2019.1 SAQ 6 – renal function tests

Outline the clinical laboratory tests of renal function.  What are the limitations of each test?

BT_PO 1.69 Describe the physiological effects and clinical assessment of renal dysfunction

T/F serum creatinine concentration can be used to infer the GFR *

T/F there is a linear relationship between GFR and serum creatinine concentration **

T/F age has a significant bearing on the interpretation of creatinine ***

T/F creatinine slightly underestimates GFR #

T/F serum urea is a useless test ##

T/F urea and creatinine are the only blood tests that reflect renal function ###

* why is creatinine a suitable thing to measure for this purpose?
** can you draw the curve?
*** how is age factored in to the calculation of GFR via (i) eGFR, and (ii) Cockcroft-Gault? why is this necessary?
# explain your answer
## why / why not?
### how many others can you come up with?

1. Guyton 13th ed, Chapter 28
2. Miller 8th ed, Chapter 52
3. Mouton R, Holder K. Laboratory tests of renal function. Anaes & Int Care Med, 2006; 7: 240-243.

BT_PO 1.69 Describe the physiological effects and clinical assessment of renal dysfunction

Late, AND only 4 today. They range from a simple fact (in Hemmings and Egan), through to more complex statements. Statement 4 is TRUE, more important is that you know why.

Neonates have a higher serum potassium due to poor renal excretion  TRUE/FALSE

Urea concentration is a measure of renal function independent of hepatic function  TRUE/FALSE

Creatinine is a reliable indicator of renal function in an 80 year old TRUE/FALSE

Positive pressure ventilation contributes to intra-operative oliguria  TRUE/FALSE


BT_PO 1.62 Explain the physiology of renal blood flow

T/F  renal blood flow is about 1 L/min, or 20% of cardiac output

T/F  renal blood flow is so high because of the high metabolic demands of the kidneys

T/F  renal blood flow is autoregulated between aterial pressure of approx. 70 – 170 mmHg via alterations in the diameter of the efferent and afferent arterioles

T/F  tubuloglomerular feedback is an additional renal mechanism for autoregulating renal blood flow – it is mediated via adenosine release from the macula densa

T/F  renin ultimately leads to efferent > afferent constriction, which increases RBF

T/F  depending on the dose being given, a noradrenaline infusion may (i) increase RBF by increasing MAP; or (ii) decrease RBF by causing afferent & efferent vasoconstriction

1. Guyton Ch 27
2. Vander

2018.2 SAQ 8 – functions of the kidneys

Briefly outline the functions of the kidneys.

The “renal and fluids/electrolytes” group of LO’s are BT_PO 1.61 to BT_PO 1.82. Quite a few of these are applicable to this SAQ.

The following are random facts about the kidneys – they are not an attempt to suggest a structure or outline for answering the SAQ.


T / F  in the absence of ADH, 20% of the glomerular filtrate would remain in the collecting ducts, producing a urine output of 30 L/day

T / F  erythropoietin is synthesised in the kidneys – this is the only endogenous source in the human body

T / F  some drug metabolism occurs in the kidneys, but this is limited to CYP450 enzymes only

T / F  the kidneys are important for long term regulation of blood pressure, via regulating sodium and water balance

T / F  during a metabolic acidosis, increased hydrogen ion excretion by the kidneys occurs mainly as ammonium

T / F  in response to hypotension, renin is released from the macula densa

T / F  the kidneys are capable of gluconeogenesis

T / F  aldosterone provides the main control over renal potassium excretion

1. Kam & Power 3rd edition, Chapter 7
2. Vander 8th edition (use this book to supplement Kam & Power as needed to enhance understanding)


BT_PO 1.66 Outline the endocrine functions of the kidney

And it will come as a surprise to no-one reading the posts this week to find I’ve been reading Ganong again.

