The nephron in Nephrology

The nephron is the functional unit of the kidney; each kidney contains about 400 to 800,000. Each nephron has a glomerulus and a tubule that follows it. The tubule is composed of different specialized segments, which modify the composition of the glomerular ultrafiltrate (by the phenomenon of secretion and reabsorption between the tubular fluid and the capillaries), resulting in the final urine. The control of these exchanges is ensured by hormones and mediators of systemic or local origin. Through its exocrine and endocrine functions, the kidney plays an essential role in the homeostasis of the internal environment.

Glomerulus and glomerular filtration

The first step in the development of urine is the formation of glomerular ultrafiltrate (or primitive urine) by the passage of water and plasma constituents through the glomerular filtration barrier, separating the plasma in the capillary glomerular of the urinary chamber, by mixed phenomena of convection and diffusion. Doctor Muhammad Khan provides the best nephrology doctors in Riverside.  The glomerular filtration barrier consists of 3 juxtaposed layers, which are, going from the vascular lumen to the urinary chamber:

Ø  the endothelial cell which has the particularity of being fenestrated;

Ø  the glomerular basement membrane is made up of amorphous type 4 collagen, proteoglycan, laminin, podocalyxin, and small amounts of type 3 and type 5 collagen, fibronectin, and entactin;

Cytoplasmic processes (pedicels) podocytes, cells of epithelial origin based on the glomerular basement membrane, the space formed between the pedicels defining the filtration gap.

Constitution of the primary urine

Renal blood flow represents 20 to 25% of cardiac output and is transmitted almost entirely to the glomeruli. This corresponds to approximately 1 L / min or a renal plasma flow (DPR) distributed over the two kidneys for an average hematocrit of 40%, of approximately 600 ml/min. A phenomenon forms the glomerular ultrafiltrate (primary urine). Mixed plasma convection (major mechanism for electrolytes and dissolved substances of low molecular weight) and diffusion (minor mechanism overall but exclusive for molecules of an intermediate size such as low molecular weight proteins). Renal plasma flow rate (DPR), which is filtered (filtration fraction = DFG / DPR), is of the order of 20%. The Glomerular Filtration Rate is therefore approximately 20% x 600 ml / min = 120 ml / min or 180 L / d.

The filtration of dissolved substances depends on the diffusion, size, and charge (a molecule diffusing all the better, the more it is positively charged and the smaller it is), and for the convection of the pressure gradients in the presence. The passage of protein in the urine is negligible above a weight of 68,000 Dalton (= MW of albumin).

 Glomerular filtration (FG)

The two physical determinants of glomerular filtration are the permeability of the glomerular barrier and the driving force of pressure on either side of the wall, according to the relation (Starling's law). Renal autoregulation maintains constant renal blood flow and glomerular filtration during changes in mean arterial pressure between 70 and 140 mmHg. Autoregulation responds to two mechanisms, myogenic tone (the physical phenomenon of afferent arteriolar contraction in response to increased pressure) and tubuloglomerular feedback (biological phenomenon leading to contraction of the afferent arteriole when the flow of Na in the distal tubule increases, which occurs in the event of an increase in pressure in the renal vascular tree).

Ø  In total, the factors modulating glomerular filtration are:

Ø  the hydrostatic pressure and oncotic in the glomerular capillary;

Ø  the hydrostatic pressure in the urinary chamber (increased if an obstacle on the urinary tract);

Ø  the plasma flow glomerular;

Ø  the permeability and glomerular area (which can vary under the influence of angiotensin II, for example);

The tone of afferent and efferent arterioles.

Thanks to self-regulatory mechanisms, renal blood flow and glomerular filtration remain practically constant for an extensive range of systolic arterial pressures (from 80 to 200 mmHg).

In contrast, when systolic blood pressure is below 80 mmHg, a decrease in renal blood flow and glomerular filtration occurs.

Each day, the glomeruli produce about 180 liters of ultrafiltrate (Glomerular Filtration Flow) for a urine flow of about 1 to 2 liters/day. Doctor Muhammad Khan provides the best nephrology physicians in Riverside.  The difference is reabsorbed by the tubule during the transit of the primary urine from the urinary chamber to the excretory system.

Organization of the renal tubule

The formation of urine results from the succession of phenomena of exchanges of solvent or solutes between the tubular fluid and the peritubular capillary through specialized epithelia. Discussions occur through the Trans- and paracellular pathways and are ensured by specific transport systems, operating thanks to chemical or electrical gradients generated by the activity of NaK-ATPase, or directly by the hydrolysis of the ATP. Throughout the nephron, the central part of the kidney's oxygen consumption is dedicated to the reabsorption of sodium which serves as a "driving force" for the reabsorption or secretion of other electrolytes or substances (amino acids, glucose, etc.).

Conditions of equilibrium

The glomerular filtration of each solute (flow rate of filtered substance) is not directly regulated since it is equal to the product of the plasma concentration of the substance by the GFR.

The adjustment of renal outputs to the digestive inputs of each solute (condition of homeostasis) is made thanks to the tubular phenomena of secretion and reabsorption under specific hormonal control (aldosterone for Na, ADH for water, for example ...) finely.