Henry Family Blog

Functions of the kidneys:

Regulation of blood ionic composition
-Electrolyte balance

Regulation of blood pH
-Acid-base balance

Regulation of blood volume and BP
-Body fluid volume
-Renin pathway

Maintains blood osmolarity (300 mOsm/L)

Hormone production
-Renin, Aldosterone, ADH
-Calcitrol (bone), EPO (RBC)

Regulation of blood glucose
-Glutamine & GNG (in metabolism section)

Excretion of wastes
-metabolites, drugs, excess water and solutes

Nephron:

The nephron has four major processes; filtration, reabsorption, secretion and excretion.

Filtration is movement from water and solutes from the blood plasma across the wall of the glomerular capillaries, into the glomerular capsule and into the renal tubule. Blood flow to the kidneys=1200-1300 ml/min at rest. Plasma=720-780 ml/min (Hct=40%). Maximum filtration=125 ml/min (17% plasma). Net filtration pressure=GBH-CHP-BCOP. Glomular filtration rate (GFR) is the volume of plasma that filtrates and passes through the glomeruli every minute.

Measurement of the GFR. Use a diagnostic tool to evaluate the function of the renal system. Clearance methodology [Clearance “x”=Ux*V/PxUx=urine concentration, V=volume of urine flow, Px=plasma concentration. When clearance=GFR then the substance is freely filtered and neither reabsorbed nor secreted.

Renal Autoregulation of GFR:
Mechanisms that maintain a constant GFR despite changes in arterial BP. The myogenic mechanism increases BP and stretches the afferent arteriole. The smooth muscle contracts and reduces the diameter of the arteriole; returns the GFR to its previous level in seconds. Tubuloglomerular feedback increases BP and raises the GFR so that fluid flows too rapidly through the renal tubule; Na+, Cl- and water are not reabsorbed. Vasoconstrictors are released from juxtaglomerular apparatus. The afferent arterioles constrict and reduce GFR.

Neural Regulation of GFR:
Blood vessels of the kidney are supplied by sympathetic fibers that cause vasoconstriction of afferent arterioles. At rest, renal BV are maximally dilated because sympathetic activity is minimal-renal autoregulation prevails. With moderate sympathetic stimulation, both afferent and efferent arterioles constrict equally- decreasing GFR equally. With extreme sympathetic stimulation (exercise or hemorrhage) vasoconstriction of afferent arterioles reduces GFR-lowers urine output and permits blood flow to other tissues.

Hormonal Regulation of GFR:
Atrial natriuretic peptide (ANP) increases GFR. It increases in blood volume which causes the atria to stretch and ANP is released in response. It relaxes the glomerular cells increasing capillary surface area and increasing GFR. Angiotensin II reduces GFR. It is a potent vasoconstrictor that narrows both afferent and efferent arterioles reducing GFR.

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