When the direct effect of sympathetic stimulation is experimentally removed, the effect of systemic blood pressure on GFR can be observed. Under these conditions, surprisingly, the GFR remains relatively constant despite changes in mean arterial pressure within a range of 70 to 180 mmHg (normal mean arterial pressure is 100 mmHg). The ability of the kidneys to maintain a relatively constant GFR in the face of fluctuating blood pressures is called renal autoregulation.
Renal autoregulation is achieved through the effects of locally produced chemicals on the afferent arterioles (effects on the efferent arterioles are believed to be of secondary importance). When systemic arterial pressure falls toward a mean of 70 mmHg, the afferent arterioles dilate, and when the pressure rises, the afferent arterioles constrict. Blood flow to the glomeruli and GFR can thus remain relatively constant within the autoregulatory range of blood pressure values. The effects of different regulatory mechanisms on the GFR are summarized in table 17.1.
Autoregulation is also achieved through a negative feedback relationship between the afferent arterioles and the volume of fluid in the filtrate. An increased flow of filtrate is sensed by a special group of cells called the macula densa in the thick portion of the ascending limb (see fig. 17.25). When the macula densa senses an increased flow of filtrate, it signals the afferent arterioles to constrict. This lowers the GFR, thereby decreasing the formation of filtrate in a process called tubuloglomerular feedback.
1. Describe the structures that plasma fluid must pass through before entering the glomerular capsule. Explain how proteins are excluded from the filtrate.
2. Describe the forces that affect the formation of glomerular ultrafiltrate.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.