Chapter 2 Part 2

The exciting conclusion to

Chapter Two: Renal Circulation and Glomerular Filtration Rate

- Determinants of GFR

    - First step in making urine is separation of an ultrafiltrate

    - Governed by starling forces

        - Balance of hydraulic and osmotic forces

        - GFR = LpS (P gc – P us - Osmotic Pressure Cap p)

            - Normal GFR 95 in women, 120 in men

            - Cap Hydrolic pressure remains constant 

            - glom cap Oncotic progressively rises 

                - Due to filtration of protein free fluid (protein concentration rises in the capillary)

            - Filtration gradient begins at 13 mmHg and falls to zero after filtration of 20% or RPF!

            - GFR is capped at 20% of RPF called filtration equilibrium

            - So GFR is dependent on RPF, unless you can change glomerular hydraulic pressure

        - Glomerular hydraulic pressure is controlled by balance of twin arteriole (afferent and efferent)

            - Constriction of afferent arteriole reduces RPF, GFR, and glom pressure

            - Dilation of afferent arteriole increases RPF, GFR, and glom pressure

            - Constriction of the efferent arteriole increases Glom pressure, increasing GFR

        - Besides glom hydrostatic pressure the other starlings forces are rarely relevant to changes in GFR

Letty says: referred to this NEJM review article later JC thought she was referring to something else -see #2- and then Roger referred to this again)

Normotensive Acute Renal Failure from Gary Abuelo in NEJM 2007. https://www.nejm.org/doi/10.1056/NEJMra064398 (note in this article, Dr. Abuelo acknowledges the newer terminology of the time, AKI rather than ARF but chooses not to embrace it). In figure 2, he highlights the classic examples of how autoregulation can be affected. In the table, additional examples are provided but all within the framework of alterations related to autoregulation and the interplay between the two resistance vessels.

- Regulation of GFR

    - Autoregulation

        - The ability to keep glomerular pressure constant over wide range of systemic arterial pressure

        - When pressure < 70 autoregulation fails and GFR will fall with decreases in systemic pressure

        - When pressure falls below 40-50 GFR ceases

        - At least some of this autoregulation is mediated with Ang2. Giving ACEi markedly disrupts autoregulation

        - Nitric oxide, not important

    - TGF

        - Chloride in macula densa

            - Blocked by furosemide