An Introduction to Kidney and Body Fluids Flashcards
What are we made of? Overview of fluid compartments
compartments?
40-20 rule?
• The majority of the body is made up of water (60% of body weight)
this may vary with relative proportion of muscle and lipid
this means that in a 70kg body, 42 liters is water
The body water is distributed into two different water compartments
• intracellular fluid
- this contains 40% of the body weight
- in a 70 kg body, this is 28 liters of water
• extracellular fluid
- this contains 20% of the body weight
- in a 70 kg body, this is 14 liters of water
- > the extracellular fluid can be split into two
- interstitial fluid
- this contains 11 liters in a 70kg body
- plasma fluid
- this contains 3 liters in a 70kg body
• The figures mentioned above are fairly abstract as there are cells in the capillaries and in the blood plasma which contain water as well
Body water and solutes
what is osmolarity?
are the solute composition the same? what must be kept the same?
what is the principal electrolyte of the ECF? what about ICF?
Osmolarity is the total concentration of osmotically active solutes
Solute composition of ECF and ICF differs, but osmolarity must be kept the same to avoid excessive shifts of water between ECF and ICF
Sodium is the principal electrolyte of the ECF, therefore sodium (with associated anions) is the major determinant of ECF osmolarity
K+ for intracellular fluid -> equal anion and cation
Factors Important in Movement of Fluids Between
Compartments (2)
- Osmolarity
* Starling’s principle of fluid exchange
Why is the control of body fluids important?
what needs to be controlled between compartments? why? what will happen if not controlled and issue with this?
what does there need to be adequate volume of? why?
• Cell volume
- the net movement of fluid between the compartments needs to be zero
- this is controlled by regulating the osmolarity of the fluids
- if there is a change in osmolarity, this will result in shift of fluid from one compartment to another
- large shifts between in ECFV and ICFV will disrupt tissue structure and function (through changes in the volume of the cell)
- regulation of extracellular osmolarity is important to ensure there are no large shifts between water in intracellular compartment and extracellular compartment
- > this is called osmoregulation
• Tissue perfusion
- this depends on the balance between circulating volume (plasma) and interstitial volume
- there has to be an adequate circulating volume in order to maintain blood pressure and thereby adequate perfusion of tissues
- if the ECF volume is lost, blood pressure will be lost, and therefore body tissues will be under-perfused
Salt and water balance depends on two key processes
different regulations
Osmoregulation: maintain osmotic equilibrium between ICFV and ECFV
Volume regulation: maintain adequate ECFV to support plasma volume
Dehydration
when does this happen?
what happens with volume of water and conc of Na+?
leads to?
major issue with this?
This happens when there is either decreased water intake or increased water loss or both
- When there is dehydration, the ECF water volume reduces while the solute concentration (Na+) remains constant
- This leads to an increase in osmolality of the ECF
- There is movement of water from the ICF to the ECF by osmosis thus causing a shrinkage of cells
• During dehydration, brain cells can shrink thus disrupting neural tissue and maybe also death
Oedema
what is this?
why does it occur?
what could it be a consequence of?
what can cause a massive increase in ECF volume? what dpes this less to?
what happens if there is less perfusion in the kidney?
what happens if oedema in lungs?
Oedema is tissue swelling due to excess fluid in the interstitial space
- This happens when there is fluid imbalance between plasma space and the interstitial space
- It is a consequence of congestive heart failure
- it is the loss of heart function to be able to produce enough pressure to adequately perfuse all tissues
- there will be a massive increase in ECF volume from sodium and water retention
- there will be accumulation of fluid in the interstitial space
• there will be less perfusion in the kidney
- there will be less salt excretion which again will promote water retention thus making the problem bigger
• Oedema within the lungs can lead to serious pulmonary congestion and death
Osmoregulation
calculating plasma osmolarity
equation?
what is the principle electrolyte?
Control of ECFV osmolarity in order to maintain osmotic equilibrium between ICFV and ECFV
Sodium (with its associated anions) is the principal electrolyte contributing to ECFV osmolarity
Plasma osmolarity can be estimated by:
2[Na] + 2[K] + [glucose] + [urea] (all in mmol L-1)
Typical:
2(135) + 2(4) + (5) + (5) = 288 mOsm L-1
2 because anions must have balance of cations
Why is sodium important?
why not other ions?
So you can see why a percentage change in Na will have a bigger impact on osmolarity than any other electrolyte or solute.
Everything else (proteins, small molecules, other cations) is present in much lower concentrations and can be neglected as a contributor to osmolarity.
Mechanism of osmoregulation
what are the 2 ways to chnage osmalarity of a solution?
how does the body achieve osmoregulation?
what happens when plasma osmolarity rises? falls?
There are two ways to change the osmolarity of a solution
1) Add/remove solute
2) Add/remove water
The body accomplishes osmoregulation by adding or removing water not sodium
- Plasma osmolarity rises – more water needed
Kidneys respond by producing small volume of concentrated urine (water retention) - Plasma osmolarity falls – too much water
Kidneys respond by producing large volume of dilute urine (water excretion)
Volume regulation
what detects the chnages in volume?
how is a fall in blood volume opposed?
Refers specifically to the control of the circulating (plasma) volume
Changes detected by stretch and pressure receptors in the cardiovascular system
A fall in blood volume is opposed by sodium retention; water follows osmotically, restoring volume
Note: although the total amount of body sodium may be increased, concentration (and hence osmolarity) is little changed because the retained sodium brings water with it
The kidneys and the urinary tract
what is the kidneys function?
what is it by product?
