7. Renal biochemistry. The role of the kidneys in the regulation of water and salt metabolism

Kidney is a paired organ; nephron is the basic structural unit of kidney. About 1000-1300 ml of blood is filtered in the kidneys per 1 minute. Due to the good blood supply, the kidneys are in constant interaction with other organs and tissues and can affect the state of the internal environment of the whole organism. There are the following renal functions: excretory, metabolic and homeostatic.

Excretory function is aimed at removing the end products of catabolism from the body. For example, the products of nitrogen metabolism: urea, uric acid, creatine, and the products of neutralization of toxic substances. Kidneys excrete excess substances sucked into the intestine or formed in the catabolic processes: water, organic acids, vitamins, hormones, etc., as well as xenobiotics – foreign substances (drugs, nicotine, etc.).

Performing homeostatic function, the kidneys regulate water homeostasis, salt homeostasis, acid and base balance.

Metabolic function of the kidneys involves their participation in carbohydrate, protein and lipid metabolism, synthesis of some biologically active substances, rennin, the active form of vitamin D₃, erythropoietin, prostaglandins, and kinins. These substances influence the regulation of blood pressure, blood clotting, calcium and phosphorus metabolism, the maturation of red blood cells and other processes.

7.1. Excretory function of the kidneys

The kidneys form urine from the components of blood plasma and can effectively regulate its composition. The uropoiesis process consists of three stages.

1. Ultrafiltration. Primary urine is formed during ultrafiltration. Blood, flowing through vessels of kidneys, is filtered in the glomerulus cavity through the pores of the connective tissue capsule – a special filter, which consists of three layers. The first layer is the endothelium of blood capillaries, which has large pores. All components of blood pass through these large pores, except blood cells and high molecular weight proteins. The second layer is the basal membrane, which is constructed of collagen fibers (fibrils) that form a molecular "sieve". Pore diameter is 4 nm. The basal membrane does not mess proteins with a molecular weight higher than 50 kDa. The third layer is epithelial cells of the capsule; their membranes are negatively charged, so it prevents the negatively charged albumin of blood plasma to penetrate into the primary urine. The form of three-layer pores is complicated; it does not match the form of the protein molecules of blood plasma. This mismatch prevents the penetration of the normal protein molecules into the primary urine. If the structure, shape, charge of the protein molecule is changed from a normal protein molecule, this abnormal protein can pass through the filter and into the urine. This is one of the mechanisms of blood plasma purification of defective proteins and its restoration of normal structure.

Ultrafiltrate (primary urine) is normally almost free of proteins and peptides. But on the other hand, the composition of low molecular weight non-protein components, and the content of various ions in the primary urine is the same as in blood plasma. Therefore, the primary urine is sometimes called "protein-free filtrate of blood plasma". Amount of the produced ultrafiltrate depends on the driving force for ultrafiltration – hydrostatic pressure of the blood vessels in the glomerulus (normal is about 70 mm of mercury). The driving force of ultrafiltration is opposed by oncotic pressure of blood plasma proteins (about 25 mm of mercury) and hydrostatic pressure in the cavity of the capsule of ultrafiltrate (about 15 mm of mercury). The driving force of ultrafiltration is about: 70 – (25 15) = 30 (mm of mercury); and it is called the effective filtration pressure. The energy of ATP in the process of ultrafiltration is not used.

Lowering blood pressure and / or increased hydrostatic pressure in the cavity of the capsule can lead to a slowdown, while significant changes and a complete cessation of the formation of primary urine (anuria). Approximately 1500 liters of blood pass through the kidneys with the formation of about 180 liters of primary urine (125 ml/min) per day.

Filtration ability of the kidneys is evaluated by calculating the filtration clearance (coefficient of purification) – for this, substances are injected into the blood that are only filtered but not reabsorbed or secreted (mannitol, creatinine, polysaccharide inulin).

