Question

The process of exchange between vasa recta and the loop of Henle has the name countercurrent exchange. What can we conclude from that name about their flow, by analogy with animal respiration terminology?

Answer 1

Counter current mechanism.

First, assume that the loop of Henle is filled with fluid with a concentration of 300 mOsm/L, the same as that leaving the proximal tubule ( step 1).

Next, the active ion pump of the thick ascending limb on the loop of Henle reduces the concentration inside the tubule and raises the interstitial concentration; this pump establishes a 200-mOsm/L concentration gradient between the tubular fluid and the interstitial fluid (step 2).

The limit to the gradient is about 200 mOsm/L because paracellular diffusion of ions back into the tubule eventually counterbalances transport of ions out of the lumen when the 200-mOsm/L concentration gradient is achieved. Step 3 is that the tubular fluid in the descending limb of the loop of Henle and the interstitial fluid quickly reach osmotic equilibrium because of osmosis of water out of the descending limb. The interstitial osmolarity is 1 300 300 300 300 300 300 300 300 300 300 200 400 200 maintained at 400 mOsm/L because of continued transport of ions out of the thick ascending loop of Henle. Thus, by itself, the active transport of sodium chloride out of the thick ascending limb is capable of establishing only a 200-mOsm/L concentration gradient, much less than that achieved by the countercurrent system.

Step 4 is additional flow of fluid into the loop of Henle from the proximal tubule, which causes the hyperosmotic fluid previously formed in the descending limb to flow into the ascending limb.

Once this fluid is in the ascending limb, additional ions are pumped into the interstitium, with water remaining in the tubular fluid, until a 200-mOsm/L osmotic gradient is established, with the interstitial fluid osmolarity rising to 500 mOsm/L (step 5).

Then, once again, the fluid in the descending limb reaches equilibrium with the hyperosmotic medullary interstitial fluid (step 6),

and as the hyperosmotic tubular fluid from the descending limb of the loop of Henle flows into the ascending limb, still more solute is continuously pumped out of the tubules and deposited into the medullary interstitium. These steps are repeated over and over, with the net effect of adding more and more solute to the medulla in excess of water; with sufficient time, this process gradually traps solutes in the medulla and multiplies the concentration gradient established by the active pumping of ions out of the thick ascending loop of Henle, eventually raising the interstitial fluid osmolarity to 1200 to 1400 mOsm/L as shown in step 7.

While in Fish, gills use a design called ‘countercurrent oxygen exchange’ to maximize the amount of oxygen that their blood can pick up.

They achieve this by maximizing the amount of time their blood is exposed to water that has a higher oxygen level, even as the blood takes on more oxygen. Countercurrent oxygen exchange (shown in the figure above) means the blood flows through the gills in the opposite direction as the water flowing over the gills. This flow pattern ensures that as the blood progresses through the gills and gains oxygen from the water, it encounters increasingly fresh water with a higher oxygen concentration that is able to continuously offload oxygen into the blood. The low-oxygen blood, which is just entering the gill, meets low-oxygen water. Since there is more oxygen in the water, the oxygen can flow from water to blood. Likewise, the high-oxygen blood, which has nearly passed the entire length of the gill, meets fresh, high-oxygen water, and oxygen continues to flow from water to blood.

If fish instead had blood flowing in the same direction as water through their gills (called ‘concurrent flow’), the low-oxygen blood entering the gill would first meet the high-oxygen water also entering the gill. Oxygen would quickly pass from the water into the blood, until the oxygen levels of the blood and water rapidly became the same, and oxygen diffusion into the blood would stop. The maximum amount of oxygen that the blood could pick up would be only half of the total amount of oxygen in the water. In contrast, countercurrent oxygen exchange allows the blood to pick up 90 percent of the oxygen in the water.

Thus in both processes the countercurrent mechanism system expends energy to create a concentration gradient.