Introduction to Kidney and Body Fluids Flashcards
How is water distributed in the body?
Body composition is approximately 60% water in males, 50% in females
70kg male: 0.6 x 70 = 42 L body water
60 kg female 0.5 x 60 = 30 L body water
This water is distributed in two main fluid compartments, ICF and ECF. Two thirds of the body water is intracellular, one third is extracellular
70kg male: 2/3 x 42 = 28 L ICF vol, 1/3 x 42 = 14 L ECF vol
60 kg female 2/3 x 30 = 20 L ICF vol, 1/3 x 30 = 10 L ECF vol
How are the ICF and ECF balanced?
The ICF and ECF must be in osmotic equilibrium
The cell membrane is semipermeable
Permeable to water (via water channels: aquaporins)
Impermeable to most solutes
Change in solute concentration in either ICF or ECF will generate osmotic gradient, resulting in shifts of water between compartments
Why is osmolarity of the ECF regulated?
Osmolarity of the ECF is regulated to void osmotic shifts of water between ICF and ECF volumes
Normal range 280-300 mOsm/L
Large shifts must be avoided to prevent changes in cell volume
The most serious complications are neurological
Osmoregulation is the physiological process that maintains constant ECF osmolarity
What is the ECF divided into?
ECF compartment is subdivided into:
Interstitial (or extravascular) compartment (about 75% of ECF)
Plasma (or vascular) compartment (about 25% of ECF)
Volume regulation is control of the ECF volume to ensure appropriate plasma volume (maintained by balance of Starling forces)
What are the two ways of maintaining salt and water balance?
Maintaining salt and water balance requires integration of osmoregulation and volume regulation
Osmoregulation: 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
What do renal functions include?
Renal functions include:
Osmoregulation
Volume regulation
Acid-base balance
Regulation of electrolyte balance (e.g. potassium, calcium, phosphate)
Removal of metabolic waste products from the blood
Removal of foreign chemicals in the blood (e.g. drugs)
Regulation of red blood cell production (erythropoietin)
What is a nephron?
The nephron is the functional unit of the kidney
The nephron consists of special blood vessels and elaborate tubules
Microscopic structures, 1.25 million per kidney
What are the structural features of the renal tubule?
Bowman’s capsule Proximal tubule (proximal convoluted tubule, PCT) Loop of Henle Distal tubule (DCT) Collecting duct (CD) CDs join and ultimately drain into the ureter Structural organisation of the nephron; blood vessels: Afferent arteriole Glomerulus Efferent arteriole Peritubular capillaries Vasa recta
What are the four basic processes of renal function?
Glomerular Filtration
Tubular Reabsorption
Tubular Secretion
Excretion of water and solutes in the urine
What happens during glomerular filtration?
Balance of Starling forces drive water and solute across the capillary membrane
Small molecules pass readily – large ones (proteins) and cells cannot pass
This leads to a plasma ultra filtrate in the Bowman’s capsule, the first step in the production of urine
Glomerular filtration rate (GFR)
The amount of filtrate produced by the kidneys each minute
Averages 125 ml/min (approx. 20% of renal plasma flow)
Very important clinical indicator (reduced in renal failure)
Plasma creatinine can be used as an index of GFR
What happens during tubular reabsorption?
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% from loop of Henle
Variable fraction of remaining 5-10% is reabsorbed from distal tubule and collecting duct
What happens during tubular secretion?
Tubular secretion is important for disposing of substances beyond their level in the filtrate
Eliminating toxins and metabolic by-products
Important in:
Potassium balance: excess K+ secreted in DT and CD
Acid-base balance relies on H+ secretion in DT and CD
What happens during the excretion of water and solutes in urine?
The tubular fluid remaining after filtration, reabsorption and secretion is excreted as urine.
Amount excreted = amount filtered (1)
- amount reabsorbed (2)
+ amount secreted (3)
If (2) and (3) are both zero, then for that substance
the amount excreted = amount filtered.
Can be used to estimate GFR
e.g., creatinine is filtered but not reabsorbed; can estimate GFR from plasma concentration
What happens if you drink a quantity of pure water?
The ICF and ECF must be in osmotic equilibrium
If you drink a quantity of pure water
It will be absorbed by the gut and fill the extracellular fluid compartment, now somewhat diluted
The osmolarity of the ECF goes down creating an osmotic gradient to a shift of water to the ICF
This means that both the ICF and ECF will have slightly expanded once osmotic equilibrium is reached
Now the dilution of these compartments are detected by the kidney
The kidney then would respond by increasing renal water excretion
How is osmolality monitoring?
Osmoregulation depends on monitoring the osmolality of the ECF
The sensors are osmoreceptors in the hypothalamus
Effector is hormonal response- vasopressin if there is a decrease in osmolality
