Cassandra Adsett Animation: Negotiated Study One: Further System Research: Metabolism, The Circulation of Blood and Respiration

Metabolism

During his lifetime the average man eats about fifty tons of food and yet maintains a weight of about 160 pounds
Even the most obese person retains in his body only a minute fraction of what he eats.
Most of the food we absorb into out bodies through the process of digestion must sooner or later pass out again
If we eat more food than required to provide energy or for tissue growth and repair, the excess is stored in adipose tissue as fat. This fat acts as our energy store and as an outer insulating layer
The advantage of storing excess food as fat is that it is a compact, energy rich compound and all that is needed for its synthesis is acetyl-coA and appropriate enzymes

The Circulation of Blood

We have a complex transport system that pumps and circulates blood around the body
Blood has special properties that enable it to take up oxygen in chemical combination in regions where it is plentiful - those in contact with the outside air - and then, during circulation, to release it where oxygen is being used up and is not so plentiful - deep in the bodies tissues
During circulation, blood can also transport carbon dioxide in the opposite direction, from the tissues where it is produced to the outside air, so that the result is an exchange of the two gases.
We have developed a special internal surface - the linings of the lugs - through which such an exchange can take place readily.
the total surface area provided by the lungs is about forty times that of the skin.
Circulating here, beneath the moist, thinner - than - paper lining blood can come into very close contact with the air that we breathe, and carry out the exchange.

Blood circulating in the walls of the intestines picks up water and dissolve food substances that are destined for distribution and use as fuel and construction materials throughout the whole body.
Some but very little of the waste matter produced by living cells does in fact find its way out of the body through the skin, such as sweat. The components of sweat, including water and salts, are derived from the bloodstream. By far the greater proportion of unwanted material leaves the body in the urine, a solution filtered by the kidneys from the blood circulating through them
This is the main route of excretion

The essential function of the blood, then, is to transport substances as it circulates.
The human body has a pump, the heart, and a vast network of blood carrying vessels that penetrates to every corner of the body, servicing and bathing with fluid every cell in every tissue
Arteries and veins are the largest tubes of the circulatory system, and the major routes by which the blood travels to all parts of the body from the heart, and returns to it for recirculation.
Arteries all carry blood away from the heart, and veins all carry blood toward the heart.
The blood in arteries is under high pressure, and the vessel walls are appropriately thick, muscular and slightly elastic.
The surge of pressure that follows each contraction of the heart causes the arterial walls to yield slightly, smoothing out the blood flow.
This momentary expansion echoing every heart beat, is felt as a pulse where any large artery runs near the surface of the body
The largest artery in the body is the aorta, nearly one inch wide, that arches, like the handle of a walking stick, up and backward from the left ventricle of the heart to pass down infront of the spine, giving off branches in its course to supply all parts of the body.
Arteries branch into smaller vessels, and at their ends become arterioles, which form a network in the skin, in the muscles and in the walls of the arterioles enable them to contract and so regulate the blood flow in particular regions according to needs.
Arterioles in turn divide into minute capillaries, tubules averaging only on twenty fifth of an inch in length, and a hundredth of that in diameter.
Capillaries have a wall only one cell thick, through this wall materials pass into and out of the blood.

On the return side of the circulation, capillaries join up with the smallest branches of the veins, the venules, and the venules unite to form wider and wider collecting vessels until they becomethe major veins that return blood to the heart.
Veins have thinner walls than arteries and the pressure in them are lower.
They expand and collapse to adjust to the blood volume within them.
The massaging effect of nearby muscles, and the presence of simple cup shaped valves in the larger veins, helps to prevent blood from collecting in the lower parts of the body.

The basic circulation, then, is from heart to artery, to arteriole, to capillary, to venule, to vein and back again to the heart.
But in fact, because blood must pass through the lungs to pick up oxygen and release the carbon dioxide it has gathered from tissues, there is a double circuit.
Instead of being pumped directly around the body again, blood returning from the tissues is sent through capillaries in the lining if the lungs and taken back to the heart through the pulmonary veins
Now the circuit can start again

The Heart: A Muscular Pump

Because there is a double circulation, blood in effect travels in a figure eight circuit.
The heart lies at the point of cross over and can be regarded as two linked pumps.
Because the systemic circulation is so large, the left ventricle has the greatest muscle mass; the apex beat is due to its forceful movement
Both atria and both ventricles contract at the same time in rapid succession.
This period of ventricular contraction is called systole, and is followed by a relaxation or recovery period called diastole.
Systole and diastole are both very short.
he heart goes through about seventy cycles of systole and diastole every minute, but its rate can be more than doubled to keep up with the body's oxygen needs during violent exertion.
It is the valves of the heart that turn it into a pump. They are one way valves, which means that they shut automatically of blood attempts to move against the proper direction of flow.
The caliber of the arterioles is constantly changing under the influence of the autonomic nervous system, according to the body's needs, and the rate and force of the heartbeat are also adjusted by hormones, particularly adrenaline, which circulates in the bloodstream.

