Reabsorption, Secretion and Excretion

Hello :) This post will deal with how reabsorption, secretion and excretion occur in the renal system. We'll look at the types of epithelium found in the nephron as well as how solutes and water are reabsorbed and secreted. We'll also go through a few key definitions that we'll need to know. In addition we'll discuss the process of water reabsorption and how glucose is absorbed at the proximal tubule in the nephron. Enjoy!

Reabsorption

This is the movement of water and filtered solutes from tubules into peritubular capillaries. Through this process, blood is able to recapture what it temporarily lost through filtration. Most reabsorption occurs unregulated in the proximal convoluted tubule (PCT) and returns 99% of filtered material to the blood.

The renal tubules have specialisations which affect how they absorb substances.

• The PCT has unregulated (it's not affected by hormones) reabsorption. The PCT acts as a mass absorber and has an large absorptive surface area because of its microvilli. "Leaky" tight junctions are also present here and this allows paracellular diffusion of solutes and water.
• The Distal Nephron has regulated reabsorption and secretion. The distal nephron includes the distal convoluted tubule and the collecting duct and they regulate reabsorption. This is the place where the exact amounts of water or solute that are excreted is determined, it is a very important place for the final modification of urine. The distal nephron has less microvilli than the PCT and its tight junctions are less permeable. This allows higher electrochemical gradients to be formed. The epithelial cells also have receptors in order to respond to regulatory hormones.
• The Loop of Henle (LOH) establishes an osmotic gradient to aid in water reabsorption. 

 Water and Solute Reabsorption and Secretion

In order for a solute to be reabsorbed it needs to cross two barriers: the endothelium of the capillary and the epithelium of the renal tubules. The capillary walls are quite permeable, thus the walls of the renal tubule provide the main barrier to reabsorption. The solute must pass through the apical membrane (the membrane which is in contact with the fluid in the renal tubule) and then through the basal membrane (the one in contact with the peritubular fluid between the tubule and the capillary) before it can pass through the capillary endothelium.

Solutes can pass through these membranes in several different ways:

1. Passive Reabsorption: This is when the solute diffuses down its concentration gradient and occurs when the concentration of the solute is greater in the tubular fluid than in the plasma. This requires no energy but the solute must be able to pass through the renal epithelium and capillary endothelium. 
2. Active Reabsorption: This is when the solute moves against its concentration gradient and occurs when the concentration of the solute is greater in the plasma than in the tubular fluid. A solute is considered actively reabsorbed if has been actively transported from the apical or basal membrane, it doesn't have to have been actively transported at both membranes. Importantly, this process requires energy.  There are two types of Active Transport:
1.  Primary Active Transport: the energy from ATP is used to directly transport a substance against its concentration gradient. eg. Na/K ATPase pumps. 
2. Secondary Active Transport: the transport protein couples the flow of one substance with another. One substance moves down its gradient and this releases energy while the other substance uses this energy to move against its gradient. Both solutes can go in the same direction (symport) or in opposite directions (antiport). eg: glucose transport coupled to Na+.
3. Facilitated Diffusion: this is when a carrier protein in the membrane binds to the molecule on one side and then undergoes a conformation change to release the molecule on the other side of the membrane. This occurs down an electrochemical gradient and no energy is required. However, saturation of the carriers can occur because there is a limited number of transmembrane proteins. 

Water Reabsorption

Water reabsorption is always a passive process and water always diffuses to areas of greater osmolarity (from areas of low solute concentration to areas of high solute concentration). This osmotic gradient is created by the active transport of solutes to the peritubular fluid and the plasma. Sodium (Na+) is the most important solute in terms of water movement because water follows the active reabsorption of sodium.

The reabsorption of sodium is unregulated at the proximal tubule and is driven by the Na/K ATPase pump. This pump actively transports sodium across the basolateral membrane into the blood plasma. This creates a low Na+ concentration inside the proximal tubule epithelial cell. This causes Na+ to move across the apical membrane into the cell using carrier proteins via cotransport with glucose and countertrasport with H+. Because positive Na+ ions are travelling into the cell this favours the paracellular transport of Cl-. Water then follows the Na+ ions into the blood plama.

Some Important Definitions

Transport Maximum (Tm): this is the maximum rate at which a solute can be transported (reabsorbed) because all the carrier proteins are saturated. Once the transport maximum is reached (when all the carrier proteins are being used) the excretion of solutes is increased in the urine.

Renal Threshold: this is the plamsa concentration at which Tm is reached. The transport of the solute is proportional to the plasma concentration until saturation occurs. 

This is important when we consider how the kidneys handle glucose. At the normal filtered load all the glucose should be reabsorbed . At the renal threshold, transport maximum will be reached and glucose will appear in the urine. This occurs during diabetes mellitus. Hyperglycaemia occurs and this causes the filtered load to exceed Tm and glucose appears in the urine.

Renal Clearance: This is the volume of plasma from which  a solute is completely removed from the kidneys per unit time. In other words, it is the rate at which a solute is removed from the plasma by the kidneys. Renal Clearance can be calculated using the following formula: 

Clearance = Excretion Rate / Plasma Concentration
OR
Clearance = volume of urine produced per unit of time  X urinary concentration of substance / plasma concentration of substance.

There is a difference between renal clearance and excretion rate though. The excretion rate is the quantity of the solute that is removed from the body per minute. The clearance is the excretion of the solute relative to that solute's concentration in the plasma.   

We can use renal clearance to estimate the GFR (Glomerular Filtration Rate) in an animal. A substance called creatinine, which is a by-product of skeletal muscle metabolism and is freely filtered with no reabsorption or secretion and is released into the blood at a fairly constant rate, is measured. The amount of creatinine released = the amount filtered. 

That's it for this post, if you have any questions please feel free to ask :)