S1) Introduction to the Urinary System Flashcards
What is the mass of the kidneys?
Each weighs approx. 150g
Describe the location of the kidneys
Retroperitoneal:
- Left: T11 - L2
- Right: T12 - L3
Describe the relation of the ureter to the kidney
- Each kidney has single ureter
- Ureter is smooth muscle tube running from the renal pelvis to bladder
Identify and describe the 4 functions of the kidney
- Regulation: control the [substances] in ECF
- Excretion: waste products
- Endocrine: renin, erythropoietin, prostaglandins
- Metabolism: active form of Vitamin D, catabolism of insulin, PTH, calcitonin
Movement of water is driven by osmotic forces.
Define osmolarity and osmolality
- Osmolality: number of osmoles of solute per kilogram
- Osmolarity: number of osmoles of solute per litre
Define osmole
Osmole is a measure a solution’s ability to create osmotic pressure & thus affect movement of water
E.g. one mole of CaCl2 forms a 3 osmolar solution in 1L water
Relate osmolarity to osmosis in cells
Water moves from low osmolarity solutions to high:
- If ECF Osmolarity is high – water moves out of cells
- If ECF Osmolarity is low – water moves into cells
Intra- and extra cellular fluids have different electrolyte compositions. Differences are maintained by active transport.
Demonstrate this
- Intracellular: high K+, low Na+, many large organic anions
- Extracellular: low K+, high Na+, mainly Cl- and HCO3-
- if you have a crash injury, lots of K will be released and damage the heart
Kidneys affect extracellular fluid concentrations directly.
What happens when this system fails?
- Changes in BP
- Changes in tissue fluid and cell function
- Failure to control ECF osmolarity: cells shrink/swell
What is the functional unit of the kidney?
Nephron is a filter unit connected to a long tube for reabsorption
Describe the composition of a nephron
- A glomerulus: tuft of capillaries between afferent and efferent arteriole
- A tubule: Bowman’s capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting duct
Which parts of the nephron are found in the cortex of the kidney?
- Proximal convoluted tubule
- Distal convoluted tubule
Which parts of the nephron are found in the medulla of the kidney?
- Loop of Henle: dips into and out of medulla
- Collecting duct: passes through medulla to pelvis
What is epithelium?
Epithelia are continuous sheet of cells covering exposed surfaces and lining internal cavities e.g. digestive, reproductive, urinary and respiratory
What is the role of epithelia in the kidney?
- Excrete waste products from the body
- Reabsorb needed materials from the filtrate
The kidney is one of the most active organs in the body.
Briefly, describe its metabolic demands.
- Needs blood flow of 4 ml/g/min
- 22% of cardiac output at rest
Describe the structure and function of the glomerulus
- Structure: highly specialised filter
- Function: water, electrolytes and small molecules are forced through it by constant filtration pressure in capillaries
The proximal convoluted tubule is a major site for reabsorption.
Describe the processes occurring
- PCT absorbs 100% of glucose and amino acids
- Filtrate remains isotonic with plasma (water follows)
- Reabsorbed materials leave by peritubular capillaries
Describe the cellular mechanisms of reabsorption in the kidney
- Tubules are lined with polarised epithelial cells
- Different membrane properties on luminal and basolateral membranes
Outline the role of sodium pumps in the kidney
- Extrude Na+ across basolateral membrane
- Na+ enters across luminal membrane down concentration gradient
- Energy from sodium movement drives reabsorption of other substances e.g. glucose and H2O follows osmotically
The Loop of Henle is a further site of reabsorption of salts in the kidney.
Describe its general functions
- Create a gradient of increasing osmolarity in the medulla by counter-current multiplication
- Forms concentrated urine if water has to be conserved
Which processes occur in the distal convoluted tubule?
- Variable reabsorption of electrolytes and H2O
- Removes even more sodium and chloride and actively secretes H+ (fluid leaving loop of Henle is hypotonic)
What happens to water in the distal convoluted tubule?
Water may or may not follow reabsorption of electrolytes:
- Water is reabsorbed = concentrated urine (natriuresis)
- Water is not reabsorbed = dilute urine (diuresis)
Explain the role of the collecting duct in types of urine produced
The collecting duct passes through the high osmolarity environment of the medulla (created by the loop of Henle):
- If water can cross the epithelium = concentrated urine
- If water cannot cross = urine remains dilute
Variable reabsorption occurs in the distal nephron.
Explain this in terms of sodium recovery
Sodium recovery controlled by RAAS which controls ECF volume
stimulated by sympathetic system
Variable reabsorption occurs in the distal nephron.
Explain this in terms of water recovery
Water recovery controlled by ADH which controls:
- Permeability of DCT and collecting duct to water
- ECF osmolarity
percentage of molecules recovered
<99% filtered sodium and chloride
<99% water (it follows sodium)
100% bicarbonate
100% glucose (so should be none in urine)
what is renal blood flow
- how much blood flows into the kidney (1L/min)
what is renal plasma flow
- volume of plasma that flows into the kidney (600ml/min)
- RPF = RBF x (1-hamatocrit levels)
- hematocrits levels → % of RBC that fill blood (normally 40-45%)
Glomerular filtration rate
GFR = 125mil/min
amount of filtrate produced from blood flow
normally 80% of substances come back in circulation
GFR/RPF
20%
What does the glomerular filtrate contain
- no blood or plasma
- no proteins
- composed of organic solutes
in histology how can you determine the difference between proximal and distal convoluted
- proximal has a cloudy lumen and contents
- distal is empty - distal is also bunched up near the Bowmans capsule
what is the role of podocytes
- small gaps within the membrane to allow substances to pass through
- negative charge to repel negative
label the difference layers in the glomerular capillary membrane
some reasons why GFR may drop?
- decline in number of nephrons
- decline in GRF within nephrons
kidneys secrete renin, what does renin do?
- tunica media in the thick part of the afferent arteriole wall secrets renin that changes in response to tubular fluid
- acts on angiotensin which results in vasoconstriction
- sympathetic response and results in a fall in plasma volume
Kidneys secrete erythropoietin, what is the relevance of this?
produces red blood cells
describe the reabsorption of glucose in the PCT
- normal conc → 2.5-5.5 mmol/L
- most reabsorption occurs In the PCT
- transported via active transport, uses the energy from Na/K pump
- travels via Na/Glucose carriers
how is glycosuria causes
- load of glucose is at excess and the transporters become saturated
- Tm reduced (max reabsorbed)
where are amino acids mainly reabsorbed?
- Normal conc is 2.5-7.5 mmol/L
- PCT
- reabsorbed via secondary active transport