Vitamin D is hydroxylated to calcitriol in the proximal tubules of the kidney     T/F

Calcitriol increases calcium reabsorption in the proximal tubules of the kidney     T/F

The O2 sensor to control erythropoietin production is probably a heme protein     T/F

Increasing catecholamines will stimulate erythropoietin production     T/F

Erythropoietin is also produced in the brain     T/F


2018.1 SAQ 9 Renal blood flow

Discuss the determinants of renal blood flow.

Kidneys often receive a multitude of insults in the peri-operative period, both physiological and pharmacological. If you know how normal renal blood flow is controlled, you will be best positioned to preserve it under anaesthesia.

Principles of physiology for the anaesthetist 3e by Power and Kam Ch 7, handles this topic better than many of the other recommended texts. Although this is an important topic, its coverage can be quite confusing.

BT_PO 1.62 Explain the physiology of renal blood flow

Blood flow to the kidneys is regulated to maintain glomerular filtration rather than oxygen supply to the kidney T/F

Renal blood is auto-regulated between a systolic blood pressure of about 80mmHg and 180mmHg T/F

In response to reduced presentation of sodium to the juxtaglomerular apparatus, afferent arterioles will dilate to increase glomerular filtration T/F

Renal blood flow is auto-regulated at the level of the glomerular capillaries T/F

Myogenic regulation of renal vascular resistance is rapid T/F

Proximal Convoluted Tubule

BT_PO 1.63 Describe glomerular filtration and tubular function

Regarding the proximal convoluted tubule:

Q. It reabsorbs 65% of filtered sodium and water.  TRUE/ FALSE

Q. It reabsorbs 65% filtered chloride.  TRUE/ FALSE

Q. Aquaporin channels are found on the luminal surface of the tubule cells.  TRUE/ FALSE

Q. Most of the glucose is reabsorbed via the paracellular route.  TRUE/ FALSE

Q. This is the principal site of urea secretion.  TRUE/ FALSE



BT_PO 1.71 Explain the effects on anaesthesia on renal function

I didn’t realise that this LO existed – found it when I was looking for a renal topic to post on, as there seem to be a dearth of renal posts on this site.

This is actually an important topic as relatively minor reductions in renal function are associated with worse peri-operative outcome.

I don’t have any kidney photos. Below is the best I could do for the genitourinary system. There are a whole series of these (some much more X rated) on view at MONA


BT_PO 1.71  Explain the effects on anaesthesia on renal function

Any anaesthetic agent which results in a reduction of blood pressure is likely to reduce GFR   TRUE/FALSE

Attenuation of the stress response to surgery is renal protective   TRUE/FALSE

Volatile anaesthetic agents may provide protection against ischaemia- reperfusion injury of the kidney TRUE/FALSE

IPPV improves renal blood flow TRUE/FALSE

Metabolic acidosis increases the kidneys’ vulnerability to nephrotoxins TRUE/FALSE

2017.1 : SAQ 13

Describe how the large daily volume of glomerular filtrate is altered by the kidney to form a relatively low volume of concentrated urine.

BT_PO 1.64    BT_PO 1.67

This question just asks for some basic functions of the kidney

Approximately 80% of the glomerular filtrate is absorbed  TRUE/FALSE

The medulla is hyperosmotic due to salt and urea  TRUE/FALSE

ADH plays an important role  TRUE/FALSE

Urea is actively secreted  TRUE/FALSE

The counter-current mechanism creates a hypo-osmotic medulla  TRUE/FALSE

BT_PO 1.63 Describe glomerular filtration and tubular function

Daily glomerular filtration of potassium is approximately 800 mmol/day. TRUE / FALSE

Reabsorption of potassium in the proximal tubule is fixed at 65% of the amount filtered. TRUE / FALSE

Aldosterone controls potassium excretion by altering the amount reabsorbed in the distal tubule. TRUE / FALSE

An increase in plasma potassium concentration directly stimulates the Na-K pumps in the distal tubule, to increase potassium secretion. TRUE / FALSE

During hypokalaemia, the kidneys can achieve zero potassium excretion in the urine. TRUE / FALSE