DIGESTION
We eat various types of food
which has to pass through the same digestive tract. Naturally the food has to
be processed to generate particles which are small and of the same texture.
This is achieved by crushing the food with our teeth. Since the lining of the
canal is soft, the food is also wetted to make its passage smooth. When we eat
something we like, our mouth ‘waters’. This is actually not only water, but a
fluid called saliva secreted by the salivary glands. Another aspect of the food
we ingest is its complex nature. If it is to be absorbed from the alimentary
canal, it has to be broken into smaller molecules. This is done with the help
of biological catalysts called enzymes. The saliva contains an enzyme called
salivary amylase that breaks down starch which is a complex molecule to give
sugar. The food is mixed thoroughly with saliva and moved around the mouth
while chewing by the muscular tongue.
It is necessary to move the
food in a regulated manner along the digestive tube so that it can be processed
properly in each part. The lining of canal has muscles that contract
rhythmically in order to push the food forward. These peristaltic movements
occur all along the gut.
From the mouth, the food is
taken to the stomach through the food-pipe or oesophagus. The stomach is a
large organ which expands when food enters it. The muscular walls of the
stomach help in mixing the food thoroughly with more digestive juices.
These digestion functions are
taken care of by the gastric glands present in the wall of the stomach. These
release hydrochloric acid, a protein digesting enzyme called pepsin, and mucus.
The hydrochloric acid creates an acidic medium which facilitates the action of
the enzyme pepsin. What other function do you think is served by the acid? The
mucus protects the inner lining of the stomach from the action of the acid
under normal conditions. We have often heard adults complaining about
‘acidity’. Can this be related to what has been discussed above?
The exit of food from the
stomach is regulated by a sphincter muscle which releases it in small amounts
into the small intestine. From the stomach, the food now enters the small
intestine. This is the longest part of the alimentary canal which is fitted
into a compact space because of extensive coiling. The length of the small
intestine differs in various animals depending on the food they eat. Herbivores
eating grass need a longer small intestine to allow the cellulose to be
digested. Meat is easier to digest, hence carnivores like tigers have a shorter
small intestine.
The small intestine is the site
of the complete digestion of carbohydrates, proteins and fats. It receives the
secretions of the liver and pancreas for this purpose. The food coming from the
stomach is acidic and has to be made alkaline for the pancreatic enzymes to
act. Bile juice from the liver accomplishes this in addition to acting on fats.
Fats are present in the intestine in the form of large globules which makes it
difficult for enzymes to act on them. Bile salts break them down into smaller
globules increasing the efficiency of enzyme action. This is similar to the
emulsifying action of soaps on dirt that we have learnt about in Chapter 4. The
pancreas secretes pancreatic juice which contains enzymes like trypsin for
digesting proteins and lipase for breaking down emulsified fats. The walls of
the small intestine contain glands which secrete intestinal juice. The enzymes
present in it finally convert the proteins to amino acids, complex
carbohydrates into glucose and fats into fatty acids and glycerol.
The digested food is taken up
by the walls of the intestine. The inner lining of the small intestine has
numerous finger-like projections called villi which increase the surface area
for absorption. The villi are richly supplied with blood vessels which take the
absorbed food to each and every cell of the body, where it is utilised for
obtaining energy, building up new tissues and the repair of old tissues.
The unabsorbed food is sent
into the large intestine where more villi absorb water from this material. The
rest of the material is removed from the body via the anus. The exit of this
waste material is regulated by the and sphincter.
RESPIRATION
In human beings (Fig. 6.9), air is taken into the body through the nostrils. The air passing through the nostrils is filtered by fine hairs that line the passage. The passage is also lined with mucus which helps in this process. From here, the air passes through the throat and into the lungs. Rings of cartilage are present in the throat. These ensure that the air-passage does not collapse.
Within the lungs, the passage divides into smaller and smaller tubes which finally terminate in balloon-like structures which are called alveoli. The alveoli provide a surface where the exchange of gases can take place. The walls of the alveoli contain an extensive network of blood-vessels. As we have seen in earlier years, when we breathe in, we lift our ribs and flatten our diaphragm, and the chest cavity becomes larger as a result. Because of this, air is sucked into the lungs and fills the expanded alveoli. The blood brings carbon dioxide from the rest of the body for release into the alveoli, and the oxygen in the alveolar air is taken up by blood in the alveolar blood vessels to be transported to all the cells in the body. During the breathing cycle, when air is taken in and let out, the lungs always contain a residual volume of air so that there is sufficient time for oxygen to be absorbed and for the carbon dioxide to be released.
When the body size of animals is large, the diffusion pressure alone cannot take care of oxygen delivery to all parts of the body. Instead, respiratory pigments take up oxygen from the air in the lungs and carry it to tissues which are deficient in oxygen before releasing it. In human beings, the respiratory pigment is haemoglobin which has a very high affinity for oxygen. This pigment is present in the red blood corpuscles.
Carbon dioxide is more soluble in water than oxygen is and hence is mostly transported in the dissolved form in our blood.
TRANSPORTATION
The heart is a muscular organ which is as big as our fist. Because both oxygen and carbon dioxide have to be transported by the blood, the heart has different chambers to prevent the oxygen-rich blood from mixing with the blood containing carbon dioxide. The carbon dioxide-rich blood has to reach the lungs for the carbon dioxide to be removed, and the oxygenated blood from the lungs has to be brought back to the heart. This oxygen-rich blood is then pumped to the rest of the body. We can follow this process step by step (Fig. 6.11). Oxygen-rich blood from the lungs comes to the thin-walled upper chamber of the heart on the left, the left atrium. The left atrium relaxes when it is collecting this blood. It then contracts, while the next chamber, the left ventricle, expands, so that the blood is transferred to it. When the muscular left ventricle contracts in its turn, the blood is pumped out to the body. De-oxygenated blood comes from the body to the upper chamber on the right, the right atrium, as it expands. As the right atrium contracts, the corresponding lower chamber, the right ventricle, dilates. This transfers blood to the right ventricle, which in turn pumps it to the lungs for oxygenation.
Since ventricles have to pump blood into various organs, they have thicker muscular walls than the atria do. Valves ensure that blood does not flow backwards when the atria or ventricles contract.
EXCRETION
The excretory system of human beings includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys are located in the abdomen, one on either side of the backbone. Urine produced in the kidneys passes through the ureters into the urinary bladder where it is stored until it is released through the urethra.
How is urine produced? The purpose of making urine is to filter out waste products from the blood. Just as CO2 is removed from the blood in the lungs, nitrogenous waste such as urea or uric acid are removed from blood in the kidneys. It is then no surprise that the basic filtration unit in the kidneys, like in the lungs, is a cluster of very thin-walled blood capillaries. Each capillary cluster in the kidney is associated with the cup-shaped end of a tube that collects the filtered urine. Each kidney has large numbers of these filtration units called nephrons packed close together.
Some substances in the initial filtrate, such as glucose, amino acids, salts and a major amount of water, are selectively re-absorbed as the urine flows along the tube. The amount of water reabsorbed depends on how much excess water there is in the body, and on how much of dissolved waste there is to be excreted. The urine forming in each kidney eventually enters a long tube, the ureter, which connects the kidneys with the urinary bladder. Urine is stored in the urinary bladder until the pressure of the expanded bladder leads to the urge to pass it out through the urethra. The bladder is muscular, so it is under nervous control, as we have discussed elsewhere. As a result, we can usually control the urge to urinate.
