3 Renal physiology ,
The kidney has multiple functions, including the following:
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Excretory function—elimination of small molecular “wastes” from the body
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Maintaining electrolyte balance
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Maintaining acid-base balance
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Maintaining appropriate body volumes and tonicity (osmolarity)
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Endocrine function—production of erythropoietin, renin, 1,25-dihydroxyvitamin D (calcitriol), and prostaglandins (not discussed here)
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Gluconeogenesis (not discussed here)
3.1 Anatomy of the kidney
3.2 Anatomy of a nephron
Each kidney has about one million nephrons with a structure depicted in simplified fashion below. The nephrons are in close contact with the renal vasculature as the efferent arteriolar outflow of blood from the glomeruli perfuses the tubules in the kidney cortex. These arterioles form the vasa recta, capillary loops which extend into the renal medulla adjacent to the Loop of Henle and are important to maintain sodium reabsorption in the thick ascending limb necessary to both dilute the urine and maintain osmotic concentrations in the renal papillae needed to concentrate the urine. ,
3.3 Physiology of glomerular filtration
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The renal blood flow is approximately 20% of the cardiac output at rest (1–1.2 L/min).
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The glomerular filtration rate (GFR) normally is approximately 120 mL/min or about 170–180 L/day.
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The filtration fraction, GFR/RPF (renal plasma flow), is approximately 20%, or about 10% of renal blood flow is filtered.
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GFR is determined by the pressure gradient across the glomerular capillary wall and glomerular basement membrane (GBM), the permeability of the GBM (filtration coefficient), and the filtration area. ,
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The pressure gradient driving glomerular filtration is the sum of the net hydraulic pressure (glomerular capillary hydrostatic pressure of ∼55–60 mmHg less capsular hydrostatic pressure of ∼15 mmHg or a net of ∼40–45 mmHg) favoring filtration counteracted by the blood colloid osmotic pressure (∼30 mmHg). As blood traverses the glomerular capillaries, filtration pressure equilibrium is reached as the fluid loss from the capillary lowers the hydraulic pressure and increases the colloid osmotic pressure, as shown below. ,
The rate of glomerular filtration can be regulated by changing intraglomerular pressure, which is primarily regulated by changing the tonus of the afferent and efferent arterioles (i.e., in the afferent arteriole, constriction reduces the flow and intraglomerular pressure, and dilation increases the flow and intraglomerular pressure, whereas constriction of the efferent arteriole will increase intraglomerular pressure to maintain GFR even when blood flow is reduced).
3.4 Autoregulation of GFR
Autoregulation of GFR is a mechanism that maintains relatively constant renal blood flow and GFR despite changes in systemic arterial pressure. This mechanism fails at very low (MAP <50 mmHg) and very high (MAP >150 mmHg) arterial pressures, in which case renal blood flow, and to a lesser extent, GFR, decreases or increases, respectively. These autoregulatory factors, some of which are noted above, tend to maintain GFR at normal or near normal levels over a wide range of arterial blood pressures. , ,
3.5 Tubular function of the nephron, at a glance
The glomerular filtrate of approximately 170 to 180 L/day allows the excretion of large quantities of small molecular waste products that are freely filtered and not reabsorbed by the tubules. The glomerular filtrate enters the renal tubules, and since the final urinary volume is about 1 to 1.5 L/day, about 99% of the fluid volume and the requisite amounts of electrolytes, glucose, amino acids, and proteins must be reabsorbed by the tubules to maintain body balance. , This process depends upon the active renal tubular cell transport of sodium, an energy-requiring process that creates the osmotic and electrostatic forces which drive the reabsorptive transport of electrolytes and water and the secretion of other molecules, such as hydrogen ions, potassium, and uric acid. A general depiction of this function is shown below.
3.6 Renal handling of sodium
Under common conditions, over 99% of the filtered sodium is reabsorbed overall, 65% in the proximal tubule with bicarbonate and chloride as “accompanying” anions, and to a much lesser degree, sodium is co-transported with glucose, phosphate, and amino acids. , , , In the collecting ducts, sodium reabsorption is accomplished by exchange for secretion of hydrogen and potassium cations. , , , The reabsorption of sodium and water to maintain homeostasis of body volumes is dependent upon the GFR, as well as glomerulotubular balance to increase or decrease sodium reabsorption in parallel with the GFR. A number of regulatory hormones impart changes in the GFR and glomerulotubular balance to effect sodium and water handling, thereby maintaining volume and osmotic balance.
