Consider a container of hot water (100oC), with a tube running straight through it, made of heat conducting material.
Cold water (0oC) runs through the tube. If the cold water runs slowly through the tube, then, ignoring inevitable imperfections, the water will enter the container at 0oC, and leave it at 100oC. This removes heat from the container. The temperature in the container therefore rapidly falls to 0oC, the equilibrium condition.
Now consider a similar container of hot water, with a tube of cold water running through it. This time, however, the tube is bent into a compressed U , to form a countercurrent system:
(The vertical limbs of the "U" must be contiguous; an open "U" behaves like a through-flow system.) As before, water enters the descending limb of the tube at 0oC. Its close proximity to the water ascending from the hot-water bath warms it to 100oC on its passage to the bath. During the water's ascent out of the bath, it loses that heat again to the descending limb of the loop. If the water flows through the countercurrent loop slowly enough (ignoring inevitable imperfections), the water in the ascending limb eventually gives up all its heat to the descending limb. It therefore emerges from the U-loop at 0oC, carrying no heat away from the container.
Thus, cold water in the tube is heated temporarily during its passage through the U-loop. That heat is, however, short-circuited back into the descending tube, so thatTHE WATER EMERGES FROM THE COUNTERCURRENT SYSTEM IN THE STATE THAT IT ENTERED IT. Countercurrent systems tend, therefore, to isolate environments despite intimate, temporary, contact between them.
It is important to note, however, that countercurrent systems isolate environments only if the flow through the U-loop is slow. If the cold water in the above example (Fig. 2) is rushed through the U-loop, the system behaves like a through-flow system, removing heat from the hot environment.
It is further important to note that if the water in the tube is at 100oC, and the bath is at 0oC, then the water in the tube will cool on its way down the descending limb, and heat up again on the way out of the bath, emerging, under idealistic circumstances, at 100oC. If both the bath water and the water in the tube are at 20oC, then there are no changes in temperature as the water flows through the tube. A countercurrent mechanism therefore merely maintains the status quo, it does not create differences in temperature between the tube water and the bath water.
Tocreate temperature, or other, gradients, energy needs to be expended. Thus, it requires electricity to warm an oven above the ambient temperature in the kitchen. It requires a similar quantity of electricity to cool a fridge to below the ambient temperature in the kitchen.
Physiological countercurrent mechanisms.
1. BLOOD FLOW THROUGH THE LIMBS
Seals live in cold sea water at 0oC to 15oC. These creatures tend, therefore, to lose a great deal of body heat. They cope with this by insulating themselves against the cold with a thick layer of subcutaneous blubber.
It is impossible, however, to cover their flippers with blubber, as that compromises their function as paddles or fins. Yet flippers are constructed of living tissue that requires a constant blood supply. Unless special measures are taken, the blood entering the flippers is cooled to the ambient water temperature (often 0oC), thereby dissipating a great deal of body heat to the environment, for which the animal cannot compensate by the addition of more blubber to the rest of the body. Ice-cold venous blood inevitably returns to the core from the flippers.
To avoid this, these creatures have to isolate themselves from their cold extremities by means of countercurrent mechanisms. Thus, the limb arteries are surrounded by the veins (venae comitantes) carrying the cold blood returning from the limb.
The blood in the artery gives off its heat to the cold venous blood, thereby cooling to almost the ambient water temperature before it enters the flipper. It therefore loses very little heat in the flipper. On returning via the venae comitantes, the blood warms to (nearly) body temperature before being added to the mixed venous blood in the right atrium. The heat (but not oxygen or nutrients, which cannot diffuse through the arterio-venous walls) has simply been short-circuited before the blood enters the cold environment of the flipper. The core remains warm and the flipper remains cold, despite sharing a common blood supply.
Humans possess a similar, but facultative, countercurrent perfusion system of the limbs, for use in cold weather. The arteries to limbs usually lie very deep, nearly at the centre of the limb, coming close to the surface only when traversing joints (the only place where the arterial pulse can thus be felt). They are surrounded by venae comitantes (or "deep veins"), through which the blood returns to the core on cold days. This, if the flow is slow enough, forms a very effective countercurrent system which cools the arterial blood to 20oC before entering the hand on a cold day. On its return to the heart the venous blood is warmed by the incoming arterial blood, back to nearly 37oC.
On a warm day the counter current system is switched off. The blood flow to the limbs is increased (which in itself compromises the heat short-circuiting mechanism), and the venous blood returns to the core through the superficial (subcutaneous) veins, instead of via the venae comitantes. The two limbs of the "U" tube have been separated, so that it is no longer qualifies as a "countercurrent system" (see above). The venous blood is now deliberately cooled, making use of the limbs' large surface area. The hotter the day, the more the superficial veins dilate (and the more the deep veins constrict).
