BT_PO 1.70 Explain the renal responses to hypovolaemia

Several SAQs have been asked on this topic which is of obvious relevance to anaesthetists. It requires a sound understanding of renal sympathetic nerve activity, the mechanisms for autoregulation of renal blood flow as well as the role of the renin angiotensin system. I used Stoelting for this post as I find Vander a bit too wordy.

T/F intraoperative urine output correlates well with volume status

T/F noradrenaline preferentially constricts the afferent glomerular arteriole

T/F atrial natriuretic peptide preferentially constricts the afferent glomerular arteriole

T/F angiotensin II causes constriction of both the afferent and efferent  glomerular arterioles

T/F angiotensin II activates the thirst reflex

T/F when stimulated, renal sympathetic nerves decrease blood flow more than GFR

T/F a MAP above 70 indicates that renal perfusion will be adequate

T/F renin is released from the macula densa in response to decreased renal perfusion

T/F an increase in glomerular capillary oncotic pressure will increase net filtration pressure and increase GFR

 

Renal physiology BT PO 1.65 and 1.73

BT PO 1.65 Explain the mechanisms involved in the regulation of renal function

BT PO 1.73 Describe the mechanisms involved in the maintenance of fluid and electrolyte balance

To my mind these two LOs are about the same topic so I have taken the liberty of doing a single post for both of them. There is a lot of overlap between all the LOs that relate to the functions of the kidney.

Renal Physiology

This is a fairly core subject and you should find the correct responses to the statements below in any of the recommended texts. I used Miller predominantly.

T/F tubuloglomerular feedback refers to the feedback mechanism facilitated by the macula densa to autoregulate renal blood flow

T/F glomerulotubular balance accounts for a constant fraction of filtered sodium and water being reabsorbed in the proximal tubule

T/F the above two terms are often confused

T/F glomerulotubular balance bluntens the ability of changes in GFR to markedly alter urine production

T/F renin is the mediator involved in tubuloglomerular feedback

T/F the phenomenon of pressure diuresis results because of glomerulotubular imbalance

T/F the blood flow to the juxtamedullary nephrons is not autoregulated

T/F the urinary flow rate is autoregulated

T/F body sodium content, blood volume and blood pressure are closely interrelated with the kidney having a central role in their control

T/F intrarenal prostaglandins are the only important vasodilating substances that have a role in blood pressure control in the kidney

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?

References
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

References
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

References:
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