyme lactate dehydrogenase (LDH) into lactic acid. The lactic acid is a by-product which accumulates in the muscles and blood, causing muscular fatigue which interferes with muscular contractions. When this occurs, exercise intensity has to be reduced to enable the lactic acid to be removed.

This system is used at the start of activities where duration of the exercise is short but very intense for instance from ten seconds to three minutes or when an athlete 'kicks' at the start or finish of a race. An example of a sporting activity that uses this system is the 400m. Once the exercise has stopped extra oxygen has to be taken in to remove the lactic acid by changing it back into pyruvic acid. This is known as repaying the oxygen debt.

Aerobic system

When the intensity of exercise is lower and the duration of exercise longer energy is provided by the aerobic system. This system differs from the previous two as it uses oxygen to break down glucose or fat. Protein can be used as a fuel but only tends to be used when glycogen and fat stores are particularly low.

Oxygen is required for this system so no lactic acid is produced. The only by-products are carbon dioxide and water; it is thus an aerobic alactic system, and is a slower but much more efficient process that supplies energy for a longer period of time. This system is used when performing sustained activity, for example, swimming, cycling, walking, long distance events and exercise classes.

The first stage of the aerobic pathway is the same as that of the anaerobic lactic acid system i.e. the conversion of glycogen into pyruvic acid. It is from this point forwards that all reactions that are involved in the aerobic system then take place in very specialised structures in the cells, called mitochondria (often referred to as the 'powerhouse' of the cell).

In the presence of oxygen the pyruvic acid no longer forms lactic acid; rather it is converted to a form of acetyl coenzyme A and enters the citric acid and Krebs cycle. A number of reactions occur in this cycle before the system moves into what is referred to as the electron transport chain. It is here that the majority of ATP molecules are produced. When 1 molecule of glycogen is broken down aerobically it will provide 38 ATP; while 1 molecule of fat can provide 128 ATP.

Energy continuum

All three energy systems are operational at any one time. The interaction of these can be seen in the graph below:

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The duration and intensity of the activity will determine which the predominant system is. For example, a 1500metre race represents a mixture of energy systems used when the athlete kicks at the start and finish of a race the phosphocreatine system will be used. During the race there will be a mixture of the lactic acid and aerobic system depending on the time taken to run the distance.

For some activities it is easy to decide which energy systems are involved to produce energy, for example, a marathon probably uses 95-100% of the energy from the aerobic system.

A basketball game is usually accepted as roughly 80% anaerobic, but this depends on several factors, such as:

- The fitness of the player

- The standard of the game

- The tactics being employed

- The commitment of the players.

It is important to identify the role of each energy system in relation to the sport/position the athlete takes part in. This can calculate the total energy requirement for that activity. Knowledge of how the intensity and duration of exercise affects the source of energy, along with how each system is regulated is therefore very important.