2. THE NOSE AND RESPIRATORY PASSAGES
The mammalian respiratory airways constitute a counter current system: air enters and leaves the lungs via the same passages. Use is made of this in 2 ways - both aimed at maintaining crucial differences (or gradients) between alveolar air and the atmosphere.
By using a dead-end, countercurrent ventilation system, the composition of the alveolar air can be kept very different from that in the atmosphere. Indeed, mammalian alveolar air is not very different from the composition of the atmosphere encountered by the first amphibians who ventured on land about 400 million years ago. Since that time the proliferation of green plants has removed most of the carbon dioxide that was present then, and replaced it with highly reactive (and therefore devastatingly toxic) oxygen, giving us our modern atmosphere. Alveolar air therefore represents a 400 million year-old fossil atmosphere, which is carefully protected from the modern atmosphere by a long countercurrent system extending from the furthest extremity of the snout to the alveoli. (To obtain an impression of how toxic and unpleasant present day fresh air is to our physiology, merely hyperventilate for 15 seconds. This overcomes the countercurrent system's isolating effects!) By normally replacing only 10% of the alveolar gas with fresh air per breath (and constant monitoring of the resulting mix), our tissues can happily live and metabolise in a long-past and more congenial atmosphere. This is impossible with a through-flow system.
The countercurrent system's second effect is to isolate the physical conditions prevailing in the alveoli from those in the atmosphere. The alveolar air is warm (possibly a reflection of the greenhouse effect of the carbon dioxide-rich atmosphere 400 million years ago?), and moist; the ambient atmospheric air is cold and dry. On inhalation, the cold atmospheric air cools and dries the nasal mucosa. On exhalation the warm alveolar air re-warms and moistens the nasal mucosa, thus recapturing heat and moisture for the body. Air exhaled through the nose is therefore not at 37oC as would be expected from a through-flow system, but is closer to 30oC (and colder on a frosty day). It is also no longer saturated with water vapour. This can be graphically demonstrated by attempting to fog up a mirror or one's spectacles with air exhaled through the nose: it does not work. The fogging up of a cool glass surface is effective only with air exhaled through the mouth (which is normally not part of the countercurrent mechanism).
3. THE KIDNEYS
Countercurrent systems dipping into the renal medulla are used in 2 different ways. One is aimed at merely not disturbing the osmolality gradient between the renal medulla (1000 mosmol/kg H2O) and cortex (290 mosmol/kg H2O). The other creates that difference (therefore requiring expenditure of energy).
The vasa recta which course down into the renal medulla in the form of countercurrent loops, operate in the same manner as the limb arteries and veins on a cold day. Plasma with an osmolality of about 290 mosmol/kg H2O, slowly flows into the hypertonic medulla (about 1000 mosmol/kg H2O) and re-emerges with an osmolality of 300 mosmol/kg H2O (the system not being as perfect as I have pretended it could be above), leaving the medulla hyperosmotic. A through-flow circulation would rapidly eliminate the hyperosmotic environment of the renal medulla, as, in fact, capillaries do (and are intended to do) in other tissues.
The endothelial walls of the vasa recta do not energy consuming osmotic pumps and cannot therefore create the hyperosmotic environment in the medulla. At best, they disturb it as little as is physically possible.
Loops of Henle
To create the hyperosmotic renal medulla, use is made of an energy-consuming countercurrent flow system.
The renal tubules, in the form of the loops of Henle, form a countercurrent multiplier system, which actively short-circuits osmotically active particles between the ascending and descending limbs of the loop. The passive short-circuits, described above, merely maintain existing differences. Active short-circuit pumps can create osmotic gradients between 2 contiguous environments.
The countercurrent multiplier works as follows. Chloride ions are actively pumped from the ascending limb of the loop of Henle into the descending limb. Na+ ions follow passively, down the resulting electrical gradient. Since the intervening wall is impermeable to water (there are no aquaporins in the tubular epithelium of the ascending limbs of the loops of Henle), the transfer of NaCl (without accompanying water) from the ascending to the descending tubule raises the osmolality of the tubular fluid as it flows toward the hairpin bend. After the hairpin bend, the osmolality decreases again as NaCl is pumped from the ascending loop.
This is called a multiplier system because the pumps at each level work against a modest osmotic gradient (of maybe not more than 50-100 mosmol/kg H2O). The counter current flow combines the effects at all the levels to eventually produce a 700-800 mosmol/kg H2O difference between the fluid entering the loop (at 290 mosmol/kg H2O) and that in the hairpin bend (at 1000 mosmol/kg H2O).
Thus, the loops of Henle create a hyperosmotic environment in the depths of the renal medulla. The vasa recta do not wash it away.
