Divers face dangers that can hurt or kill them if they don't know what to do. They can use up all their air, or get decompression sickness or nitrogen narcosis. A person must be trained to use the equipment and dive safely. When they show that they can dive safely they get a certification card. The biggest organization for certifying divers is PADI - Professional Association of Diving Instructors - but there are many others, depending on the country. Some tourist places have a short course without certification and then the instructor will lead the diver in a shallow dive, all in one day.
Because of special dangers, there are advanced classes for things like diving in or around underwater shipwrecks, cave diving, and deep diving (more than 60 feet or 18 meters).
Capillary
A capillary is a blood vessel. It does not have the muscular/elastic tissue of other blood vessels. It has a single celled wall to help substances be transported through organisms.
Capillaries are small, and smaller than any other blood vessels. They are about 5-10 μms big, which connect arteries and venules, and enable the moving of water, oxygen, carbon dioxide, as well as many other nutrients and waste chemicals between blood and surrounding tissues.
Blood moves from the heart to arteries, which branch and narrow into smaller arteries, and then branch more into capillaries. After oxygen has been moved to the tissue, capillaries join and widen to become small veins and then widen more to become veins, which return blood to the heart.
The "capillary bed" is the network of capillaries supplying an organ. The more metabolically active the cells, the more capillaries it will require to supply nutrients and carry away waste products.
Special arteries connect between arterioles and venules and are important in bypassing the flow of blood through the capillaries. True capillaries come from mainly from metarterioles and provide movement between cells and the circulation. The width of 8 μm forces the red blood cells to partly fold into bullet-like shapes in order to bypass them in single file.
Precapillary muscles are rings of smooth muscles at the start of true capillaries that handle blood flow into true capillaries and control blood flow through a body part or area.
The capillary wall is a one-layer tissue so thin that gas and other items such as oxygen, water, proteins and fats can pass through them driven by pressure differences. Waste items such as carbon dioxide and urea can move back into the blood to be carried away for removal from the body.
The capillary bed usually moves no more than 25% of the amount of blood it could contain, although this amount can be increased through auto regulation by making the smooth muscle relax in the arterioles that lead to the capillary bed as well as metarterioles making themselves smaller.
The capillaries do not have this smooth muscle in their own wall, and so any change in their width is passive. Any signaling molecules they release (such as endothelin for constriction and nitric oxide for dilation) act on the smooth muscle cells in the walls of nearby, larger vessels, e.g. arterioles.
Capillary's ability to move items can be increased by the release of certain cytokines, such as in an the body defending itself from germs.
Viriato Clemente da Cruz
Viriato Clemente da Cruz, an Angolan poet and politician, was born in 1928 in Kikuvo (Porto Amboim), Angola and died in Beijing, China on 13 June 1973.
He is considered one of the most important Angolan poets of his time. He wrote poems in Portuguese and Angolan languages. He took part in the fight to free Angola from Portuguese rulers.
He was educated in Luanda, the capital of Angola, which was then ruled by Portugal. As a young man in 1957 he went to Paris, where he met Mario Pinto de Andrade, another Angolan poet and politician, who helped with his political ideas. In the 1960s they helped create the group called the Popular Movement for the Liberation of Angola (MPLA—from the Portuguese name "").
The MPLA was in not in Angola at this time, but was first in Conakry (the capital of Guinea) and then later in Leopoldville, Congo (now Kinshasa the capital of the DRC). Viriato da Cruz became secretary-general of the MPLA. After some time he and others, did not agree with some people in the group and this led to fighting in the streets of Leopoldville.
Viriato da Cruz went to Beijing, China in the 1960s because of the fighting in the MPLA. At first, the Chinese government welcomed him. He was well-known because he had helped create the MPLA. The Chinese wanted Viriato da Cruz to help them bring Maoist socialism to Africa.
This was a big problem for Viriato and his family. His ideas were not the same as the Maoists. He thought that stronger countries can not bring a socialist revolution to other places. He was very brave and did not change his ideas. This was against the Chinese Maoist idea of world revolution.
Viriato da Cruz wanted to leave China and return to Africa, but the Chinese government would not let him go and they were not good to him. This may be because he could have made trouble for their work in Africa.
The last years of the life of Viriato da Cruz were unhappy and difficult. He was weak, without much food, and he died on 13 Jun 1973. His dead body was taken away in a military vehicle and his burial was without any ceremony.
Some Angolan people think that this was shocking, because Viriato da Cruz worked hard for Angola and was one of the leaders of the liberation from Portugal.
Poems (1961). Among these poems, "Namoro", "Sô Santo" and "Makézu" are well-known.
Mollusc
Molluscs (or mollusks) are an important phylum of invertebrate animals. Most of them are marine. They have huge numbers in-shore, that is, in shallow water. They are the largest marine phylum, with about 85,000 living species, 23% of all named marine organisms. They also occur in freshwater and on land.
Mollusks are extremely varied: they have great diversity. That may be why there is no word in English for the phylum as a whole. "In an evolutionary sense, mollusks are plastic material". They have much more variety than their ancient rivals, the brachiopods.
Most molluscs have shells, but some groups do not: octopods, slugs, and the gastropods known as sea slugs.
Many mollusks also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat.
The phylum is usually divided into 9 or 10 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish and octopus, are among the most neurologically advanced of all invertebrates: they have good brains and complex behaviours. Either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species, and account for 80% of the total. The scientific study of mollusks is called malacology.
The three most universal features defining modern mollusks are:
Other than these things, mollusks have so much variety that many textbooks use a "hypothetical ancestral mollusk" to summarize them (see below). This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate. It is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot".