Introduction to the kidney and body fluids Flashcards
Explain body water and fluid compartments
- Body composition is approximately 60% water
in males, 50% in females
This water is distributed in two main fluid
compartments, ICF and ECF. Two thirds of the
body water is intracellular, one third is
extracellular
Explain how the ICF and ECF must be in osmotic equilibrium
-the cell membrane is semi permeable
-permeable to water
-impermeable to most solutes
- Change in solute concentration in either ICF or ECF will generate
osmotic gradient, resulting in shifts of water between compartments
Explain how the osmolarity of the ECF is regulated to avoid osmotic shifts of water between ICF and ECF
- 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
Explain how regulation of the ECF volume is important to ensure effective circulating volume
ECF compartment is subdivided:
- 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 is the definition 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
What are renal functions
- Osmoregulation
- Volume regulation
- Acid-base balance
- Regulation of electrolyte balance
(eg potassium, calcium,
phosphate) - Removal of metabolic waste
products from blood - Removal of foreign chemicals in
the blood (e.g. drugs)
Explain the nephron
- The nephron consists of special
blood vessels and elaborate
tubules - Microscopic structures, 1.25
million per kidney
Explain the structural organization of the nephron: 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
ureter
What are structural organization 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
Explain 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,
Explain tubular re absorption
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
Explain tubular secretion
Tubular secretion is important for disposing of substances beyond their level in the filtrate
Eliminating toxins and metabolic byproducts
Important in
* Potassium balance: excess K+ secreted in DT and CD
* Acid-base balance relies on H+ secretion in DT and CD
explain the excretion of water and solutes in the 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
amount excreted = amount filtered.
Can be used to estimate GFR
e.g., creatinine is filtered but not reabsorbed;
What is the equation for total body water balance and what happens if there are changes in water balance
Balance= input- output
Changes in water balance :
-changes in body fluids osmolality
-shift of water between ICFV and ECFV
What are physiological responses to water restriction
Loss of water (skin, lungs)
* Plasma osmolality rises
Response is
* Increased thirst
* Increased secretion of hormone, ADH (antidiuretic hormone, also
known as vasopressin)
ADH increases renal water reabsorption
* Decreased urine volume
* Increased urine osmolality
What are physiological responses to increase in water intake
Increase in water absorption through GIT
* Plasma osmolality falls
Response is
* Decreased thirst
* Reduced secretion of ADH
Results in
* Urine volume increases
* Urine osmolality decreases
How is plasma osmolality maintained
Plasma osmolality is maintained (280 - 300 mOsmol/Kg) in face of
changes in water intake by retaining or by excreting water
Done by ADH system, kidneys and behavioural thirst response
Describe volume regulation
ECF volume is determined by the amount of sodium in this compartment
Sodium intake and excretion must be balanced to maintain constant ECF volume
A fall in blood volume is opposed by hormonal signals promoting sodium retention; water follows osmotically, restoring volume
Main volume sensors are in the cardiovascular system
What are hormonal systems in sodium balance
Renin-angiotensin-aldosterone system (RAAS)
* Increases renal Na reabsorption
* Increases ECF volume
Cardiac natriuretic peptides (ANP)
* Decreases renal Na reabsorption
* Decreases ECF
