EXPLAIN THE ELECTRICAL ACTIVITY OF THE HEART DURING A HEART BEAT

A heartbeat involves a contraction phase systole and a relaxation phase diastole.
These contractions of the heart are stimulated by electrical impulses which originate from the nodes that are present in the heart; sino-atrial (SA node) and the atrioventricular (AV node). These nodes are clusters of nerve of cells.
It is the SA node that initiates the heart to contract. Due to the fact the heart is made up predominately of cardiac muscles, it has the ability to contract in a variety of directions unlike striated muscle that can only contract one way.
The SA node is situated at the upper corner of the RA towards the outside of the atrium. The SA node stimulates contraction by the use of electrical impulses. When the SA node stimulates the contraction, the electrical impulses spread from the SA node, through the myocardium of both the atriums.
When these impulses reach the AV node, which is positioned at the top of the RV towards the interventricular septum, the AV node holds the electrical impulse static. This means contraction stops allowing blood to pour into the ventricles. This only lasts for a moment, and then the AV stimulates another electrical impulse. From the AV node the electrical impulses spread along fibres called ‘his fibres’ till they reach the interventricular septum. Here the impulses continue down the his fibres of the septum, till they reach the bottom of the ventricles. The impulses branch out; left and right around the ventricles along different fibres called purkinje fibres. This causes the ventricles to contract, and pumps the blood out of the ventricles. When all the ventricles have contracted the impulse is lost and the ventricles relax again (diastole phase). Until the SA node sends another electrical impulse and the whole process will begin again ‘another heartbeat’.

Image found at http://www.phschool.com/science/biology_place/biocoach/cardio1/electrical.html

The hearts ability to beat with no interference from the CNS (central nervous system) is due to cardiac muscle being myogenic which means it is self stimulating. The heart’s contraction is controlled by the SA node. This is what stimulates contractions, not impulses from the brain. However, sometimes the heart does need to beat faster, such as during exercise. To speed up the heart the CNS has the ability to stimulate the SA node to contract.
During times when the heart needs to speed up, the sympathetic nervous system stimulates the heart to speed up via phrenic nerves. When the heart needs to slow down the parasympathetic nervous system will take control via the vagus nerves. Without intervention from the CNS then the heart will always beat at the same rate. Parker (2007) states “The heart controls its own rhythm but its rate is controlled by the central nervous system”.

DESCRIBE THE STRUCTURE OF THE HEART AND EXPLAIN THE CARDIAC CYCLE

Image found at http://www.medicalook.com/diseases_images/heart-diseases1.jpg

Structure of the heart
The heart is a hollow, muscular vital organ. It is approximately the size of a clenched fist. Abrahams (2002 pg114) states “Roughly the shape of a pyramid on its side, the heart is said to have a base, three surfaces and an apex”. The heart sits near the left lung, in the cardiac notch. It has to the ability to move slightly during breathing.
The heart contains 4 chambers; 2 on the left and 2 on the right. The chambers are called
RA (right atrium)
RV (right ventricle)
LA (left atrium)
LV (left ventricle)
The left and right sides of the heart are separated by a thick muscular wall called the ‘interventricular septum’. The two sides of the heart needs to separated because they have different functions. The left half of the heart is responsible for pumping oxygenated blood through the systemic circulatory system, whereas the right half of the heart is responsible for pumping deoxygenated blood to the lungs via the pulmonary circulatory system. These occur simultaneously and this is way humans are referred to as having ‘double circulatory system’ due to the heart supplying to different circulatory systems with blood.
The chambers of the heart are separated by valves. Blood flows into the atriums and then past the valves into the ventricles. The valves only allow blood to flow into the ventricles at certain times and they will open to allow this to happen. At other times the valves stay closed. The valves that separate the atriums and ventricles are generically called the atrioventricular valves. The atrioventricular valve that separates the RA and the RV is called the tricuspid. The atrioventricular valve that separates the LA and the LV is called the bicuspid or mitral.
The atria of the heart are for receiving blood and the ventricles of the heart are for pumping blood out of the heart. Due to this fact, atria of the heart have thin walls, whereas the ventricles of the heart have thick walls. In fact the left ventricle has thicker walls than the right ventricle. This is because the left ventricle needs to be pump blood at high pressure so blood efficiently travels around the body. This is a longer route than the blood in the right ventricle has to travel; the heart is positioned within in the thoracic cavity where the lungs are also present.
The blood can be pumped easily out of the heart due to the fact that the majority of the heart is made of muscles. The heart consists of 3 layers; the outer layer is called epicardium, the inner layer is called the endocardium and the middle layer (which is the thickest) is called myocardium. This is cardiac muscle that has the ability to contract simultaneously in various directions. Within the hearts walls are nodes; sino-atrial and the atrioventricular node. These nodes are responsible for the stimulating and keeping the pace of the contractions of the heart.
When blood is in the right and left ventricles, it is held in the ventricles due to another set of valves, generically called semi lunar valves. Blood can only leave the ventricles when the ventricles contract and the valves open.
Blood from the RV, when ventricular contraction occurs passes the semi lunar valve called the pulmonary valve, and enters the pulmonary artery leaving the heart.
Blood from the LV, when ventricular contraction occurs passes the semi lunar valve called the aortic valve and enters the aorta, leaving the heart.
It is the movement of all the valves at specific times that make the ‘lub dub’ sound of our heart beat as they close.
Blood enters the heart via large blood vessels. The RA has 2 large blood vessels that enter it. These are called the vena cava superior and the vena cava inferior. These are naturally delivery deoxygenated blood back to the heart from the body. The LA has only one blood vessel of delivery, this is called the pulmonary vein.

