An Introduction to the Kidneys and Body Fluids Flashcards
What does osmolarity/osmolality refer to?
Refer to the same thing - the total concentration of osmotically active particles in a solution
How can osmolarity be measured/calculated?
Osmolarity can be measured directly in a lab - has the units mOsmol/Kg pure water
Osmolarity can be calculated from a blood test by adding the [ ] of main solutes - has the units mOsmol/L of solution
Why do osmolarity/osmolality values differ slightly?
Values expressed as osmolality or osmolarity will slightly differ as one L of solution has slightly less water than one L of pure water
Give an example to show that in electrolytes, anions and cations contribute to molarity independently
E.g. 100 mmol/L NaCl = 100 mmol/L
Na+ + 100 mmol/L Cl- = 200 mmol/L
Give an example to show that non electrolyte solutes contribute to osmolarity the same as molar concentration
E.g. 5 mmol/L of glucose gives 5 mOsmol/L
Give an example of how the principle of electroneutrality allows us to not need to count anions when we have cation values
E.g. 140 Na+ = 140 of an anion (A-) = 280 for anion and cation
What is the body composition of water in males and females?
Body composition is approximately 60% water in males and 50% in females
How is body water distributed in the main compartments?
This water is distributed in two main fluid compartments - ICF and ECF - 2/3rds of body water is intracellular and 1/3rd is extracellular
Explain why ICF and ECF must be in osmotic equilibrium
- The cell membrane is semipermeable
- It is permeable to water via aquaporins but it impermeable to most solutes
- Change in solute concentration in either ICF or ECF will create an osmotic gradient which shifts water between compartments
- The ICF has K+ with associated anions as its main electrolyte while ECF has Na+ with associated anions as its main electrolyte
Why must large shifts in water between ICF and ECF be avoided?
To prevent changes in cell volume and the most serious complications can be neurological
What is osmoregulation
A physiological process that maintains constant ECF osmolarity
Why is regulation of the ECF volume important?
To ensure effective circulating volume (adequate blood volume and pressure for perfusion of tissues)
What is the ECF compartment subdivided into?
- Interstitial (extravascular) compartment (75% of ECF)
- Plasma/vascular compartment (25% of ECF)
What is volume regulation and how is it maintained?
Volume regulation is control of the ECF volume to ensure appropriate plasma volume (maintained by balance of starling forces between extravascular and vascular)
Summarise by defining osmoregulation and volume regulation
- Osmoregulation is control of salt concentration by adjusting the amount of pure water in the body
- Volume regulation - control of the amount of salt and water in the ECF and hence ECF volume
- Many organs and systems involved but kidney is central to both processes
State the central and byproduct functions of the kidneys
- Homeostasis is the central function of the kidney
- The production of urine is a by product of this central function
State the renal functions
- Osmoregulation
- Volume regulation
- Acid base balance
- Regulation of electrolyte balance e.g. potassium, calcium, phosphate
- Removal of metabolic waste products from blood
- Removal of foreign chemicals in the blood e.g. drugs
- Regulation of red blood cell production (erythropoietin)
Describe the structural organisation of the nephron
Renal tubule -
- Bowman’s capsule
- Proximal convoluted tubule
- Loop of Henle
- Distal convoluted tubule
- Collecting duct - drains into ureter
Blood vessels -
- Afferent arteriole
- Glomerulus
- Efferent arteriole
- Peritubular capillaries - surround proximal and distal tubules
- Vasa recta - surrounds the loop of Henle
State the 4 basic processes of renal function
- Glomerular filtration
- Tubular reabsorption
- Tubular secretion
- Excretion of water and solutes in the urine
Describe glomerular filtration (what drives it, what it forms)
- Balance of starling forces drive water and solute across the capillary membrane
- In the glomerulus there is hydrostatic pressure due to blood pressure which is opposed by oncotic pressure and hydrostatic pressure of bowman’s capsule and interstitial space - net hydrostatic pressure favours filtration
- Small molecules pass readily - large ones and cells cannot pass
- This leads to a plasma ultrafiltrate in the bowman’s capsule the first step in the production of urine
Define glomerular filtration rate and why it is an important measure
- The amount of filtrate produced by the kidneys each minute
- Averages 125 ml/min
- Very important clinical indicator (reduced in renal failure)
What can be used as an indicator of GFR?
Plasma creatinine can be used as an index of GFR
Describe tubular reabsorption and where it occurs
- Many substances are filtered and then reabsorbed from the tubular lumen into the peritubular capillaries
- About 70% of filtered salt and water reabsorbed from proximal tubule
- 20-25% is from the loop of Henle
- Variable fraction of remaining 5-10% is reabsorbed from distal tubule and collecting duct
Describe tubular secretion - what it does and what it is important in
- This is important for disposing substances beyond their level in the filtrate
- Eliminating toxins and metabolic by products
- Important in potassium balance - excess K+ secreted in distal tubule and collecting duct and in acid base balance relies on H+ secretion in distal tubule and collecting duct