Respiration

Every minute we breathe in and out between ten and fifteen times
Brain cells are particularly sensitive to oxygen lack; starved of the gas for just a few minutes, they dies and cannot be replaced. We must breath to stay alive.
Two systems in particular are responsible for supplying all the body's cells with oxygen and, at the same time, for disposing of the waste carbon dioxide and water resulting from the cells metabolism.
These are the respiratory system, of airways and lungs; and the circulatory system, of heart, arteries, capillaries and veins.

Air enters and leaves the body through the nose and mouth.
At the entrance to the nasal cavities, paired passageways separated by a bone sectum, are the nostrils.
Coarse hairs act as a trap of large dust particles that may be inhaled
Within the nasal cavities the air that is breather in passes over the thin, glistening membrane lining the walls, and is warmed and moistened.
The mucus membrane is richly supplied with blood vessels and is capable of raising the temperature of the air from near freezing, on a cold day, to almost that of the body by the time it reaches the back of the throat.
Warm, moistened and filtered, the air now passes into the pharynx, the cavity behind the nose and mouth.
The air is sucked down into the larynx, the box like structure that supports the vocal chords and then goes into the lower respiratory tract.

During swallowing, the entrance to the trachea is momentarily closed by a flap called the epiglottis to prevent food entering but the tube itself is always kept open by c-shaped struts of cartilage.
At its lower end, the trachea divides into the bronchi, one of which enters each lung, there to divide and subdivide and lead eventually to several hundred million air sacs, called alveoli.
Here the exchange of gases between the inhaled air and the blood flowing in networks of capillaries takes place.

The lungs fill most of the chest cavity, they are sponge like structures that are inflated and deflated by the movements of the chest wall and the rise and fall of the diaphragm- the sheet muscle that divides the chest from the abdomen below.
Each lung is enclosed in a double layer of fine membrane called pleura; the outer pleura is closely attached to the walls of the chest cavity and to the upper surface of the diaphragm, and the inner pleura is attached to the lungs.
During normal breathing it is mainly the diaphragm that does the work.
As the muscle contracts, stimulated by the slow rhythmic impulses carried by nerves from the brain, the volume of chest cavity is increased.
At the same time the ribs, hinged where they join the spine, and link to the sternum, or breastbone, at the front, move upward and outward, pulled by the muscles between them, the intercostal muscles.
As the volume of the chest increase, so the pressure within drops, and air is suck into the lungs through the trachea, restoring the pressure. This is inspiration

Expiration is normally an effortless process. The diaphragm and intercostal muscles relax, and the elasticity of the spongy lung tissue forces air out as the volume of the chest cavity decreases.
With muscular effort, considerably more air can be squeezed from the lungs.
The respiratory system is able to adapt to changes in the body's needs. As the rate at which energy is used up increases, so the rate and depth of respiration are automatically adjusted.
The quantity involved is called the tidal volume
During physical exertion as when you are running for a bus, the amount of air needed may be some twenty times as much as during rest.
The rate of respiration may be more than doubled , and, at the same time, the amount of air per breath is greatly increased.
With effort up to five pints of more air can be taken in during inspiration
Breathing is normally an involuntary activity, but it can, at least within certain limits, be controlled voluntarily.

Coughing, sneezing and hiccuping, sighing, yawning, crying and laughing are all unusual forms of respiration and occur in response to physical or emotional triggers.
Strong smells or dust particles, for example, can irritate the nasal lining and cause sneezing, while the cough is a response to the presence of irritant material, such as dust in the bronchi, or to excess mucus produced in response to infection or noxious fumes.
Hiccups result from sudden, sharp contractions of the diaphragm, stimulated by the vagus, a nerve which is also involved with the digestion. That is why hiccuping can be associated with eating certain foods.
Crying and laughing are both long inhalations followed by a burst of short, staccato exhalations and are respiratory reactions to emotional stimuli.