Cardiac Cycle
The cardiac cycle is the sequence of events including contraction and relaxation of the heart, which means that blood is pumped around the body. The cardiac cycle can be clearly heard because the common name for this is ‘a heart beat’. When the heart beats once, this is one cardiac cycle. The cardiac cycle usually lasts less than a second.
According to Vascular Concepts (2010) “Each heartbeat or cardiac cycle is divided into 2 phases”.
Blood enters both atria at the same time. The muscle in the heart is relaxed and the atrioventricular valves are open and the semi lunar valves close making a lub sound. This is called diastole phase.
The second phase is the systole phase. The sino-atrial node of the heart signals the atria to contract, forcing more blood into the ventricles. The contraction reaches the atrioventricular node and this signals the ventricles to contract. As they contract, the atrioventricular valves (tricuspid and bicuspid) snap shut (making the dub noise) and the semi lunar valves open, due to the pressure of the blood inside the ventricles. The blood with the contraction is forced out of the heart.
The contraction slowly stops. The heart is relaxed. The atrioventricular valves open allowing blood to flow freely to the ventricles and this is now called the diastole phase. The whole process has begun again.

DESCRIBE THE STRUCTURE OF ARTERIES AND VEINS AND CAPILLARIES AND RELATE THIS TO THEIR FUNCTION

Humans like all multicellular organisms need oxygen for cellular respiration, without this process occuring then we would die.
Oxygen needs to be able to travel to every cell of the body quickly and efficiently. Blood also needs to remove unwanted substances from the body and to distribute substances that the body needs, such as nutrients. This major task can only be carried out due to the vast array of blood vessels that the body has. These blood vessels make a large transport network throughout the body together with the heart and the blood itself they form the circulatory system.
However there are 3 main types of blood vessels; arteries, veins and capillaries. They all are part of the circulatory system, but they are all structurally different but have some similarities such as; they all are hollow tubes that have some form elasticity. They are different for a reason; they needed to be different because they all have different functions.



Image found at http://www.hcc.uce.ac.uk/physiology/circulation02.htm

Arteries
The function of arteries is to carry oxygenated blood through the body’s systemic circulatory system.
Arteries have thick muscular walls. This allows them to be strong and stay open when blood is pumped into them from the heart at high pressure. The lumen (the hollow centre) of the arteries is quite small. However, the endothelium (the lining of the lumen) is folded. The endothelium being folded is necessary. In conjunction with the elastic tissue that is encased in the walls of the artery, this allows for expansion. Expansion is needed when blood is pumped through the arteries due to the high pressure of the blood. Expansion may also be needed if the cardio vascular system needs to work harder, such as during times of exercise.
Veins
The function of veins is to transport deoxygenated blood back to the heart during systemic circulation.
Veins are structurally different to arteries. Although they contain some elasticity, there is not an abundant amount of elastic fibres in the veins wall, unlike arteries. They also have only a thin muscle wall and not a thick muscle wall like arteries. They do not need their walls to be tolerant of high pressure blood flow. This is because deoxygenated blood does not travel at high pressure; it has not just left the heart. The lumen is large. It has a smooth endothelium, since there is no need for expansion. However, when blood is returning back to the heart, it has to defeat gravity. This is done by the veins due to the use of valves. These valves stop the blood flowing backwards.
Capillaries
. According to Ivy Rose (2010) capillaries function “Is to supply tissues with components of, and carried by the blood and also to remove waste, from the surrounding cells as opposed to simply moving the blood around the body (in the case of other blood vessels)”.

