Functional Anatomy and Control of Renal Blood Flow

Describe the functional anatomy of the kidneys and renal blood flow.

Functional Anatomy

The functional unit of the kidney is the nephron. Nephrons:

• Are composed of the glomerulus, proximal tubule, loop of Henle, distal tubule, and collecting duct
• Are divided by their location into:
  • Superficial cortical nephrons
    Have short loops of Henle.
  • Juxtamedullary nephrons
    Have long loops of Henle, and the efferent arteriole forms the vasa recta for the kidney.
  • Mid-cortical nephrons
    May have either long or short loops.

Control of Renal Blood flow

The kidneys:

• Receive 22% of cardiac output at rest
• Extract only 10% of delivered O₂
• Have a high renal blood flow exceeds that required for metabolism
  High flow is instead needed to produce the large volume of glomerular filtrate (125ml.min^-1) required for excretion of waste.

Autoregulation

Renal blood flow is autoregulated over a wide range of mean arterial pressures (60-160mmHg) via:

• Myogenic autoregulation
• Tubuloglomerular feedback

Myogenic autoregulation:

• Describes the intrinsic constriction of the afferent arteriole in response to an increased transmural pressure
• This increases vascular resistance in proportion to the increase in pressure, keeping flow constant

Tubuloglomerular feedback is more complicated, and describes the constriction or dilation of the afferent arteriole in response to adenosine or NO (respectively) release from the macula densa:

• The macula densa lies in the wall of the ascending limb of the loop of Henle
• It detects change in tubular flow rate (probably via changing Na⁺ flux across its membrane)
  • Increased flow in the loop indicates an increased perfusion pressure, prompting release of adenosine and constriction of the afferent arteriole
  • Decreased flow indicates a decreased perfusion pressure, reducing adenosine release and prompting the release of NO and renin, which causes the afferent arteriole to dilate

Notably, flow to juxtamedullary nephrons is not autoregulated. High blood pressure increases juxtamedullary flow, increasing GFR and impairing renal concentration, resulting in a pressure diuresis.

Neuronal Control

The kidneys are innervated by noradrenergic sympathetic nerves, which causes:

• Afferent and efferent arteriolar constriction
  This increases capillary hydrostatic pressure (increasing filtration) and also increases capillary oncotic pressure (decreasing filtration).
  • This leads to an overall slight reduction in GFR

Hormonal Control

Renin:

• Is released from the juxtaglomerular apparatus by β₁ stimulation
• Catalyses the production of angiotensin I from circulating angiotensinogen
  Angiotensin I is then converted into Angiotensin II by circulating ACE.
  • The actions of the RAAS are described in more detail in the endocrine functions of the kidney.

References

1. Barrett KE, Barman SM, Boitano S, Brooks HL. Ganong's Review of Medical Physiology. 24th Ed. McGraw Hill. 2012.
2. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
3. Hall, JE, and Guyton AC. Guyton and Hall Textbook of Medical Physiology. 11th Edition. Philadelphia, PA: Saunders Elsevier. 2011.