Mammals_Concept_3

The heart and lungs of any mammal can be found within the chest or thoracic cavity, protected and supported by a cage of flexible bones — the ribs. The flexibility allows for expansion and contraction, caused by the inhalation of air into the lungs and its exhalation out again. The heart is between the lungs in a central position, slightly to the left (see Figure 4).

The muscle that never rests

The heart is the hardest-working muscle in the body: it is continually contracting and relaxing from before birth to the point of death, Generally speaking, the smaller the mammal, the faster the heart rate.

The mammalian heart is effectively two pumps: one side receiving blood from the body and pumping it to the lungs, the other receiving blood from the lungs and returning it to the body, The efficiency of this design is fairly amazing and if exercised properly, nearly maintenance-free. The structure of the heart is quite simple (see Figure 5), though the number of blood vessels connecting it to the rest of the body can make it appear confusing.

Blood flows towards the heart from different parts of the body through tubes called veins that become increasingly large as they merge, finally entering the heart through a huge vein called the vena cava (on a human heart this will be at the top on the right).The first chamber that the blood enters is a 'waiting area' called the right atrium. In between contractions, blood is able to flow through a one-way valve into the main chamber or right ventricle. When the heart beats, the muscle tissue surrounding it contracts, squeezing the ventricle. The blood that has flowed into it is forced out again; but it can,t go out the way it came in, because the pressure on the valve leading back to the atrium has closed it — so the only way out is through a strong-walled blood vessel or artery to the lungs. This too has a valve leading to it, preventing blood from flowing back into the right ventricle.

As the blood passes through the lungs, an exchange of gases takes place . From the lungs, the blood returns to the left-hand side of the heart Here, it collects in the left ventricle before being forced out by the contraction of the heart into the main artery or aorta. The aorta is basically a distribution centre to which arteries leading to different parts of the body are connected. As the blood leaves the left side of the heart it is under considerable pressure, so the artery wails have to be thick and muscular. For protection, arteries tend to run deep inside the body.

The keys to the flow of blood through the heart are the valves (see Figure 6), Also, to ensure that blood continues to flow in the right direction as it returns to the heart, all of the larger veins have valves along their length to prevent backflow^’.

You can demonstrate this on yourself; on the underside of your wrist, you will probably be able to make out a series of blue lines. If you press down on one of these and draw your finger slowly up your arm, following the line, you will probably see the line disappear as the blood is pressed out of it. Then you release your grip, the line will reappear.

Fortunately, all of the actions of the heart are completely automatic: no effort or thinking is required! The heartbeat is controlled by the autonomic nervous system (see Concept 5), which regulates processes within the body that are not under conscious control. The body produces natural stimulants to increase blood flow when necessary (for example, during exercise). The sounds that the heart makes as it beats are caused by valves closing: in the 'tub-dub' sound, the 'tub' is the closing of the valves between the atria and the ventricles; the sharper-sounding 'dub' is the snapping shut of the valves leading to the arteries.

Because of this repeated squeezing action, blood travels around the body in a series of squirts. This uneven flow or pulse of blood can be felt at various places around the body where an artery is close to a hard body part (bone, cartilage or tendon), making it vibrate. The best places to experience these pulses on yourself are on the inside of the wrist (just behind the thumb) and on the neck (at the inner sides just under the jaw). The normal 'at rest' adult heart rate is 70 to 80 beats per minute.

Heart problems

There are three main ways in which the heart can be damaged and function less well. The first, a defect often spotted at birth is a 'hole in the heart'. This does not mean that there is a hole in the external wall of the heart, allowing blood to leak out into the thoracic cavity: if this were the problem, it is unlikely that a live birth would be possible.The hole' is a leak between the two ventricles, which allows oxygen-poor blood to cross over and be pumped back around the body a second time, or allows oxygen-rich blood to take a second trip through the lungs. Any mammal suffering from this condition is unlikely to be able to be as fast or as strong, or have as much endurance, as others of the species.

The heart valves of ageing mammals can also become less efficient and allow blood to flow backwards, so that the heart has to do more work to move the blood

Humans tend to be the only mammals with heart problems that are self-inflicted through poor or inappropriate diet (see Chapter 4). If we eat too much animal fat and take insufficient exercise, the inner surfaces of our blood vessels can become coated with fatty plaque. This reduces the bore (internal width) of the vessels and so restricts the flow of blood, leading to a build-up of pressure. This high blood pressure can strain the heart muscles, causing them to 'flutter' rather than contract strongly — a 'heart attack'. When this happens, there are two medical options: to insert a new blood vessel that can bypass' the blocked parts; or to insert and inflate a balloon in the restricted section to compress the plaque against the vessel walls, so that blood flow can increase once the balloon has been removed.

