An introduction to kidneys & body fluid Flashcards
what % of body weight does Total body water take up?
60%
what % of body weight does Intracellular water take up?
40%
what % of body weight does extracellular water take up?
20%
What does osmosis of water determine?
The movement of water between ICF and ECF
What is the purpose of osmoregulation?
To prevent large fluid shifts between these compartment.
How is intracellular fluid treated?
Water outside all the cells of the body is not really one compartment, - but can be treated as one.
What two sub-compartments does the ECFV have?
- Plasma (3L)
2. Interstitial fluid (11L)
What do Starling forces determine?
Starling forces determine fluid and solute movement between plasma and interstitial fluid
What is the plasma compartment sometimes called?
Effective circulating volume:
It has to be defended to maintain adequate blood pressure for effective tissue perfusion. Normally it’s around 20% of the ECF.
What is osmolarity?
The total concentration of osmotically active solutes
Why must osmolarity of ECF and ICF be kept the same?
To avoid excessive shifts of water between ECF and ICF
What is the principal electrolyte of the ECF?
Sodium is the principal electrolyte of the ECF, therefore sodium (with associated anions) is the major determinant of ECF osmolarity
How to work out the osmolarity of extracellular Na?
e.g. 140 mmol/L + 140 mmol/L , so 280 mmol/L, - this is because you assume the same amount of anions to the sodium cations.
Why is the ECF the compartment that can be regulated?
- The ECF is continuous
* the ICF in reality is 10^14 individual cellular compartments
ECF Na+ concentration
135-145 mmol/L
ICF Na+ concentration
5-10 mmol/L
ECF K+ concentration
3-5 mmol/L
ICF K+ concentration
130-150 mmol/L
Why is the control of body fluids important?
- Osmoregulation
2. Volume regulation
Why is the control of body fluids important in terms of osmoregulation?
Cell structure: Differences in osmotic pressure between IC and EC will lead to volume shifts, leading in turn to cell and tissue damage and function . – done by regulating the total solutes in the ECF
Why is the control of body fluids important in terms of volume regulation?
Depends on balance between circulating volume (plasma) and interstitial volume: this is done by regulating the total volume of the ECF
Salt and water balance depends on which two key processes
- Osmoregulation: maintain osmotic equilibrium between ICFV and ECFV
- Volume regulation: maintain adequate ECFV to support plasma volume
What is the principal electrolyte contributing to ECFV osmolarity?
Sodium (along with its associated anions)
How can plasma osmolarity be estimated?
2[Na] + 2[K] + [glucose] + [urea] (all in mmol L-1)
- don’t double glucose and urea as they are not an electrolyte, so they don’t have charges
What are two ways to change the osmolarity of a solution?
- Add/remove solute
2. Add/remove water
How does the body accomplish osmoregulation?
By adding or removing water not sodium
How does the body respond to a rise in plasma osmolarity?
More water is needed so the kidneys response by producing small volumes of concentrated urine (water retention)
*also triggers thirst, a powerful behavioural response which will counteract the rise in osmolarity.
How does the body respond to a fall in plasma osmolarity?
There is too much water and so the kidneys respond by producing large volumes of dilute urine (water excretion)
What does volume regulation involve?
The control of the circulating plasma volume
How are changes in plasma volume detected?
Changes detected by stretch and pressure receptors in the cardiovascular system
How is a fall in blood volume opposed?
By sodium retention; water flows osmotically, restoring volume
- total amount of sodium will be increased but concentration (therefore osmolarity) isn’t really changed because the sodium brings water
Kidneys
Produces urine
Ureter
Transports urine towards the urinary bladder
Urinary bladder
Temporarily stores urine prior to elimination
Urethra
Conducts urine to exterior
Primary kidney function
- Homeostasis: salt and water balance, this is done by regulating the amount of urine to conserve/excrete water, and its concentration (the amount of salt)
- production of urine is a by-product
Urinary tract
Important for temporary storage and then to remove urine from the body
List functions of the kidney
- Osmoregulation
- Volume regulation
- Acid-base balance
- Regulation of electrolytes balance (eg 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)
What is the functional unit of the kidney?
The nephron
What does each nephron consist of?
- special blood vessels and elaborate tubules
- microscopic structures:
- Blood vessels
- The glomerulus
- Bowman capsule
- The renal tubule
How many nephrons per kidney?
1.25 million
What occurs at the nephron?
Urine production begins here
Describe arrangement of blood vessels and blood flow through the kidneys
SEE DIAGRAMS
What blood vessel is the whole kidney supplied by?
A major artery, the renal artery which subdivides many times into fine arterioles supplying each nephron, the afferent arteriole.
What blood vessel is the kidney drained fro?
The renal vein, which is supplied by venules coming away from each nephron, joining together into larger veins which merge into the renal vein
Structural organisation of the renal nephron?
SEE NOTES
What are the four basic processes of urine formation?
