REGULATION OF RENAL BLOOD FLOW

Renal blood flow is regulated mainly by autoregulation. Autoregulation is the intrinsic ability of an organ to regu­late its own blood flow. Some vital organs in the body, such as brain, heart, and kidneys, have autoregulation mecha­nisms.

Two mechanisms are involved in renal autoregulation: Myogenic response: In the afferent arteriole, increased

blood pressure would increase the stretch, and the arteriole would respond by contracting. In this way, renal blood flow would be decreased. A reduction in blood pressure would cause less tension, and the blood vessel would dilate, hence increasing renal blood flow. Tubuloglomerular feedback: The macula densa plays an important role in tubuloglo- merular feedback, which controls the renal blood flow and GFR. Macula densa cells sense changes in volume delivery as well as NaCl concentration to the distal tubules.

18.13.1 Urine Formation

Urine formation consists of three processes: glomerular filtration, tubular reabsorption from the renal tubules into the blood, and tubular secretion from the blood into the renal tubules. Filtration is the function of the glomerulus, whereas reabsorption and secretion are the functions of the tubular portion of the nephron.

18.13.2 Glomerular Filtration

Most substances must be cleared from the blood, espe­cially the end products of metabolism such as urea, cre­atinine, uric acid, and urates. These substances are poorly reabsorbed and therefore excreted in large amounts in the urine.

The formation of urine begins when an ultrafiltrate of plasma passes through the fenestrated capillary endo­thelium, the basement membrane, and the epithelium of Bowman’s capsule. The glomerular capillaries are imper­meable to proteins, so the glomerular filtrate is protein-free and devoid of cellular elements, including RBCs. Other constituents of the glomerular filtrate are similar to the con­centrations in the blood plasma

Size of solute is an important parameter for their filtra­tion. Filtration of solutes is inversely proportional to their size. The barrier present in glomerular membrane is selec­tive in determining the molecules to filter, based on their size and electrical charge. Negatively charged large mol­ecules are filtered less easily than positively charged mol­ecules of equal molecular size. Albumin is restricted from filtration, because of its negative charge and the size. In cer­tain kidney diseases, the negative charges on the basement membrane are lost and as a result of this loss, some of the lower-molecular-weight proteins, especially albumin, are filtered and appear in the urine.

18.14