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## Tubular Secretion In this process certain substances move into the filtrate of PCT and DCT from blood plasma. This includes waste that escapes during ultra-filtration and remained in the blood. These substances are absorbed actively, and include urea, creatinine, hydrogen ions, potassium ions, s...

## Tubular Secretion In this process certain substances move into the filtrate of PCT and DCT from blood plasma. This includes waste that escapes during ultra-filtration and remained in the blood. These substances are absorbed actively, and include urea, creatinine, hydrogen ions, potassium ions, some hormones and drugs if present. Tubular secretion mostly performed by proximal convoluted tubule (PCT) but some of the K are also secreted from DCT and collecting duct due to reciprocal exchange of Na with K. It adjust the pH of urine. ## Counter Current Mechanism The counter current mechanism is biological processes intend to allow maximum exchange of molecules between two fluids of different concentration which are moving in opposite directions. This mechanism involves loop of Henle and the environment of medulla. In medulla region of the kidney, the ascending limb of loop of Henle is permeable to Na+, K+ and Cl ions while impermeable to water. The descending limb of loop of Henle is permeable to water. Ascending limb cells have specialized ionic co-transporter protein, each allow one Na with one K and two Cl hence allow lot of ions to move out from the entire limb. When these ions are actively reabsorbed from the ascending limb and accumulated in medulla, it makes medulla environment hypertonic. This movement of ions is also facilitated by steroid-based hormone called aldosterone secreted from the cortex region of adrenal gland. Movement of water molecules is facilitated by anti-diuretic hormone (ADH) secreted from posterior lobe of pituitary gland. Another compound that increases the osmotic gradient in inner medulla is the Urea, it enters in the medulla from the collecting duct and along with other ions, helps the reabsorption of water. This process is called counter current multiplier. Now there is another counter current mechanism performed in between nephron loop of Henle and the peritubular capillaries or vasa recta. The loop of Henle is surrounded by peritubular capillaries containing blood. These capillaries are permeable to both water and ions so due to high ionic concentration in the interstitium, water is diffusing out and solutes diffuse inside the capillaries that travel along the descending limb of the loop of Henle. This movement of water and solutes, counter current mechanism, helps in the medulla from the collecting duct. The mechanism involves the flow of fluid in opposite directions and the concentration of solutes in these fluids. The process helps to maintain the concentration gradient, which is important for reabsorption of water and solutes in the kidneys. The counter current mechanism also helps to regulate the urine concentration and volume. The process is essential for the functioning of kidneys and helps to maintain the body's fluid and electrolyte balance. A diagram of a kidney is shown to the right of the text. The diagram shows the parts of the kidney including the loop of Henle, the collecting duct, the peritubular capillaries and the vasa recta.