59 - Clearance tests. Urine volume and composition. Micturition. Endocrine and metabolic functions of the kidneys. Regulation of renal functions.

sections

clearance and clearance test

urine volume and composition

micturition

endocrine function of kidneys

regulation of renal functions

clearance and clearance test

• clearance - volume of plasma that is completely cleared from that particular substance per unit of time by the kidneys. the rate at which the kidneys remove a substance from the blood.

• Clearance tests are used to see how well the kidneys are functioning.

• The substance used in a clearance test must be: possible to filtrate freely, the filtrated amount in the tubuli should not be changed by secretion or reabsorption, mustn’t change the kidney function or used in their metabolism.

• Creatinine clearance is calculated by the creatinine concentration in the plasma, concentration of creatine in the collected blood sample and urine flow rate.

  • clearance rate is: C = (U x V)/P where C is clearance rate, U is the urine concentration of the substance and V is urine flow rate and P is the plasma concentration of the substance.

urine volume and composition

• Volume: 0.6-2.6 litres, average: 1.4l

• Composition:

  • water - inorganic salts and organic compounds, hormones, and different metabolites varying by what is consumed:

  • urea

  • carbon, nitrogen, oxygen and hydrogen.

micturition

Micturation = Urination

• expelling urine from the urinary bladder

• usually under voluntary control inless person is incontinent

• average capacity of the bladder is 600-800 ml

• micturition reflex center in spinal cord

• when urinary bladder wall is stretched, stretch receptors →produce action potential - parasympathetic nerve fibres will stimulate internal urethral sphincter, contraction of the urinary bladder and relaxation of the internal sphincter

• by conscious activity of the brain motor nerve fibres of the external urethral sphincter, when the external sphincter relaxes urination occur

endocrine function of kidneys

1. Erythropoietin: , stimulates RBC production. -stages of kidney failure anaemia

2. calcitriol: increase gut absobtion of calcium and phosphate

3. Renin: synthesized in response to insufficient blood supply to the kidneys, it is part of the renin-angiotensin-aldosterone system.

   • renin catalyzes the conversion of angiotensinogen into angiotensin I

   • renin activates aldosterone to cause vasoconstriction Metabolic function of kidneys

   • perform gluconeogenesis in starvation

regulation of renal functions

• an increase of blood pressure will lead to vasoconstriction of vas

• decrease of blood pressure leads to vasodilation of vas afferens

• the hydrostatic pressure in the kidney is kept at a constant value of 50mmHg and ensures a constant filtration rate despite blood pressure changes