Filtration clearance is the volume of blood plasma, which is completely cleared from the nonreabsorptable substances for 1 min. Filtration clearance (FC) is calculated as follows:

FC = (A-urine  V) / A-blood

where: A-urine is the concentration of substances in the urine, the A-blood – the concentration of substances in the blood; V – the rate of formation of urine in ml/min. In a healthy person FC is about 125 ml/min.

Primary urine, which contains all the low molecular weight components of blood and a small amount of low molecular weight proteins, is subjected to reabsorption in the proximal tubule.

2. Reabsorption is a movement of substances from the lumen of the tubule into the blood. Almost all the proteins got in the ultrafiltrate, and other substances necessary for organism are subjected to reabsorption. Therefore, daily losses of protein and peptide components of urine do not exceed 100-150 mg/day, although to 8-10 grams of protein per day can be filtered to the primary urine. 85% of the ultrafiltrate is reabsorbed in the proximal tubule. About 99% of water is also reabsorbed there; the same situation with the nutrients (glucose, amino acids), many minerals, and partially with the end products of nitrogen metabolism (urea, uric acid).

There are two mechanisms of reabsorption: simple diffusion and active transport. By active transport, Na ions are reabsorbed with sodium pump – the membrane enzyme Na, K-dependent ATPase. Many substances such as glucose and amino acids are reabsorbed in the complex with ions of Na, i.e. energy for the transport of these compounds is released as a result of the ATPase action. Reabsorption of calcium and magnesium occurs similarly. This process involves Ca²⁺, Mg²⁺-dependent ATPase. In addition to ATPase, active transport processes involves specific transport carrier proteins, called translocases. They have the ability to selectively bind to a substance that is reabsorbed, and have a limit of capacity (level of saturation of protein). Limit of kidney working capacity is determined by the limiting concentration of reabsorbed substance from the primary urine. This quantity is called the renal threshold for reabsorption (RTR). RTR is the lowest concentration of reabsorbed substance at which we have the transport maximum of reabsorption (T_max). T_max characterizes the state of the renal tubules and is equal to the rate of transport of substance by the transfer protein in its saturation with the substance being transported. Thus, for glucose, for example, RTR is 12.10 mg/dl. In normal glucose concentration in blood, transport systems are not fully saturated with glucose, so glucose does not appear in the urine, i.e. it is completely reabsorbed.

Isotranslocases are different from each other by Michaelis constant (K_m) like isozymes. For example, early proximal tubule, where the translocases with K_m = 6 mmol/l is situated, the glucose concentration is high in the filtrate. At the end of the proximal part, where most of the glucose is reabsorbed, K_m of translocases is 0.35 mmol/liter. With these translocases having different affinities for glucose, almost all glucose is reabsorbed from the primary urine. 179 liters of water, about 1 kg of NaC1; about 340 g NaHCO₃; about 170 g of glucose, etc. are reabsorbed per day.

3. Secretion. Tubular selective secretion is similar to reabsorption, but goes in the opposite direction: from the blood into the tubules lumen. Secretion occurs mainly in the distal tubule. The process of secretion as well as the process of reabsorption takes place with an expenditure of ATP (active transport) and is characterized by the value of transport maximum (tubular maximum). This value may serve to characterize carrier proteins, providing transport of substances. Frequently the reabsorption and secretion occur simultaneously – for example, the secretion of K⁺ occurs under the influence of Na⁺, K⁺-dependent ATPase. The difference is that K⁺ is secreted, but Na⁺ is reabsorbed Na. H⁺ and NH₄⁺ are also secreted. The secretion rate can be identified by excretion of different dyes with the urine from the body, which can be only secreted by the kidneys secretion. To do this, dyes must be pre-entered into the blood.

As a result, the daily secondary urine volume is between 1,000 and 2,000 ml, in which the following elements are dissolved: from 12 to 6g of urea, about 1g of creatinine, approximately 1g of ammonium salts, approximately 0.5-1.0g of other products of nitrogen exchange (normally the urine may contain creatine, hippuric acid, indican and pigments); about 5-7g of mineral salts, products of detoxification of toxic compounds (in small amounts).

There is the maintenance of water and salt balance and acid-base balance in process of the kidney excretory function.