Bless you, Bob lee
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oozing on the muggy shore of the gulf coast
l...@fbtc.net
--- Begin Message ---Greetings neoteric and prolocutors of the CS art, TRANSPORT TISSUES Blood and lymph are circulating tissues that help other parts of the body commmunicate and interact. They consist of specialized cells dispersed in the cardiovascular system,lymphatic system and intercellular substance. The blood is contained in a set of closed vessels and does not come into immediate contact with the cells, except those cells lining the walls of the blood vessels. A medium of exchange between the blood and the cells is therefore necessary. Tissue fluid serves this purpose. The walls of the capillaries are thin, and some of the fluid passes out into the spaces between the tissue cells. Tissue fluid is derived from the plasma of the blood mainly by diffusion and filtration. There is sometimes assumed an active secretory process on the part of the endothelial cells of the capillaries. Some suggest that the term *lymph* be applied to the fluid within the lymph capillaries and lymph veins and that the fluid outside the vessels,in the tissue spaces, be called tissue fluid. Usually the term lymph is applied to the tissue fluid in general,i.e.,to the fluids in the tissues and to the excess drained off in the lymph tubes. *Lactaels* are lymph channels in the small intestine. During digestion they are filled with *chyle*, a milk-white fluid composed mainly of emulsified fat. The composition of tissue fluid is similar to that of blood plasma. It is a colorless or yellowish fluid possessing an alkaline reaction, a salty taste and a faint odor. More on lymph further on. CAPILLARIES The exchange of substances between the blood and cells of the body occurs through the capillaries. Functionally,the capillaries are the most important vessels of the blood vascular system. Each capillary is about 0.5 to 1.0 mm long and has approximately the same diameter as a red blood cell(0.01 mm). This requires the cells to pass single file through the capillary, which allows for an efficient exchange of gases to occur. The blood cells actually have to stretch and squeeze to pass through the capilllary and the direct contact with the capillary wall(endothelial cells)makes it easy for gases and nutrients to pass from the blood cells to the cells of the tissue.Although capillaries are very small, they are so numerous that it is estimated that their total surface area within the body is 60 square meters (642sq.ft). This provides a generous surface through which exchange can occur. The walls of the capillaries consist of a single layer of the endothelial cells of the *tunica intima*. The tunica media and the tunica externa are not present ,as in the arteries and veins. The cells of the endotheliam are generally elongated along the axis of the capillary and tapered at each end. Many substances are able to pass through the cell wall directly,as the white blood cells,allowing them to reach areas of infection outside of the bloodstream. This passage of the white cells through the unruptured walls of the capillaries is called *diapedesis*. At any given moment,approximately 5 per cent of the total circulating blood is flowing through the capillaries, performing the ultimate function of the system, the exchange of nutrients and metabolic end products. The capillaries permeate every tissue of the body, no cell is more than 0.005 in.(two or three cells) from a capillary. Therefore diffusion distances are very small and exchange is highly efficient. The thin capillary membrane behaves as though it were perforated by small pores through which water and solute particles smaller than proteins may readily move. The permeabily of the capillaries is termed *leaky*, with liver capillaries the leakiest and brain capillaries the tightest. No capillary is so leaky that it will allow erythrocytes to escape (pass thru). Blood enters the capillary network from the arterioles. Most tissues appear to have two types of capillaries,*true* capillaries and *thoughfare* channels. True capillaries exit and enter the channels in a vast mat of capillaries,millions of them. At each capillary exit from a channel there is a *precapillary sphincter* muscle which continually opens and closes so that the flow thru the capillary is controled. In more active tissue more sphincter muscles are open (when exercising), thus providing more blood flow thru the the tissue. Millions of these sphincter are opening and closing every second controling the flow of blood. Eating and exercising shortly there after causes the digestive and muscular sphincters to be open at the same time,the general result is a muscular cramp due to reduced blood flow in the muscle tissue and lack of oxygen and nutrients in the tissue. As the blood flows from arteries to arterioles to capillaries the cross section area for the blood flow becomes larger,and the flow becomes slower. The significance of this phenomenon is very great,blood flows through the capillaries so slowly (0.07 cm/sec) that there is adequate time for exchange of nutrients and metabolic end products between the blood and tissues. There is no active transport of solute across the capillary wall,materials crossing primarily by simple passive diffusion. Because fat-soluble substances penetrate cell membranes easily, they actually pass directly through the endothelial capillary cells. In contrast,many ions and molecules are poorly soluble in fat and pass thru pores between adjacent endothelial cells. In any case ,all nutrients and metabolic end products diffuse across the capillary wall with great speed. Other means of ions and molecules passing thru cell membranes will be discussed later on. What is the sequence of events involved in capillary-cell transfers? Tissue cells do not exchange material directly with the blood. The interstitial fluid (we can call it lymph)always acts as the middleman. Thus ,nutrients diffuse across the capillary wall into the interstitial fluid (lymph),from which they gain entry to cells. Conversely, metabolic end products move first across cell membranes into the lymph, from which they diffuse into the plasma of the blood. Thus, TWO membrane-transport processes must always be considered,that across the capillary wall and that across the tissue cell membrane. The cell-membrane step may be by diffusion or by carrier mediated transport,but, the transcapillary movement is always by diffusion. Since to achieve NET transport of any substance by diffusion, a *concentration gradient* is required,transcapillary diffusion of nutrients and metabolic end products PROCEEDS primarily in ONE DIRECTION because of diffusion gradients for these substances between the blood and lymph fluid (interstitial fluid). Bless you Bob Lee -- oozing on the muggy shore of the gulf coast l...@fbtc.net-- The silver-list is a moderated forum for discussion of colloidal silver. To join or quit silver-list or silver-digest send an e-mail message to: silver-list-requ...@eskimo.com -or- silver-digest-requ...@eskimo.com with the word subscribe or unsubscribe in the SUBJECT line. To post, address your message to: silver-list@eskimo.com List maintainer: Mike Devour <mdev...@id.net>
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