These are the smallest blood vessels of the circulatory system. There are thousands of capillaries in the human body and usually found in close proximity to each other. Due to this fact they are commonly referred to as capillary beds. To achieve their function, capillaries are only one cell thick. This thickness is necessary for diffusion of metabolic substances and other substances. For diffusion to occur and be effective the diffusion pathway needs to be short i.e. one cell thick.
Due to capillaries being the smallest blood vessels, this also aids diffusion. Blood cells can only travel through capillaries in single file. This enables optimum diffusion in and out of the cells. Due to their function capillaries carry oxygenated blood and deoxygenated blood.

It is important to point out there are other blood vessels that are present in the systemic circulatory system. There are arterioles and venules. Arteries are structurally identically to arteries, just narrower and venules are structurally the same are veins, just narrower.
The pathway of blood flow, from the heart is
Arteries-Arterioles-Capillaries-Venules-Vein

EXPLAIN THE TRANSPORT OF OXYGEN AND CARBON DIOXIDE IN THE BLOOD

• Air( carrying oxygen) is inhaled and enters the lungs
• Travels through the lungs to the alveoli, oxygen is diffused out of the air, and into the surrounding capillaries.
• The oxygen binds with the haemoglobin of the RBCs to form oxyhaemoglobin
• The newly oxygenated blood enters the pulmonary circulation system, which leads from the lungs to the heart.
• The blood enters the left side of the heart from the pulmonary vein into the left atrium, and down past the bicuspid valve into the left ventricle
• From here, the blood is pumped out of the heart past the aortic valve into the aorta.
• The aorta is the largest blood vessel of delivery of oxygenated blood to the body. At this point the blood is on the pathway of systemic circulation. Here the blood goes around the body to where it is needed. Through arteries, arterioles and capillaries.
• At this point, of being in the capillaries, it will eventually reach cells that require oxygen for cellular respiration
• The oxygen diffuses out of the capillaries and into cells, at this point carbon dioxide diffuses into the blood. This carbon dioxide is converted to carbonic acid. The blood in now deoxygented blood.
• The blood carries on its journey in the capillaries but this time it enters the venules, and veins making its way back towards the heart
• Blood enters the right side of the heart by the vena cava (inferior or superior). Into the right atrium, and then past the tricuspid valve into the right ventricle.
• It is pumped out of the heart, past the pulmonary valve to the pulmonary artery.
• From here the blood goes to the lungs ending up in the capillaries surrounding the alveoli.
• This is the point of gaseous exchange; the carbon dioxide is diffused out of the blood into the alveoli. From here the carbon dioxide is released into the air passageway of the lungs for exhalation. Oxygen is diffused out of the alveoli into the capillaries surround the alveoli.

This is the beginning of another circle of new freshly oxygenated blood entering the body, going to the heart, around the body to be taken where needed for respiration. At this point carbon dioxide will diffuse into the blood this is taken to the heart and then to the lungs to be diffused out of the body by exhalation for the whole process to begin again and again.