There is a third medullary element, however, which tends to undo these effects by its through-flow arrangement. This is the collecting ducts system. When ADH (antidiuretic hormone) is present in the blood, collecting duct fluid enters the medulla with an osmolality of about 290 mosmol/kg H2O. Coursing through the medulla its osmolality rises to 1000 mosmol/kg H2O (as it loses a little more than 2/3 of its water to the hypertonic environment), and is then discharged into the renal pelvis. This would therefore rapidly eliminate the difference in osmolality between the cortex and medulla, saturating the medulla with water.
Removal of water from the renal medulla
Clearly the water discharged into the medulla from the collecting ducts has constantly to be removed from the medullary interstitium, if the system is to continue operating as a urine concentrating mechanism.
According to the textbooks the vasa recta fulfil this function. A passive countercurrent flow will, however, no more remove water from the medulla than it will remove excess NaCl, urea, heat, hypoxia, or any other property from the medulla (see introductory remarks). It is designed to leave the status quo undisturbed. If the status quo is changed (by an external agency) a passive counter current system will simply maintain (i.e. not disturb) the new situation. It possesses no propensity to return the system to some previous or preferred situation.
Indeed, the fact that the osmolality of the plasma returning from the medulla along the ascending limbs of the vasa recta is higher than it was when it entered, means that the blood in the vasa recta has LOST water in the medulla, and not picked it up! If they were the route by which water is removed from the medulla then the plasma would emerge from the vasa recta hypotonically, not hypertonically (as is actually the case).
There is only one way in which the vasa recta could remove water absorbed from the collecting ducts from the medulla. That is to expend metabolic energy, thus creating, in effect, a second countercurrent multiplier. This is very unlikely.
The mechanism for water removal from the medulla suggested in the textbooks, relies on the fact that a portion of plasma's osmotic pressure (the oncotic pressure) is inexchangeable with that of the medulla. Thus, like elsewhere in the body, plasma has a small but significantly higher osmotic pressure than the surrounding tissue fluid, and therefore tends to dry the tissues out. This is however more than balanced by the higher hydrostatic pressures in the capillaries than in the tissues, which forces water out at a slightly higher rate than the oncotic pressure reabsorbs it. Tissues everywhere therefore tend to become hydrated, and require a lymphatic drainage system to avoid oedema. The vasa recta could, therefore, remove water from the medulla, but only if the hydrostatic pressure in these vessels is equal to that in the medullary interstitium. Even if this could be achieved, which is unlikely if there is to be any flow through these vessels, the water absorption capacity of the vasa recta will be small.
The most likely site of water resorption from the medullary interstitium is the loop of Henle. Energy is expended to create an hyperosmotic fluid in the hairpin bends of the loops of Henle. This hyperosmotic fluid will absorb water from the interstitium, thus dehydrating the medullary tissues, and correcting any hydration effects caused by the operation of ADH on the collecting ducts.
The water fluid which emerges from the ascending limbs of the loops of Henle is, therefore, as expected, hypotonic compared with the state in which it entered the loops.
The proposed mechanism is as follows. Salts pumped from the ascending limb into the descending limb of the loop of Henle cause the osmolality of the tubular fluid to rise from 290 to 1000 mosmol/kg H2O as it approaches the hairpin bend. This draws water into the tubule, drying out the interstitium. The thick portion of the ascending limb is impermeable to water, ensuring that water drawn into the tubule, particularly near the hairpin bend (where the osmolality is at its highest), is trapped within the tubule. This water is then carried to the cortex, giving the fluid emerging from the ascending limb of the loop of Henle an osmolality of about 50 mosmol/kg H2O.
Being at a lower osmolality than the osmolality of the tissue fluids of the cortex interstitium, the distal tubular fluid readily loses water to its surroundings (in the presence of ADH), from where it is washed away by the through-flow system of cortical capillaries. (In the absence of ADH the distal tubule and collecting duct are impermeable to water. No further osmotic adjustment of the tubular fluid therefore occurs after the loop of Henle, and a large volume of urine with an osmolality of 50 mosmol/kg H2O is discharged into the renal pelvices.)
When large amounts of sugar (glucose, or intravenously infused mannitol) are filtered, so that not all of it is reabsorbed from the proximal tubular fluid, water is osmotically retained in the proximal tubule. The flow rate which is normally reduced from about 120 ml/min at the beginning of the tubule to 25 ml/min at its end, is now reduced to, say, only 50 ml/min. This represents a 100% increase in the flow rate through the countercurrent multiplier. The countercurrent multiplier's efficacy is therefore compromised. The osmolality at the tip of the loop of Henle tends towards 290 mosmol/kg H2O (as if it were a through-flow system), and the kidney's urine concentrating and diluting powers are lost. The person is then said to have an osmotic diuresis: urine with an osmolality approaching 290 mosmol/kg H2O is produced at a higher than normal rate.