Organs are parts of the body having specific tasks. An example is the intestine, in which food is broken down and the nutrients are absorbed in the blood. Another important organ is the liver, which utilizes the components of food. Other organs are the kidneys, the spleen, the heart, and our biggest organ, the skin. They are all made up of different kinds of tissues, such as muscular tissue, fatty or supportive tissue, and blood vessels. The tissue in each organ is specifically designed to perform the tasks assigned to that organ. Thus, the muscular tissue of the heart is different and performs different functions as compared to that in the upper thigh. Each organ interacts with other organs to keep the body healthy.
Most of our organs are protected by the skull and the body skeleton. Some of our most sensitive organs lie embedded in the powerful cranial bone of the skull—the brain, the eyes, and the hearing organs. The mouth cavity protects the only movable bone of the skull, the lower jaw. The body skeleton with the ribs fanning out of the backbone protects the area of the chest. Internal organs such as the heart, lung, liver, and spleen are protected behind the rib cage, and the backbone covers the vital spinal cord.
An adult has 206 bones, which together weigh about 9 kg. A baby has 350 bones. Many bones in a baby’s skeleton are not fully grown, such as those of the skull. Cartilages are present at these points, and turn into bone at a later point. Bones are made up of different materials, and provide strength and elasticity. The bone mass is covered by a periosteum (bone skin), which contains nerves, blood vessels, and bone-building cells. The bone marrow, which makes the red blood cells, is present inside long bones. The longest bone in our body is the thigh bone.
Urine is a waste product, which is formed in the kidneys after the purification of blood. Kidneys contain millions of very tiny canals, called nephrons, which filter out harmful substances as well as excess water from the blood to form primary urine. This liquid still contains some useful components that are reabsorbed into the body in the kidneys. The remaining fluid that can no longer be used flows through the ureters into the bladder. Once the bladder is full, we need to go to the toilet and relieve ourselves.
Stress, an unhealthy way of life, bacteria, and viruses are the most important causes of intestinal problems. Bacteria and viruses reach the body via food or water and cause sickness, vomiting, and diarrhoea. Diarrhoea is caused because the body fights against these invaders and wants to get rid of them as quickly as possible. Sometimes, with diarrhoea, we have fever too, since an increase in the body temperature accelerates the healing processes of the body.
After the absorption of the nutrients in the small intestine, the 1.5 m long large intestine absorbs water from the remaining undigested food and converts it into a solid mass. The solid mass of waste matter is stored in the lowermost part of the large intestine, the rectum, and is then excreted as ‘stools’ through the anus.
The function of the small intestine is to digest the food into simpler nutrients, and to absorb these nutrients. If food stays longer in the small intestine, it will have more time to get digested completely and will also enable the body to absorb all the nutrients. The very long length (more than 6 m) of the small intestine enables the food to stay there longer. The finger-like ‘intestinal villi’ on the inner wall of the small intestine help in absorbing the nutrients in the bloodstream. The liver and pancreas produce enzymes that travel to the small intestine to help break down proteins, fats, and carbohydrates into sugar. This process is known as metabolism.
After the food has been chewed and mixed with saliva, it reaches the stomach through the oesophagus. The food is prepared for actual digestion in the small intestine, which is connected to the large intestine, and ends in the rectum. The undigested remains of the food and urine are excreted from the body. We can survive without water only for a few days, but the body can manage without food for quite some time. If the starvation goes on for a very long time, the body becomes weak and prone to diseases.
The different cells of our body have very different tasks, but they are all similar in their structure. They have a cell wall or a cell membrane, which protects the inner matter of the cell and controls what goes in the cell and what comes out of it. The nucleus inside the cell controls all the processes in the cell. The energy for these processes comes from mitochondria, the power house of the cell. They are responsible for cell respiration. The endoplasmic reticulum takes care of the conversion of substances and for detoxifying the cells. All these organelles are embedded in the liquid cytoplasm inside the cell. If we drink less fluid, the cytoplasm will dry out and the cell may die.
When we breathe, the oxygen reaches our lungs via the mouth, nose, and trachea. The lungs have a number of very tiny cavities known as air sacs which are covered by a network of very fine blood vessels. An exchange of gases takes place here. Impure blood reaches these capillaries and releases the waste gas carbon dioxide in the air sacs. This is exhaled out of the body, and freshly inhaled oxygen is absorbed into the air sacs. This oxygen is released into the blood via the capillaries.
The heart, a hollow organ with four chambers, can be imagined as an unbelievably strong pump. It is about the same size as a fist. Electrical signals coming from the brain cause the heart muscle to contract about 70 times in a minute in an adult. The contractions are more frequent in children and while we are playing sports. These contractions pump the blood through the whole body through the circulatory system. This system consists of two types of vessels: arteries with blood rich in oxygen and veins with blood poor in oxygen. While the arterial blood supplies oxygen to every cell in the body, the venous blood is sent to the lungs where it is purified by removing the waste product carbon dioxide and absorbing new oxygen.
Our body consists of millions of cells, which must be supplied with water properly. This supply is the responsibility of the blood. The red blood cells transport vital oxygen around the body. The blood also absorbs nutrients from the digestive tract, distributes them to the organs, and in return, takes away the waste products from the organs. Blood contains platelets that seal the wounds and white blood cells that help us fight disease-causing germs.
Animals do make use of tools for getting food or for protecting themselves. Apes reach for sticks or stones when threatened, sea otters break their favourite food, the shells, with the help of stones, and a species of finches picks out insects from tree trunks, with a long cactus quill. For generations the young ones of animals have watched the traits of the adults and imitated them exactly. Humans, on the other hand, may change the traits according to personal requirements. Humans advance and discover new things by adapting and learning. It is hard to believe that the hand axe made by prehistoric man out of stone has today given rise to sharp knives, chainsaws, and many other things.
The Neanderthals were people who lived 130,000-30,000 years ago, but then died out due to unknown reasons. One theory is that other lines of humans were stronger and eliminated them. The Neanderthals were about 1.70 m tall, had a heavily built body and were very powerful. They needed this power and strength to hunt woolly mammoths and rhinoceros with their home-made spears and knives. They made clothes of animal skins, lived in caves, and used fire. Their name is derived from the place where their remains were found, the Neander valley near Dusseldorf.