DESCRIBE THE STRUCTURE OF A RED BLOOD CELL AND EXPLAIN HOW THIS RELATES TO FUNCTION

RBCs (Red blood cells) are manufactured in the red bone marrow of bones. In adults, RBCs are only manufactured in specific bones; thoracic, vertebrae, cranial, and the ends of femurs and humerous bones. In infants, every bone in their body has the ability to manufacture RBC.
RBCs are the most common blood cells in our body. Typically blood contains 25 trillion RBCs, which are continuously being replaced at a rate of approximately 3 million per second.
The lifespan of a RBC is short- 120 days. This means that they will be working at their optimum capacity over this time but after this time they are worn out and need replacing. When they need replacing, phagocyte cells will digest them. The bone marrow will release matured RBCs into the blood stream, to replace the ones that have reached the end of their life span. So there is no noticeable loss in our bodies.
RBCs are only released into the blood when they are mature. Whilst maturing a macrophage cell is attached to them. This macrophage cell is of particular importance. When a RBC is mature, its nucleus is squeezed out. The macrophage cell ingests this nucleus. This is an extremely important defence mechanism of the body. If a RBC still had a nucleus whilst working in the body, and antigen specifically a virus would be able to take over the RBC for its only purpose i.e. cause infection. The virus would be able to replicate and spread. Having no nucleus means that a virus cannot invade and disrupt the RBC’s vital role in the body.
RBCs are called so due to them being red in colour. The reason that they are red in colour is that they contain haemoglobin. Haemoglobin contains iron pigments, which gives its red colour. When RBCs are matured 90% is haemoglobin.
Haemoglobin is of vital importance. Haemoglobin easily combines with oxygen, and for every molecule of haemoglobin, 4 molecules of oxygen can attach to it.

Haemoglobin + 4 oxygen = oxyhaemoglobin

This is a large quantity of oxygen for one single molecule to hold. This therefore makes haemoglobin a highly efficient substance to use, to carry oxygen. When haemoglobin becomes saturated with oxygen this is called ‘oxyhaemoglobin’. Oxyhaemoglobin is therefore the source of oxygen in the body. It is able to be the source of oxygen in the body simply because the reaction to combine oxygen with haemoglobin is reversible. Oxygen can be released easily from the haemoglobin, out of the RBC and diffused to where it is needed such as the muscles during exercise. Oxygen is of vital importance, it is needed by every cell in the body for cellular respiration. Without oxygen, all our body systems would eventually shut down and we would die.
(When oxyhaemoglobin is present in a RBC the red colour of the RBC becomes more intense)
RBC is not only important for transporting oxygen due to the ability of its haemoglobin; it also helps transport carbon dioxide and delivers it to the lungs to expel from the body by exhalation. Carbon dioxide is poisonous; it needs to be taken as efficiently as possible to be excreted via exhalation.
To transport carbon dioxide safely and efficiently this is done by dissolving carbon dioxide to form the solution carbonic acid. To convert carbon dioxide to carbonic acid would naturally take a long time. Therefore RBCs contain an enzyme called carbonic anhydrase that acts as a catalyst, so this conversion is quick to occur. Due to the presence of this specific enzyme the majority of carbon dioxide will be carried in the RBC. Occasionally plasma (also found in blood) will also carry carbon dioxide. This is not an ideal situation due to the fact that if too much is carried in the plasma this leads to high amounts of lactic acid being formed.
So RBCs need to be present in our blood in high quantities to deliver oxygen to all our cells in our bodies for respiration and to take away waste carbon dioxide to be excreted. This is why infants can manufacture RBCs in every bone in their bodies. They need a huge amount of mature efficient RBCs to deliver the oxygen to the body systems for the rapid period of growth that all infants under go. To be able to achieve this, RBCs must be able to move efficiently through our blood vessels in our bodies. These blood vessels widen and narrow, and twist and turn. RBCs specialised shape enables this to happen

RBCs are round discs which are bi-concave. This allows them to be flexible, and twist and turn through all the blood vessels easily. Bianco C (2010) “An RBC can change shape to an amazing extent, without breaking, as it squeezes single file through capillaries”. So whether transporting oxygen or carbon dioxide this can be done efficiently.
RBCs are also important to maintain health. We need iron in our bodies. Iron in the haemoglobin is responsible for the vast uptake of oxygen one haemoglobin molecule can do. If we had limited numbers of RBCs are levels of iron in our body would be low. We would not have enough oxygen in our bodies and not enough oxygen means our bodies would not be able to produce energy through respiration. One physiological effect of this on our bodies would be feeling tired all the time. This would be medically described as being ‘anaemic’ and could require iron tablets

DESCRIBE THE CONSTITUENTS OF PLASMA AND EXPLAIN THEIR FUNCTIONS

Blood is a tissue consisting of several types of cells. These cells are:-
• RBC (Red Blood Cells). These are also called erythrocytes. Their basic functions are to transport oxygen to the body where it is needed and transport carbon dioxide to the lungs to be eliminated by the body.
• WBC (White Blood Cells). These are also called Leucocytes. There are various types of these WBCs such as lymphocytes, neurophils. However they all have one thing in common they are part of our immune system and are there to fight disease and infection.
• Platelets. The function of platelets is preventing our body from bleeding and upsetting the homeostasis of the body. They allow this to happen by helping to form blood clots at points of injury (externally and internally) such as a cut on the finger.