Kidney function is homeostasis - The central function of the kidney is salt and water balance
The production of urine is a by-product of kidney function.
The urinary tract is important for temporary storage and then to remove the urine from the body
Functions of kidney
8 different functions
• the production of urine is a central process of kidney function.
- the urinary tract is important for temporary storage and then to remove the urine from the body
- regulation of salt and water balance
- control of blood pressure
- 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)
The nephron is the functional unit of the kidney
what is it?
what structures allow urine production to begin here?
- the nephron is the basic repeated unit of the kidney
- contains tiny tubes and interrelated blood capillaries
• urine production begins here with the help of many
structures:
- blood vessels, the glomerulus, bowman
capsule, the renal tubule
The kidneys have a very strong vasculature
describe the vasculature
how it recieves blood? what it branches off to? what does this supply?
what drains the blood?
• each kidney receives blood via the renal artery
which subdivides until it branches down to
arterioles called afferent arterioles
• afferent arterioles supply the bowman capsule
of nephron
• there is an elaborate network of capillaries
associated with the tubule of the nephron
• the nephron is drained by a venule ultimatey
draining the kidney via the renal vein
Nephrons are structurally organized in the kidney
describe the organisation of the nephon
what does the cortex contain?
what does the medulla contain?
• the nephron has a blind ending at one end
called the bowman capsule containing the
glomerulus which contains a series of curved tubules
• the tube continues down and comes back up again,
continues on and eventually drains into collecting ducts which come along to form the ureter
- the cortex of the kidney contains the bowman capsules and the tubules
- the tubules dip down to the medulla of the kidney
bowmans capsule -> glomerulus -> PCT -> thin loop of henle -> thick loop of henle -> DCT -> collecting duct -> ureter
The four basic processes of renal function
Glomerular Filtration
Tubular Reabsorption
Tubular Secretion
Excretion of water and solutes in the urine
glomerular filtration
where is blood filtered from the capillaries into the kidneys?
what drives this process? what causes an inward pressure?
which process is favoured and what does this lead to?
what is the glomerular filtration rate? what is it average to and % of renal flow?
when is this rate reduced?
what can be used as an index for the GFR? measured where and why is it produced? what does its conc depend on?
the blood is filtered from the capillaries into the kidneys at the bowman capsule
the entire blood volume is filtered out everyday several times
there are several forces that drive this filtration:
- > the capillary and bowman capsule are both lined up with endothelial cells
- hydrostatic pressure forces fluids and solutes through the glomerular capillary membrane
- here the small molecules pass readily into the bowman capsule whilst the larger proteins remain inside
-> this causes an inwards (towards the capillaries) osmotic pressure
the net gradient between the hydrostatic pressure and the osmotic pressure favors filtration
- thus the filtrate enters the bowman capsule
the glomerular filtration rate is monitored by doctors
- it is the amount of filtrate the kidneys produce each minute
- it averages to 125 ml/min (approx 20% of renal plasma flow) (6 liters/hour)
- the filtration rate is reduced in renal failure
- plasma creatinine levels can be used as an index to measure GFR
- it is easily measured in plasma
- produced as a byproduct of muscle metabolism at a
constant rate
- its concentration in the plasma will depend on the filtration rate
Reabsorption
where does this take place? from where to where?
what will the filtrate from the earlier process contain? how much of solutes is reabsorbed?
when may glucose not be reabsorbed?
how does reabsroption take place? what can happen to water?
- this happens in the proximal region of the tubule
many substances are filtered and then reabsorbed from the tubular lumen into the peritubular capillaries - the filtrate from the earlier process (glomerular filtration) will contain virtually all the solutes of the blood except for the large proteins
- the important solutes now need to be reabsorbed
180 liters of water is filtered each day but only 1.5 liters is excreted (rest is all reabsorbed)
for mostly all the solutes, 99% of them are reabsorbed - glucose may not be completely reabsorbed in pathological conditions such as diabetes and therefore there will be sugar in urine
- reabsorption takes place by active transport into the network of capillaries around the kidney tubules
- the epithelial cells around the kidney tubule will reabsorb many solutes which have been filtered into the kidney
- water can also be drawn back from the tubular fluid
Secretion
importance of this? (4)
many substances are filtered and then reabsorbed from the tubular lumen into the peritubular capillaries during tubular reabsorption
tubular secretions are used for fine tuning and the secretion of additional solutes that need to be excreted
- this involves secretion of substances that were not filtered, or substances that were reabsorbed but need to be filtered
this process also takes place in the peritubular capillaries - > tubular secretion is 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
Excretion of fluid and solutes in the urine
what 3 things determine what is excreted?
what is the eqaution
what is excreted depends on the amount filtered, the amount reabsorbed and the amount secreted
- the formula for total excretion = amount filtered - amount reabsorbed + amount secreted
Control of Sodium and Water Balance
what controls body water and what controls body sodium
• Body water is controlled by osmoregulation
by controlling the osmolality of ECF
• Body sodium is controlled by volume regulation
by controlling the volume of water in the body
• Control is based on negative feedback systems
Changes in water balance:
effect?
Changes in body fluid osmolality
Shift of water between ICFV and ECFV
The Control of Water Balance
how is this done?
This is done by controlling the osmolality of the ECF
• Concentration or osmolarity can be changed in two ways
- change the amount of solute
- change the amount of water
• The body regulates osmolarity by changing the amount of water
• Controlling sodium is important in controlling the overall volume of water especially extracellular body water
- by retaining sodium, water is retained with it
- regulation of sodium is the defense of the circulating
volume
