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Describe the balance of Starling's forces across the glomerular capillaries that drives the movement of plasma and the pathway that plasma takes as it becomes filtrate.Answer: Blood enters the glomerular capillaries through the afferent arteriole. Due to the resistance of the efferent arterioles, the pressure within the glomerular capillaries is relatively high (60 mm Hg). That glomerular capillary hydrostatic pressure is working against Bowman's capsule hydrostatic pressure. Hydrostatic pressure within Bowman's space is also much higher than other vascular beds, approaching 15 mm Hg, which results from the small, rigid, enclosed structure of Bowman's capsule. This higher pressure creates a net hydrostatic pressure that movesfluid out of the glomerular capillaries. At the same time, oncotic pressure on either side of the capillary endothelial cell will affect fluid movement across the capillary endothelial cells. Oncotic pressure is created by differences in osmolarity across a membrane that are, for the most part, dependent upon protein concentrations. Since proteins do not leave the glomerular capillaries, oncotic pressure is higher in the glomerular capillaries (29 mm Hg) than in Bowman'scapsule (near zero). The net effect of oncotic pressure alone is to pull fluid back into the glomerular capillaries. However, the net filtration pressure (glomerular filtration pressure) substantially favors the movement of protein-free plasma into Bowman's space. The pathway that the fluid must follow to enter the Bowman's space restricts the movement of protein out of the capillaries (as well as preventing cells from exiting). That fluid is being pushed out of the fenestrated glomerular capillaries and must only cross a thin filtration barrier before entering the Bowman's space. This filtration barrier includes the fenestrated capillary endothelial cells, a basement membrane that restricts the movement of proteins out of the capillaries, and the slit pores through the podocytes. A slit diaphragm controls the size of the slit pores, which limits the surface area of the glomerular capillaries over which filtration can occur.Diff: 7 Page Ref: 51527
3) In spite of the large volume of fluid that enters the kidneys every day, glomerular filtration rate (GFR) is highly regulated to minimize changes that might alter urine output. Describe the intrinsic mechanisms involved in the maintenance of GFR in the presence of a stimulus that would elevate GFR.
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Summer, Official, Kidney anatomy, Page Ref, Nephron, Glomerulus, weekly quizzes, Afferent arterioles, PCB 2099, Maureen Walter