All these blood cells therefore need to be able to move freely through our bodies and reach specific destination points. This would be difficult if there was not something else present in blood, a fluid that enables movement. blood cells are relatively large molecules and would not easily move through blood vessels if they could not be carried in a fluid. This fluid is called plasma.
Plasma
Plasma is a pale yellow clear liquid, which is approximately 95% water. Plasma accounts for approximately 55% of a human’s blood volume.
So being liquid, it therefore has the ability to flow freely, through blood vessels and the blood cells can float in it and therefore move freely.
Plasma does not only help the blood cells in the blood to move freely to where they are needed, plasma also contains an array of other substances. It transports them, by carrying them in solution (dissolved) or suspension (held in the plasma) to specific areas in the body either to be used by the body or taken to be excreted by the body.
Examples of substances found in plasma are:-
Sugars, Vitamin, Minerals and Lipids
These are nutrients. Which are diffused into the bloodstream during digestion. These nutrients are in transit and are taken where needed and used or taken to be stored such as some vitamins and minerals will be taken to the liver to be stored. If there is excess of certain nutrients such as vitamins they will be taken to the kidneys to be excreted.
Carbon Dioxide
Carbon dioxide is a waste product of cellular respiration. Generally carbon dioxide is transported in just the RBCs, in the form of carbonic acid. However, if there is an excessive amount of carbon dioxide that needs to be removed from our body i.e. during exercise, plasma will carry this. It will transport it to the lungs, and this will be diffused into the lungs during gaseous exchange and exhaled into the atmosphere.
Chemicals
Such as medication, nicotine, alcohol and illegal drugs
These enter our bodies, through digestion, intravenously, and inhalation. These chemicals are not naturally found in our bodies. Sometimes chemicals can be helpful such as medication and sometimes chemicals can be harmful to us such as alcohol. Plasma transports these chemicals to where they are needed in our bodies, or will transport them to be taken out of our systems, such as alcohol will be taken to the liver to be broken down and then removed by the kidneys.
Proteins
Many important proteins which are vital for life and help to maintain homeostasis in our bodies are carried in plasma.6-8% of plasma are protein.
The majority of proteins in the blood are called serum albumin and serum globulins.
Serum albumin plays an important role of binding small molecules together in the plasma so that they can be transported in plasma to their destination. It also helps to regulate osmotic pressure. So water does not leave the plasma and hence the plasma will not be able to allow blood to move freely through the blood vessels.
Serum globulins are more specific. They too are there to bind molecules together, but are more specialised than serum albumin. There are various types of serum globulins such as alpha globulins that transport thyroxin (the hormone released from the thyroid responsible for many metabolic reactions in the body) and retinol (Fat soluble vitamin A).
Plasma also contains other hormones, and enzymes. These will readily be taken to specific places in our body to be used for their specific function.
A protein called fibrinogen is also present in plasma. Fibrinogens’ specific role is to aid clotting. It is therefore important that it remains within close proximatey to the platelets of the blood so that clotting can occur immediately. It is separated from the platelets due to plasma, but when it is needed the blood transports in to the specific area and the fibrinogen and platelets will react to form a clot.

These are just a small amount of what plasma carries, it carries practically everything the body needs and what it does not need and takes it to their specific destination. Everything it carries is usually in transit and is removed from a ‘source and deposited to a ‘sink’. Plasma is vital for transportation in the body not only does it carry substances, due to it being mainly water; it allows blood to flow freely. The Franklin Institute (2010) states “It might seem like plasma is less important than the blood cells it carries. But that would be like saying that the stream is less important than the fish that swims in it. You can't have one without the other.”

