Renal System Flashcards
Why do we need the Kidneys?
To control what is in the blood and how much blood we have
What do the kidney’s do?
Filter the blood/plasma and reabsorb what the body needs back into the blood. Excretes what we don’t need
What gets excreted by the kidneys?
Excretes waste products, drugs, toxins and excess water, ions and acid/bicarbonate in the urine
What do the kidney’s regulate and eliminate?
REGULATE:
- Blood osmolarity
- Blood volume & pressure
- Red blood cell production
- Blood pH
ELIMINATE:
- Metabolic (nitrogen)
- Drugs/toxins
What is a nephron?
The functional units of the kidneys - they are the powerhouse of the kidneys
each kidney contains about 1 million nephrons (they are found in the renal medulla)
Whats the pathway of blood in the Nephron?
- Blood arrives in the afferent arteriole
- Into the glomerulus where filtration takes places
- Blood flows out the efferent arterioles and into the peritubular capillaries
What are the basic functions of the nephron?
- Filtration
- Reabsorption
- Secretion
What happens in filtration?
Substances (eg. glucose and water) move from the blood through the ‘sieve’ and into the nephron tubule.
What happens in reabsorption?
Movement of useful substances out of the nephron tubule back into the blood (stay in the body)
What happens in secretion?
Removal of waste products (eg. drugs) from the blood into the nephron tubule and end up in the urine (leave the body).
What cells are the kidneys lined with?
The renal tubules are lined with epithelial cells
Whats the order of reabsorption into the blood?
Lumen –> apical membrane –> through/between cells –> basolateral membrane –> interstitial space –> blood stream (peritubular capillaries)
What are the two transport pathways in the kidneys?
Para-cellular pathway
Trans-cellular pathway
What is the para-cellular pathway?
- ‘between’ cells
- transport proteins not required
- only possible in cells with ‘leaky epithelia’
What is the trans-cellular pathway?
- ‘through’ cells
- Apical & basolateral membrane (requires permeability and driving force)
- can be automatic or controlled by hormones
- cells with ‘tight epithelia’ allows transcellular reabsorption only
How does water move through the kidneys?
Water can only move via both the trans and the para-cellular pathway.
If leaky epithelium - water can go straight through both
If tight epithelium - water can only go through the trans-cellular pathway using channels (aquaporins)
What are the layers of the Glomerular capsule?
- Inner layer
- Outer layer
- Capsule space in between
Why do the glomerular capillaries have gaps?
Have gaps between them to allow smaller substances to get filtered through
What are the filtration slits?
Cells branch out to form finger like projections that interconnect and have gaps in between called filtration slits
What is the filtration membrane?
The membrane that substances have to go through to get filtered to then go to the renal tubule. Molecules can get through due to concentration gradient.
Only small molecules can fit through (ie water, ions, glucose, amino acids and hormones) and some are too big to be filtered (cells and proteins)
What is the Glomerular Filtration Rate?
The volume of plasma/blood filtered per minute. 125ml/minute = 180L/day
The kidney’s are filtering the entire blood supply 36 times a day.
Only 1% becomes urine, the rest (99%) is reabsorbed back into the blood stream.
What is renal clearance?
The amount of blood plasma that is completely cleared of a substance per minute
Filtration - reabsorption + secretion = clearance
How is renal clearance and glomerular filtration rate (GFR) connected?
The clearance of certain substances gives an indication of how much blood is being filtered by the nephrons per minute (GFR).
GFR gives and indication of how well the kidneys are functioning.
What is Creatinine Clearance?
The amount of blood the kidneys can make creatinine-free is called the creatinine clearance. This is about 125ml/minute (similar to GFR).
Is creatinine filtered, reabsorbed and secreted?
- Freely filtered
- Not reabsorbed
- Not secreted
- All filtered is excreted in the urine
- All of the plasma that has been filtered is cleared of creatinine
What are the two main parts of the renal tubule?
- Proximal Tubule
- Distal Tubule
They have different functions and characteristics
What is the proximal tubule?
This is where the bulk of reabsorption happens.
- 90% water
- 90% sodium
- 100% of nutrients
This is an automatic process
Leaky epithelia: transcellular and paracellular
What is the process of reabsorption in the proximal tubule?
- The glucose and amino acids utilise the sodium electrochemical gradient to cross the apical membrane and enter the cell.
- Needs to exit the cell and cross the basolateral membrane, via facilitated diffusion (uses transporters)
- Reabsorption of glucose and amino acids causes sodium reabsorption as well
- Where sodium goes, water follows via osmosis (both paracellular and transcellular pathways)
- Due to the build up of positive charge, chloride (negative) follows via the paracellular pathway
What is the Distal Tubule?
