Renin Release and Kidney Function

Renin Release

• Primary stimuli for renin release include reduction of renal perfusion pressure and hyponatremia
• Renin release is also influenced by angiotensin II and ADH

Aldosterone Release

• Renin release stimulates aldosterone release
• Aldosterone's effect is predominantly on the distal tubular network, increasing sodium reabsorption in exchange for potassium

Vitamin D Production

• Kidneys are primarily responsible for producing vitamin D3 from dihydroxycholecalciferol

Urine Formation

• Glomerular filtration is the first step in urine production
• Filtration occurs in the renal corpuscle of the kidneys across the endothelial capsular membrane (Bowman's capsule)
• The resulting fluid is called the filtrate
• Filtration is a passive process
• Glomerular filtration rate (GFR) is about 125 ml/min in a normal adult, resulting in approximately 180 liters/day
• Filtrate has the same composition as blood plasma, without plasma proteins due to the endothelium and basement membrane's permeability

Plasma Urea

• Plasma urea concentration can increase due to various renal diseases with glomerular, tubular, interstitial, or vascular damage
• Measurement of plasma creatinine provides a more accurate assessment of kidney function than urea
• Non-renal factors can affect urea level, such as mild dehydration, high protein diet, and treatment with cortisol or its synthetic analogous
• Prerenal azotemia is caused by decreased perfusion of the kidneys, resulting in increased blood urea without a parallel increase in plasma creatinine
• Postrenal azotemia is caused by conditions that obstruct urinary outflow through the ureters, bladder, or urethra, resulting in increased plasma urea and creatinine

Urea Nitrogen/Creatinine Ratio

• The urea nitrogen/creatinine ratio is used to identify azotemia as prerenal or postrenal
• A normal person on a normal diet has a reference interval for the ratio between 12 and 20
• Factors affecting the ratio of plasma urea to creatinine include:
  • Urea tubular reabsorption increases at low rates of urine flow
  • Osmolality measurements in plasma and urine

Urinalysis

• Urinalysis is important in screening for disease and is a routine test for every patient
• It comprises a range of analyses usually performed at the point of care
• Examination of a patient's urine should not be restricted to biochemical tests
• Urinalysis using disposable strips involves biochemical testing of urine components, such as protein, glucose, and blood
• Fresh urine sample is essential for valid results
• Appearance of urine can indicate various conditions, such as:
  • Blood
  • Colour (haemoglobin, myoglobin)
  • Turbidity (infection, nephrotic syndrome)
• Specific gravity of urine is a semi-quantitative measure of concentration

Proteinuria

• Injury to glomerular integrity results in the filtration of large molecules normally retained, marked as proteinuria
• Proteinuria may be due to:
  • Abnormality of the glomerular basement membrane
  • Decreased tubular reabsorption of normal amounts of filtered proteins
  • Increased plasma concentrations of free filtered proteins
  • Decreased reabsorption and entry of protein into the tubules consequent to tubular epithelial cell damage
• Measurement of individual proteins, such as β2-microglobulin, can be used in the early diagnosis of tubular integrity
• Severe glomerular damage can result in red blood cells being detectable in the urine (haematuria)
• Haematuria can occur as a result of lesions anywhere in the urinary tract
• Quantitative urine protein measurements should always be made on complete 24-hour urine collections
• Albumin excretion in the range 25-300 mg/24 hours is termed microalbuminuria

This quiz covers the primary stimuli for renin release, its influence on aldosterone release, and the role of the kidneys in producing vitamin D3. Understand the effects of renin and aldosterone on the distal tubular network and sodium reabsorption.