DESCRIBE THE ROLE OF THE NERVOUS SYSTEM IN GENERATING BREATHING RHYTHM

The majority of time that we breathe, this is an involuntary action. We do not have to tell our bodies to breathe, it just will.
However breathing unlike other involuntary working systems such as the heart, can also be voluntary. We can consciously control our breathing if we need to. We can speed up our breathing and hyperventilate (rapid breathing) slow it down, or stop completely (hold our breath). It is important to note, that although we have the ability to do this, our bodies desperately require us to breathe so that we intake oxygen. If there becomes a major need for our bodies to have oxygen i.e. not enough available oxygen already in our body systems then involuntary breathing will over-ride voluntary breathing. This is why we cannot hold our breath indefinitely. We can do it for a short while, but we must breathe again to allow oxygen into our bodies for respiration.
At rest most people have a breathing rate of 15-25 breathes per minute. This breathing rate allows us to take sufficient oxygen into our bodies so that all our body systems get the specific amount of energy they require to work appropriately.
Occasionally, this breathing rate needs to increase; our bodies need more oxygen than at rest, such as during times of exercise.
The amount we breathe is controlled by the CNS (central nervous system) and the peripheral nervous system. This unconscious control is delivered by the ANS (autonomic nervous system). This is controlled by the CNS which works in conjunction with the peripheral nervous system. Autonomic means that this is involuntary; all body systems that are involuntary are controlled by the ANS.
Abrahams (2002 pg 242) states “The autonomic nervous system is divided into two parts; the sympathetic and the parasympathetic system. Both systems generally supply the same organs but with opposing effects”
The Sympathetic Nervous System
This is referred to as the ‘fight or flight’ response. This nervous system usually becomes active when there is usually a need to increase activity of specific body systems. For example during exercise, the body requires a greater amount of oxygen to allow for increased muscle contraction. To increase the amount of available oxygen the body has, the breathing rate needs to increase and also the heart beat as this will allow for the increased amount of oxygen in the blood readily reach where it is needed i.e. the muscles.
The Parasympathetic Nervous System
This is referred to as the ‘rest and digest’ response. This part of the nervous system is regulating our body systems the majority of the time. It keeps a constant pace for the body at rest. After exercise, it has the ability to override the sympathetic nervous system, when the majority of waste products e.g. lactic acid has been removed from the body. It will lower the activity of the heart and respiratory system to return the body to a normal resting state.

So as you can see these two nervous systems work in harmony together, if there was only one present then we could breathe too rapidly all the time which would have a devastating effect on our health. Also we would never be able to increase our normal activity level because we would not be able to cope with the demands of the activity for very long without feeling completely exhausted.
So we know that breathing is usually controlled by the autonomic nervous system, and can be variable i.e. our breathing rate can change. I will now discuss how the nervous system is actually controlled and how it copes with the variations of breathing.

The CNS is controlled by the brain. It is specific areas of the brain that signal what is needed to be done and transmits this information to appropriate areas. For breathing these specific areas are called the respiratory centres of the brain.
Medulla oblongata is part of the brain that is responsible for vital life functions. This includes breathing. Here lies a respiratory centre
There is also an area of the brain called the pons. Very little is known about the pons , the only known parts of the pons are the pneutaxic and the apneustic. However what is known about the pons is that is also contains a respiratory centre. This respiratory centre holds the standard rate of breathing. This standard rate of breathing is variable so that we can breathe faster depending on our situation.



Our respiratory centres in our brains need to know when to increase breathing rate, and when to decrease breathing rate. For all our body systems, there are sensors inside our bodies to detect changes and transmit this information to the specific part of the brain such as the medulla oblongata, which in turn will process this information and allow our body systems to adapt to the changes so they work efficiently.
During breathing, the sensor associated with this is found in aortic arch. The aortic arch is close to the left lung and the heart. When this specific sensor detects low or high oxygen levels or low or high carbon dioxide levels, it will send a signal to the respiratory centres in the brain to act appropriately. This would be to speed up the breathing rate, controlled by the sympathetic nervous system or slow down the breathing rate, controlled by the parasympathetic nervous system. It also transmits information to the brain ,approxiamately 20 times a minute regardless whether or not there is change in oxygen levels of the blood. this helps maintain the typical at rest breathing rate.