- Glomerular Filtration
- Tubular reabsorption
- Tubular Secretion
- Excretion of water and solutes in the urine
Describe glomerular filtration
- The first step in the production of urine
- Hydrostatic pressure forces fluids and solutes through (pressure difference in the afferent and efferent arteriole)
- Small molecules pass readily: large ones (proteins) and cells cannot pass
- This leads to a plasma ultra filtrate in the Bowman’s capsule
What is GFR?
The amount of filtrate kidneys produce each minute
* averages 125 ml/min (approx 20% of real plasma flow)
What can be used as an indication of GFR?
Plasma creatinine: breakdown product of creatinine phosphate in muscle
Clinical significance of GFR
Reduced in renal failure
Where does reabsorption occur?
Many substances are filtered and then reabsorbed from the tubular lumen into the peritubular capillaries
Volume of H2O filtered a day?
180 L/day
Volume of Na+ filtered a day?
25,000 mEq/day
Volume of HCO3- filtered a day?
4,500 mEq/day
Volume of Cl- filtered a day?
18,000 mEq/day
Volume of Glucose filtered a day?
800 mM/day
Volume of H2O excreted a day?
1.5 L/day
Volume of Na+ excreted a day?
150 mEq/day
Volume of HCO3- excreted a day?
2 mEq/day
Volume of Cl- excreted a day?
150 mEq/day
Volume of Glucose excreted a day?
~ 0.5 mM/day
Volume of H2O reabsorbed a day?
178.5 L/day
Volume of Na+ reabsorbed a day?
24,850 mEq/day
Volume of HCO3- reabsorbed a day?
4,498 mEq/day
Volume of Cl- reabsorbed a day?
17,850 mEq/day
Volume of Glucose reabsorbed a day?
799.5 mM/day
Percentage proportion of filtered H2O that is reabsorbed?
99.2%
Percentage proportion of filtered Na+ that is reabsorbed?
99.4%
Percentage proportion of filtered HCO3- that is reabsorbed?
99.9%
Percentage proportion of filtered Cl- that is reabsorbed?
99.2%
Percentage proportion of filtered Glucose that is reabsorbed?
99.9%
What is tubular secretion important for?
- Disposing of substances not already in the filtrate
- Eliminating undesirable substances such as urea and uric acid
- Ridding the body of excess potassium ions
- Controlling blood pH
Describe excretion of fluid and solutes in the urine
SEE NOTES
What is the control of water balance (osmoregulation) based on?
The osmolarity of the ECFV?
Describe what occurs when there is changes in water intake/excretion
Changes in body fluid osmolarity
I
SENSOR
I
EFFECTOR
I
Changes in renal water excretion: intake ~ OUTPUT
I
back to beginningWhere are neural sensors?
Neural sensors that are sensitive to the plasma osmalarity are in the hypothalamus.
What kind of effector will it be?
The effector will be a hormonal response to rebalance intake and output
How is total body water balance calculated?
Input - Output
Types of water input
- Drinks: 1000 ml
- Food: 700 ml
- Metabolism: 300 m l
2000 ml/day
Types of water output
- Gut: 100 ml
- Insensible loss: 900 ml (lungs/skin)
- Renal excretion: 1000 ml
2000 ml/day
What leads to changes in water balance?
- Changes in body fluid osmolality
- Shift of water between ICFV and ECFV
Why should the total body water balance be zero?
Otherwise there will be a change in osmolality
What are examples of insensible loss?
Breathing with the lungs and through the skin
How can we control output?
Cannot control the output from the gut and insensible loss. – but can control renal excretion
What are some physiological responses to water restriction?
- Loss of water (sweat, breathing).
- Thirsty, but no water is available to drink
- Plasma osmolality rises
- Response is increased secretion of hormone, ADH (antidiuretic hormone, also known as vasopressin)
What does secretion of ADH result in?
- Decreased urine volume
- Increased urine osmolality
- Will start to feel thirsty
What are some physiological responses to increase in water intake?
- Increase in water absorption through GIT
- Plasma osmolality falls
Response is reduced secretion of ADH
What does reduced ADH secretion result in?
- Urine volume increases
- Urine osmolality decreases
- Less water reabsorbed
Renal output when water intake is excessive
20 L/day
Renal output when water intake is restricted
0.5 L/day
Osmolality when water intake is excessive
50 mM
Osmolality when water intake is restricted
1200 mM
What can happen when there is EXCESSIVE water intake?
Worst case scenario:
- water will move from ECF to ICF compartment
What determines ECF volume?
Amount of sodium
What must be balanced to maintain constant ECF volume?
Sodium intake and excretion
Where are the main volume sensors located?
The cardiovascular system
What is a fall in blood volume opposed by?
Hormonal signals promoting sodium retention; water follows osmotically, restoring volume
How is total body Na+ concentration calculated?
Intake - Elimination
Why is control of sodium balance important?
Sodium is the major electrolyte of the ECFV
The maintenance of sodium balance is based mainly on?
The control off the ECFV
What is the sodium retaining system?
Renin-angiotensin-aldoesterone system (RAAS)
What is the sodium eliminating pathway?
Cardiac natriuretic peptides (ANP)
Hormonal and renal responses to increases/reductions in sodium intake
SEE NOTES
