Number three, dedicated to Katarina. :-)
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oozing on the muggy shore of the gulf coast
l...@fbtc.net
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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
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oozing on the muggy shore of the gulf coast
l...@fbtc.net
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