Breathing

The mammalian lungs are a very effective way of extracting oxygen from air. The tissue that they are made from appears very spongy, and is full of very small chambers or sacs called 'alveoli'. Air enters the lungs from the 'trachea^lor breathing tube, and passes through bronchial tubes' that subdivide into smaller and smaller tubes, eventually terminating in bunches of alveoli (see Figure 7).

The walls of these tiny sacs are very thin — only a couple of cells

— and are lined with a slimy 'mucus' into which gases can dissolve. The bunches of alveoli are surrounded by small, thin-walled blood vessels called capillaries, which transport blood to and from the lungs.

The design ofthe lungs provides a very large surface area within a limited volume. Surface area is important, because the surface is where the exchange of gases takes place. Oxygen contained within the air diffuses into the red blood cells, and the carbon dioxide carried by the red blood cells diffuses back into the air to complete the exchange.

The act of breathing in is caused by the diaphragm (a sheet of muscle that separates the thorax from the abdomen) tightening and pulling down. Something has to fill the extra space that has been made in the thorax, so air is drawn into the lungs causing them to expand. As the diaphragm relaxes it once more arches upr compressing the lungs and causing them to expel ain If you hold yourself around your chest, you can feel how your chest expands as you breathe — but this is not the only method of breathing! Tummy breathing' allows the lungs to inflate with the chest moving as little as possible: you move your abdominal organs out of the way instead. By combining both methods — sticking your chest and tummy out — you can really fill your lungs (which is very helpful if you are singing or playing a trumpet).

Although the act of breathing is to some extent an automatic response, similar to your heartbeat, it is possible to exert much more control over it. You can physically make yourself stop breathing for a minute or so, before the automatic responses take over. You breathe in as an automatic response to a build-up of carbon dioxide. In a relaxed, resting state you do not need a great deal of oxygen, so your breathing is shallow and the exchange of gases is quite small in proportion to the capacity of your lungs. Every so often the carbon dioxide builds up to such an extent that a near-complete change of air in the lungs is required: this results in a sigh or a yawn, which amounts to a single deep breath. A good deep breath of 'fresh air' makes you feel more alert mainly because an over-saturation of carbon dioxide makes you feel drowsy. During mouth-to-mouth artificial respiration, it is as much the high proportion of carbon dioxide being breathed into the lungs as the inflation of the lungs that restarts the breathing reflex.

The oxygen/carbon dioxide exchange that takes place in the alveoli is not a complete one. During normal breathing, only a quarter of the available oxygen breathed in diffuses into the blood supply; so where a normal oxygen concentration of 20% is breathed irm the concentration in the air on the way out is still usually about 16% — otherwise, artificial respiration couldn't do much good!

Blood and respiration

Mammalian blood has four main components:

White cells — these come in a variety of forms, and attack infections within the blood.

Platelets — small bits of cells that form a clot or scab when a blood vessel is damaged (especially when they are exposed to air).

Plasma — the yellowish liquid, about 90% water, that carries the blood cells and other material around the body.

Red cells — these transport gases for respiration; they contain the pigment haemoglobin, which gives the blood its red colour

In the lungs, oxygen diffuses into the red blood cells. Once the oxygen has been delivered to where it is needed (see Concept 6), the red blood cells carry the waste carbon dioxide back to the lungs.

Respiration and energy

The purpose of respiration is to deliver energy in a usable form to all parts of the body.The blood delivers carbohydrate (sugar) and oxygen to the body tissues, where they are combined to release energy. This process is known as aerobic respiration. Its main waste product, carbon dioxide, is taken back to the lungs by the blood. Other waste products of body chemistry, such as urea, are filtered out as the blood passes through the kidneys.

There are times, usually during strenuous exercise, when aerobic respiration cannot supply as much energy as the most active muscles need. At such times, energy release can take place without oxygen for short periods (anaerobic respiration). The waste product of this process, lactic acid, is toxic: it causes muscle fatigue, To get rid of it, oxygen has to be used up; so there is an 'oxygen debt' to be paid back causing the period of puffing and panting and increased heart rate that follows sudden exercise. During sustained exercise, the oxygen debt may be paid back — resulting in a new surge of aerobic energy (the 'second wind'),

Why you need to know these facts

The mammalian heart—lungs system is a very effective means of distributing the energy obtained from food to different parts of the body and making use of it. Sugars obtained from food are combined with oxygen to release energy that allows mammals to perform their various functions, The waste products of this process are then recycled or expelled. Understanding these processes will allow children to appreciate the need to maintain their own bodies and instil a sense of wonder at the complexities of nature.

Vocabulary

Aerobic respiration — the release of energy in body tissues, using oxygen.

Anaerobic respiration — the short-term release of energy without the use of oxygen, leading to a build-up of lactic acid.

Arteries — the tubes carrying blood away from the heart

Atria (plural of atrium) -z the smaller heart chambers through which the blood passes to reach the ventricles.

Capillaries — very fine (small-bore) blood vessels,

Diaphragm — a muscle sheet that allows air to be drawn into the lungs.

