BIOLOGY Volume 3 - A Guide to General Biology - 2004

20. EXCRETION AND OSMOREGULATION

20.5. The Human Kidney

20.5.6. Loop of Henle

The function of the loop of Henle is Water conservation. The longer it is, the more concentrated the resulting urine becomes. This is a beneficial adaptation for terrestrial life. Only birds and mammals possess a nephron that includes a loop of Henle, making them the only vertebrates capable of producing urine more concentrated than Blood. In humans, the concentrations of blood and urine can differ by a factor of 4–5. The drier an animal's natural habitat, the longer its loop of Henle. For example, in the semi-aquatic rodent, the beaver, the loop is short, producing large amounts of dilute urine, whereas desert dwellers—such as the kangaroo rat and jerboas—feature long loops of Henle, yielding small volumes of urine that are 6–7 times more concentrated than human urine; these animals do not drink at all, satisfying their fluid needs entirely from water present in their food and generated as a byproduct of cellular Respiration.

Together with the vasa recta (straight capillaries) and the collecting duct, the loop of Henle creates and maintains a gradient in the renal medulla, running from the cortex to the tip of the renal pyramid (see Fig. 20.12).

This gradient is established through varying salt concentrations between the cortex and the papilla. As a result, water leaves the nephron by osmosis within the medullary region, and the fluid ( forming the urine) inside its lumen becomes more concentrated.

The loop of Henle comprises three functionally distinct sections:

1) the descending limb with a thin wall;

2) the thin segment — the lower part of the ascending limb with a thin wall, similar to the descending limb;

3) the thick segment — the upper, thick-walled portion of the ascending limb.

The descending limb is highly permeable to water and most solutes. Its function is to facilitate their diffusion across its wall. Both PARTS OF THE ascending limb are almost entirely impermeable to water. Cells of the thick segment actively reabsorb sodium ions, potassium ions, chloride ions, and others from the nephron lumen. Under normal conditions, water moves osmotically wherever ions go, but in this case, such movement is prevented by the cellular impermeability to water. Consequently, the fluid in the ascending limb becomes heavily diluted by the time it reaches the Distal convoluted tubule.

The Loop of Henle: A Countercurrent Concentrating Mechanism

As previously mentioned, a salt concentration gradient is maintained in the renal medulla. It spans an increase from roughly 300 to 1200 mOsm/L (units are omitted hereafter for simplicity). Let us examine the underlying causes. Consider a theoretical initial scenario where the entire loop of Henle is filled with a fluid at a concentration of 300 (the normal concentration for tissue fluid and blood), which is in equilibrium with the surrounding medullary tissue fluid. Our objective is to establish the aforementioned gradient. Let us imagine the process starting in the thick ascending segment. Here, Active Transport of sodium ions from the nephron cells into the tissue fluid occurs via the sodium-potassium pump (Section 5.9.8, active transport). Sodium ions then diffuse from the lumen of the ascending limb into its wall, replenishing the local sodium deficit. In doing so, they pass through a carrier protein that simultaneously transports potassium and chloride ions. As a result, these ions are co-transported against their concentration gradient and subsequently diffuse into the medulla alongside sodium ions. The entire process is driven by the sodium-potassium pump, leading to an accumulation of sodium, potassium, and chloride ions in the tissue fluid surrounding the nephron. The concentration of these ions decreases in the ascending limb while rising in the medulla, as water cannot accompany them across the impermeable wall. Meanwhile, the descending limb remains highly permeable to water and less permeable to ions. This described mechanism maintains a concentration difference of approximately 200 units between the ascending limb of the loop of Henle and the medulla, as illustrated in Fig. 20.25.

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Fig. 20.25. Transport of water and ions from the loop of Henle into the renal medulla. A. The hypothetical situation that would occur with static fluid in the loop of Henle. B. The actual situation resulting from fluid movement through the loop of Henle. The numbers indicate solute concentrations (mOsm/L).

Box 20.1. METABOLISM/2.html">THE CONCEPT OF Osmolarity

The concentration of dissolved substances (ions or molecules) can be expressed in terms of solution osmolarity. This parameter is traditionally used by animal physiologists. Since every structural unit of a substance (ion or molecule) contributes to the osmotic potential, and one mole contains an identical number of these units across all substances, osmolarity is defined as the total number of moles of all particles per 1 dm³ of solution and is measured in osmoles. For instance, 1 mole of KCl dissociates in water into 1 mole of K⁺ and 1 mole of Cl^-, yielding a total of 2 moles of particles. Consequently, the osmolarity of this solution is 2 osmoles.

Mediums with identical osmolarities possess identical osmotic potentials.

When comparing solutions, the number of osmoles (or typically milliosmoles in the case of glomerular filtrate) is referenced per 1 liter of fluid (mOsm/L).

Normally, the osmolarity of Blood Plasma and tissue fluid is close to 300 mOsm/L.

In urine, it ranges from 300 to 1000 mOsm/L. The osmolarity of tissue fluid in the renal medulla reaches 1200 mOsm/L, whereas seawater averages about 1000 mOsm/L.

When the salt concentration in the medulla is high, water exits the descending limb via osmosis, rendering the luminal fluid more concentrated. If this fluid did not move along the loop of Henle, the situation illustrated in Fig. 20.25A would arise, where the pump cannot raise the medullary concentration above 400 due to ions leaking back into the thick segment of the ascending limb. However, under physiological conditions, fluid flows continuously through the loop of Henle. The higher it ascends within the thick ascending segment, the more sodium ions are pumped out, and the more diluted it becomes. A concentration gradient is thus established in the ascending limb. Because the pump maintains a 200-unit difference between the ascending limb and the medulla, an identical gradient forms in the medullary tissue fluid. Simultaneously, increasing amounts of water are drawn out of the descending limb, elevating the solution concentration from top to bottom. This ultimately produces the condition depicted in Fig. 20.25B.

Some ions exit the descending limb, but water is removed at a much faster rate. It does not dilute the tissue fluid because it is carried away by the vasa recta running parallel to the loop of Henle. The blood composition within them Changes in the same manner as the solution surrounding the nephron. Blood flow here is slow, gradually establishing an equilibrium state at all levels. It should be noted, however, that this equilibrium is dynamic—meaning it is disrupted if the sodium-potassium pump fails, blood flow stops, or fluid movement within the nephron ceases.

The loop of Henle Functions as a countercurrent multiplier. It is termed 'countercurrent' because the fluid in the two limbs flows parallel to each other but in opposite directions—first downward, then upward. The multiplying effect becomes evident when comparing Fig. 20.25A and 20.25B. The latter demonstrates how a pump, capable of maintaining a mere 200-unit difference across the nephron wall, generates a luminal gradient spanning from 300 to 1200. This is achieved through the continuous removal of sodium and other ions from the ascending limb of the loop of Henle, as well as the fact that the outflow of solutes is replenished by their influx into the descending limb from the proximal convoluted tubule.