Outline the functional anatomy of the kidneys (40% of marks). Outline the regulation
of renal blood flow (60% of marks).
Candidates who scored well weighted their answers according to the marks allocation outlined in the question and adopted a good structure. A number of candidates confused the roles of tubuloglomerular feedback and the renin angiotensin aldosterone pathway.
Functional anatomy of the kidneys in under 4 minutes? Sure:
- Paired abdominal (retroperitoneal) organ
- Blood supply: single renal artery
- Venous drainage: single renal vein
Structural organisation, from the outside in:
- Cortex: cortical labyrinth and medullary rays
- Outer medulla: inner stripe and outer stripe
- Inner medulla: forms the tip (papilla) of the renal pyramids
- Minor calyces (tips of renal papillae) joint to form major calyces
- Major calyces join to form the renal pelvis.
Structure of the nephron
- The renal corpuscle: glomerulus and Bowman's capsule
- The juxtaglomerular apparatus, containing the macula densa
- The proximal tubule (convoluted and straight segments)
- The distal tubule (Loop of Henle: descending thin limb, ascending thin limb and ascending thick limb), and distal convoluted tubule
- The collecting duct system
Regulation of renal blood flow
- Renal blood flow remains constant over a MAP range of 75-160 mmHg
- This regulation is produced by:
- Myogenic response (50% of the total autoregulatory response)
- Tubuloglomerular feedback (35%)
- Other mechanisms involving angiotensin-II and NO (<15%)
- Intrinsic myogenic mechanisms:
- Vasoconstriction in response to wall stretch
- This is a stereotyped vascular smooth muscle response, not unique to the kidney
- Tubuloglomerular feedback
- This is a negative feedback loop which decreases renal blood in response to increased sodium delivery to the tubule
- The mechanism is mediated by ATP and adenosine secreted by macula densa cells, which cause afferent arterolar vasoconstriction
- Sympathetic regulation of renal blood flow
- Sympathetic tone regulates the range of renal blood flow autoregulation
- Autoregulation typically maintains stable renal blood flow over a wide range of systemic sympathetic conditions
- Massive sympathetic stimulus (eg. shock) overrides autoregulation and markedly decreases renal blood flow
- Glomerula filtration rate is less affected (out of porportion to blood flow) because the efferent arterioles vasoconstrict more than the afferent in response to a sympathetic stimulus.
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