Heart the muscle that pumps the blood around the body

Lungs — the organs that extract oxygen from the air.

Pulse — the rhythmic flow of blood along the arteries.

Red blood cells — cells that carry oxygen and carbon dioxide in the bloodstream.

Valves — flaps of muscle that are positioned to prevent the reverse flow of blood,

Veins — the tubes carrying blood towards the heart.

Ventricles — the large, muscular chambers of the heart.

Amazing facts

The average human heart beats 2.5 billion times in its life. This is roughly the same number of heartbeats that a mouse living to the ripe old age of seven years will manage (at about 500 beats per minute.)

Whales have been known to hold their breath for over hours while diving for food.

The average lung capacity of an adult human is 5 to 6 litres.

A whale heart has been known to weigh almost 700kg.

A human heart weighs less than i kg and is about the size of a fist.

A rat will take 100 to 200 breaths per minute, while a horse or an elephant will take only five. Humans take 6 to 20 breaths per minute — that's 23,000 per day, or enough air to fill a room 2.5m³.

Humans make new red blood cells at a rate of 2 million per second. Human blood contains 500 red cells for every white cell, e Air rushes out of your nose at 1 60km/h when you sneeze — about the speed of a hurricane.

Common Misconceptions

Mammals breathe in oxygen and breathe out carbon dioxide.

This is too great a simplification.The air that we breathe in will have a greater oxygen content and a smaller carbon dioxide content than the air we breathe out. Lungs do not extract all of the oxygen content from the air in each breath and replace it entirely with carbon dioxide.

You yawn when you are tired.

You are likely to yawn or sigh when the carbon dioxide content of the air in your lungs reaches a point where your body attempts to force a complete refill of the lungs. This is more likely to happen when you have been fairly inactive for a while, and have been breathing rather shallowly — which may be the case if you are tired, but also if you are trying to keep still or if there is too much carbon dioxide in the air around you.

Questions

What are hiccups?

These short sharp breaths occur when the diaphragm, which controls the expansion and contraction of the lungs, goes into a spasm and makes regular jerky movements. This can be brought on by a temporary loss of breath. It usually wears off after a couple of minutes.

If somebody's heart stops in a TV hospital drama, they give them an electric shock. Why?

All muscles work — tense and then relax — because of very small electrical impulses sent to them through nerves (see Concept 5). The electric shock given to patients by a defibrillator makes the heart muscles tense up; when they relax, the body's own automatic system may be able to start working again. Some people with heart problems are fitted with a pacemaker, which gives a regular small electric shock to the heart to make it beat regularly.

Teaching ideas

To avoid distressing animals, most activities relating to heart rates and breathing are best carried out by the children on themselves — under controlled conditions where any medical problems are known and allowed for. Emphasise that the children should never subject animals to abnormal or stressful conditions. Taking their own pet dog out for a normal *run on the common' and observing its respiration and heart rates before and after exercise would be acceptable. Making a hamster run for five minutes in a wheel and then passing it around a group, so they can feel its heart rate, would not be!

Obtaining mammal hearts and lungs from a butcher to cut up and observe in close detail is problematic in terms of hygiene, ethics and practical issues; the sensibilities of vegans or vegetarians must be taken into consideration. Besides, the dangers of cutting tools and the sheer messiness of such an activity in the average primary classroom make it inappropriate in almost all cases. Videos and computer-based simulations offer an appropriate alternative to first-hand sources.

Modelling the heart (explaining, modelling)

If we simplify the heart to a two-chamber model (ventricles only), its main actions can be modelled by four children, with the rest of the class acting as the blood', Each ventricle is made up of two children, facing each other with their arms outstretched and touching. Another child (as some blood) 'flows' from the body' between two hands into the area contained within the arms, then is pushed by the arms to make him or her 'flow' through the other pair of hands (towards the 'lungs). The arms then reach back to collect the next child and push him or her through. Another pair of children act as the other chamber, pumping the *blood' that returns from the lungs' back into the 'body',

Lung capacity (testing, measuring)

You will need a large plastic sweet jar, a large bowl of water (bigger than the sweet jar) and a length of PVC tubing Ask the children to measure both their lung capacity and their normal respiration flow, as follows:

1. Calibrate the jar by pouring in water 50ml or 100ml at a time and marking each level on the side.

2. Now fill the jar completely with water (no air at all) and invert it in the bowl of water. Insert the tube into the jar; take a very deep breath and blow into the tube. The amount of air in the jar is your lung capacity.

3.A normal breath into the tube will show you how much air is normally used: only a small proportion of the lungs^’total capacity.

Journey in the blood (explaining, hypothesising)

The children can describe the cycle of a red blood ceil or an oxygen particle within the mammalian respiration cycle. They can use drawings, an imaginative story, a cartoon strip or a drama, as they prefer (and according to their abilities).

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