This is the fine-tuning reabsorption.
- 1-10% water
- 1-10% sodium and chloride ions
- acid and bicarbonate for pH control (depending on what the body needs)
This process is more regulated and is controlled by hormones
Tight epithelia: transcellular movement of water and ions
What hormone regulates water reabsorption?
Anti-diuretic hormone (ADH)
What hormone regulates sodium reabsorption?
Aldosterone (+renin)
What is diuresis?
Increased urine production
What is the distal tubule controlled by?
Hormonal regulation
What is anti-diuretic hormone (ADH)?
- The control of water reabsorption
- peptide hormone: water soluble
- from the posterior pituitary
- rapid response
What is aldosterone?
- controls sodium reabsorption
- steroid hormone: lipid soluble
- released from the adrenal gland
- long term response
What is renin?
- made in the kidneys
- it stimulates the release of aldosterone from the adrenal gland
How is water reabsorbed in the distal tubule?
- Needs a driving force to make the water move: osmotic gradient
- The osmolarity increases from renal cortex to medulla
- The distal tubule has tight epithelia which means that there is trans-cellular movement of water
- water travels via aquaporins
- this is controlled by ADH
What happens when you have an increase in blood (ECF) osmolarity?
Water moves out of the cells - the cells shrink
What happens when you have an decrease in blood (ECF) osmolarity?
Water moves into the cells - the cells swell
How does ADH control water reabsorption when there is an increase in ECF osmolarity?
Increase in ECF osmolarity –> detected by osmoreceptors in the hypothalamus –> increases release of ADH from the posterior pituitary into the blood –> increased aquaporins in apical membrane of distal tubule cells –> increased water reabsorption/decreased urine volume –> ECF osmolarity returns to normal
What does pale yellow/clear urine mean?
- well hydrated
- low ADH levels
What does dark yellow urine mean?
- dehydrated
- high ADH levels
How is water reabsorption controlled during high intensity exercise?
- Increased sympathetic nervous system –> decreased blood flow to kidneys –> decreased filtration rate in glomerulus –> decreased urine production
- increased secretion of ADH –> increased aquaporins in apical membrane of distal tubule cells –> increased water reabsorption –> decreased urine production
How is sodium reabsorbed in the distal tubule?
- The electrochemical gradient is the driving force for sodium entering the cell
- Tight epithelia so sodium uses the transcellular pathway using sodium channels to enter and Na+/K+ ATPase pump to exit the cell
What is isosmotic fluid loss?
- The loss of both water and ions at the same time.
- This means the osmolarity isn’t changing so there is no gradient for water to move in or out of the cell
- Only a change in ECF and therefore blood volume and pressure
How does aldosterone control sodium reabsorption when there is a decrease in blood volume/pressure?
Decrease in blood volume/pressure –> detected by pressure receptors in the kidney causing renin release –> renin stimulates release of aldosterone from the adrenal cortex –> increased sodium channels in apical membrane of distal tubule cells –> increased sodium reabsorption/decrease sodium excretion in urine –> blood volume returns to normal
THE RELEASE OF ALDOSTERONE TRIGGERS THE RELEASE OF ADH
Increased release of ADH from the posterior pituitary –> increased aquaporins in apical (top) membrane of distal tubule cells –> increased water reabsorption –> blood volume returns to normal
Why is ADH fast and aldosterone slower?
Blood osmolarity is an emergency so therefore a fast response
Blood volume/pressure is less urgent so therefore a slower response
How do the kidneys respond to an increase in blood osmolarity?
An increase in blood osmolarity means that water will move out of the cells (cells shrink). Increased ADH release to increase the amount of aquaporins which increases water reabsorption to replace the lost water. This decreases ECF osmolarity and the cells return to normal size.
How do the kidneys respond to a decrease in blood osmolarity?
A decrease in blood osmolarity means that water will move into the cells (cells swell). Decreased ADH release to decrease the amount of aquaporins which decreases water reabsorption to remove the excess water. This increases ECF osmolarity and the cells return to normal size.
What happens in dehydration in sports?
Sweat is hyposmotic = less salty than plasma
Dehydration during exercise:
- lose more H2O than Na+
- ECF osmolarity increases
There are no recorded deaths from dehydration during sports
What is hyponatremia and what is it caused by?
Low amounts of sodium in your ECF (under, sodium)
Caused by: over-hydration + feedforward ADH release at the start of exercise
This combination will lead to the body retaining (keeping) too much water which causes the cells to swell and potentially burst.
What are the recommendations to prevent overhydration/hyponatremia?
