A. Functional Anatomy of the Renal Microcirculation

1. Portal system (capillary beds in series), paralleling the nephron

renal ==> afferent ==> glomerular ==> efferent ==> proximal peritubular ==>
arteries    arterioles     capillaries           arterioles      capillaries

vasa ==> distal peritubular ==> collecting duct ==> renal venules
recta        capillaries                    capillaries and veins

B. Values

1. Normal (resting) blood flow: 1.2 L/min (20-25% of cardiac output

Note: the high blood flow is required to furnish the kidney with sufficient plasma for filtration, reabsorption, etc.

2. Range: the highest renal blood flow occurs at rest; progressively increasing activity or stress progressively reduces renal blood flow; under extreme conditions (e.g. shock), renal blood flow can be reduced to almost zero

3. Oxygen requirements

a. the major use of oxygen is to furnish energy for active transport, particularly Na⁺ and co-transported substances

b. resting renal blood flow is more than sufficient to satisfy the kidney's oxygen requirements and therefore RBF could be reduced without sacrificing renal function. (Evidence: low oxygen extraction: C_a - C_v): low: 1-2 ml O₂/dl or 10-20ml O₂ / Liter blood). However, excessive reduction in RBF can lead to permanent damage to the kidney.

Control of Renal Blood Flow

1. Autonomic: major normal influence on renal blood flow

a. sympathetic vasoconstrictors: very effective
b. adrenal medulla: reinforces direct sympathetic constriction

Note 1: the above contribute to the reduction of renal blood flow in stress

Note 2: Although the nerves innervating the kidney aid in regulation of renal blood flow and renal transport, they are not essential for normal renal function (note renal transplant)

2. Pressure: although required, less important in the normal range because of autoregulation

a. autoregulation definition: intrinsic ability to adjust resistance in order to maintain blood flow relatively constant even when perfusion pressure changes (does not require innervation) -- implies vascular resistance must increase when perfusion pressure increases

Note autoregulation of glomerular filtration rate (GFR), also

b. autoregulation effective over limited range; at sufficiently low systemic arterial pressure, renal blood flow is reduced, compromising renal function and even kidney survival

c. mechanisms: myogenic (inherent in vascular smooth muscle) reflex, local vasomotor agents