To transmit this information from the brain to the respiratory system, special cells are used. These are called neurons. These neurons pass information from the brain, down the spinal cord and back again or from the sensors to the brain for the information to be processed. Then, down the spinal cord following nerve paths to stimulate specific activity. The brain and the spinal cord is the CNS. The nerve pathways branch out all through the body; this is the peripheral nervous system. Neurons in the brain reach the medulla oblongata and from here pass information to the pons. Information passes back to the medulla oblongata, back down the spinal cord and to the peripheral nervous system to carry out the required response.

When we breathe, the diaphragm is supplied with spinal nerves from the C3 C4 and C5 and the intercostal muscles are supplied spinal nerves T1-T12. Our CNS sends signals through this pathway, making our respiratory muscles contract at specific intervals, controlling our breathing rate. When the sensor in the aortic arch detects changes in oxygen levels or carbon dioxide levels in our blood. This information is sent to the brain via neurons. The respiratory centres in the brain process this information and via neurons send information to the respiratory muscles to either contract at a rapid rate or lower the contraction rate i.e. increasing the breathing rate or decreasing the breathing rate or keep the breathing rate at a constant
.
It is important to note, that during conscious breathing, the respiratory systems in the brain are over ridden by the cerebella cortex (another area of the brain). According to Gogoi (2010) “. The portion of the brain aiding the thinking process (cerebral cortex) has a major role to play. It sends signals to the rib muscles and the diaphragm for temporarily overriding the respiratory center signals"

EVALUATE THE CONDITIONS NECESSARY FOR EFFECTIVE GASEOUS EXCHANGE

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Gaseous exchange takes place in the lungs. The exact location is the alveoli situated in the lungs at the ends of the bronchioles.
Alveoli are tiny hollow sacs, which cluster together; hence they are often described as having a grape like appearance. Surrounding the alveoli is a network of capillaries commonly called capillary beds. Capillaries are the smallest blood vessels inside the body and they can carry both oxygenated blood and deoxygenated blood depending where they are in the body.

The alveoli walls are one cell thick, they are made of flattened epithelial cells. The capillaries too are also one cell thick. This allows for a short diffusion pathway, the gases only have to travel through 2 cell membranes to their destination, whichever direction they need to go i.e. oxygen from the lungs can readily diffuse into the capillaries into the red blood cells; carbon dioxide can readily diffuse out of the capillaries, into the lungs ready to be exhaled into the atmosphere If the diffusion pathway was longer than this, then for gaseous exchange to take place the process would take longer. Also, some of the gases maybe lost to other tissues which may cause problems to the body.

Lungs are moist, what keeps the lungs moist is mainly due to the presence of water.
Water is present in the alveoli. It needs to be present for effective gaseous exchange because it allows the gases to be transported in solution form making it easier to diffuse; oxygen and carbon dioxide are dissolved.
This water diffuses in and out of the alveoli and the epithelial cells, and some water is always present in exhalation. If the alveoli did not have this constant osmotic movement of water then alveoli could become dry. A dry environment is not at all effective for gaseous exchange; it makes it difficult for gases to exchange through cells, even if they are only one cell thick.

According to Abrahams (2002 pg 111) “Two other type of cells are found in the alveoli: macrophages (defence cells), which engulf any foreign particles that get down the respiratory tract; and cells which produce surfactant” Surfactant is a soapy solution that lowers the surface tension of the alveoli lining. This prevents the alveoli collapsing. Keeping the surface area of the alveoli large, therefore allowing optimum gaseous exchange. If the alveoli collapsed then the surface area will decrease hence less gaseous exchange will be able to take place.

A steep concentration gradient of the gases needs to be maintained. Gases will always move from a high concentration to a low concentration, in a constant strive for an equilibrium. The capillary beds contain RBC (red blood cells) which may contain little or no oxygen in them. When this is the case, they need more oxygen. The air in the lungs contains plenty of oxygen; to meet the body’s requirements. So gaseous exchange of oxygen will readily occur. This is true for the exchange of carbon dioxide. The RBCs contain high amounts of carbon dioxide; a waste product of cellular respiration. Carbon dioxide is poisonous to us; we need to eliminate it from our body system. The lungs have very small amounts of carbon dioxide, so carbon dioxide will readily diffuse out of our capillaries and into our lungs to be expelled by exhalation.