- Athletes - don’t over drink during competition, drink electrolytes
- Organisers - restrict fluid availability during cycling & running legs of race
Why is a change in blood volume less urgent for the kidney’s?
Because the cardiovascular system will rapidly adapt to changes in volume and pressure.
How do the kidneys respond to a decrease in blood volume?
Decrease in blood volume (loss of isosmotic fluid)
Fast CVS response to: restore blood pressure
Slower aldosterone response to: replace isosmotic fluid, increase in sodium and water reabsorption in the kidneys, blood volume replaced
What is an acid?
- A molecule that can release a hydrogen ion (H+)
- H+ donor
- eg. Hydrochloric acid (HCL)
What is a base?
- A molecule that can accept a hydrogen ion from an acid
- H+ acceptor
- eg. bicarbonate (HCO3)
What happens as acids increases?
More H+ will be released into the blood causing the pH to become lower
What happens as bases increase?
They bind more H+, removing it from the blood causing the pH to become higher
What is pH?
A measure of hydrogen ions in a solution. This indicates how acidic (or basic/alkaline) a solution is
What is the normal range of pH in the ECF?
7.35-7.45
What is acidosis?
Too much acid is in the blood - pH less than 7.35
What is alkalosis?
Not enough acid is in the blood - pH more than 7.45
What pH is lethal?
Below 6.8 or above 7.7
How is blood acidity increased (decreased pH)?
- Bi-products of cell metabolism (ATP and Lactic Acid production) - increases CO2 and H+
- Decreased ventilation (hypoventilation) - increases CO2
- Losing base: lose HCO3 (diarrhoea)
How is blood acidity decreased (increased pH)?
- losing acid: lose H+ (vomiting)
- medications & diuretics: lose H+
- hyperventilation: lose CO2
What is the pH range of urine?
4.5 - 8
How can you make your urine more acidic?
A diet rich in meat, sugar, cheese and alcohol
How can you make your urine more alkaline?
A diet rich in citrus fruits, vegetables, legumes and low sugar
Why do we want a more alkaline urine?
A more alkaline urine will help maintain good bladder health. Alkaline urine inhibits most bacterial growth that could be associated with diseases
What happens if pH goes outside the range?
Proteins denature/don’t function properly
How much acid does the body produce per day?
~13000 mmol
What are the 2 ways to control acid to ensure healthy body functioning?
Buffered and removed
What 3 systems work together to control the pH of the body?
- Buffering systems
- Respiratory systems
- Urinary (renal) system
What is a buffer?
A buffer is a substance that minimises changes in blood pH when H+ concentration is altered. It’s a neutraliser.
Example: HCO3 which is created and maintained by the kidneys
How does HCO3 work as a buffer?
When H+ concentration increases in the blood, HCO3 binds to H+ molecules to minimise change in blood pH.
What is the Bicarbonate-Carbonic Acid Buffering System?
- Acids from cell metabolism dissociate to release H+
- HCO3 can bind to H+ to buffer it by forming carbonic acid
THIS PROCESS IS REVERSIBLE: - Carbonic acid can dissociate into H+ and HCO3 (left to right)
- Carbonic acid can also dissociate into CO2 and H2O (right to left)
How does the respiratory system correct acidosis?
- Carbonic acid dissociates to become CO2 and H2O
- CO2 can then be exhaled out at the lungs (hyperventilation)
- This reduces the acid load back to normal in the blood - correcting the acidosis
This is the short term response
How does the renal system correct acidosis?
- Carbonic acid dissociates to become H+ and HCO3
- H+ is secreted into kidney tubules to be excreted out of the body in urine
- HCO3 is reabsorbed back into the blood
- This removes acid and replenishes the HCO3 buffer stores
How does the respiratory system correct alkalosis?
-Reducing ventilation or hypoventilation (breath hold) causes CO2 to build up
- CO2 and H2O join to form carbonic acid
- Carbonic acid dissociates to become H+ and HCO3
- The most important change is the increased H+ (acid), causing pH to decrease back to normal
How does the renal system correct alkalosis?
- Carbonic acid dissociates to become H+ and HCO3
- HCO3 is secreted into kidney tubules to be excreted out of the body in the urine
- H+ is reabsorbed from the kidney tubule back into the blood
- This removes base and increases acid in the blood
Summary of how the 3 systems correct acidosis?
Buffer - extra H+ binds to HCO3 to create carbonic acid
Kidneys - excrete H+ and keep HCO3
Lungs - exhale CO2 (increase ventilation)
Summary of how the 3 systems correct alkalosis?
Buffer - extra HCO3 binds to H+ to create carbonic acid
Kidneys - excrete HCO3 and keep H+
Lungs - retain CO2 (decrease ventilation)
