39) An introduction to kidneys & body fluids Flashcards
1
Q
What is the water composition of the body?
A
- The body composition is about 50-60% water
2
Q
Where is the water distributed in the body?
A
- The water is distributed in two main fluid compartments (separated by the cell membrane) known as the intracellular fluid (ICF) and the extracellular fluid (ECF)
- 2/3rds of the water is intracellular and 1/3rd is extraacellular
3
Q
Why is there no osmotic movement of water between the ICF and ECF?
A
- The ICF and ECF are in osmotic equilibrium which prevents the osmotic movement occurring
- Without this equilibrium osmotic movement can occur leading to cells growing and shrinking
- Therefore large shifts in osmolarity must be avoided to prevent changes in cell volume
- Large shifts in osmolarity can cause neurological complications
4
Q
What is the cell membrane?
A
- The barrier between the ICF and the ECF
- It is semi-permeable as it is permeable to water (via water channels called aquaporins) and impermeable to most solutes
5
Q
What is the main salt in the different components?
A
- In the intracellular component the main salt is K+ with its anion
- In the extracellular component the main salt is Na+ with its anion
6
Q
How are osmotic shifts avoided?
A
- Through a physiological process that maintains constant ECF osmolarity (called osmoregulation)
7
Q
How is an effective circulating volume achieved?
A
- Regulation of ECF volume ensures effective circulating volume and hence maintains adequate tissue perfusion
8
Q
How is the ECF subdivided?
A
- Interstitial (or extravascular) compartment (about 75%)
- Plasma (or vascular) compartment (about 25%)
9
Q
What is volume regulation of the ECF?
A
- Controlling the amount of salt and water in the ECF in order to maintain plasma volume
- The plasma volume is regulated by starling’s forces
10
Q
How is salt and water balance maintained in the body?
A
- It is maintained through 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 the ECF volume
- Although many organs and systems are involved, the kidneys are central to both processes
11
Q
What vessel supplies the kidneys with blood?
A
- The renal artery
12
Q
What vessel drains blood from the kidneys?
A
- The renal vein
13
Q
How is urine made and excreted?
A
- They have a rich blood supply from which they produce urine
- This urine flows down the ureters for temporary storage in the bladder before it is excreted by the urethra
14
Q
What is the function of the kidneys?
A
- Osmoregulation
- Volume regulation
- Acid-base balance
- Regulation of electrolyte balance
- Removal of metabolic waste products from the blood
- Removal of foreign chemicals in the blood
- Regulation of red blood cell production
(All these roles can be categorised under homeostasis)
15
Q
What is the nephron?
A
- The functional unit of the kidney which consists of special blood vessels and elaborate tubules
- They join together and are drained (via draining ducts) into calyxes which are finally drained into the ureter
16
Q
What are the different components of the nephron?
A
- The blood vessel component
- The renal tubule component
17
Q
Describe the structure of the renal tubule component.
A
- The renal tubule begins at the Bowman’s capsule
- The Bowman’s capsule is then continuous with the rest of the renal tubule starting with the proximal tubule
- It wiggles around a little bit before descending deeper into the cortex and reaching the medulla where it turns and ascends back up to form a loop (called Loop of Henle)
- Whilst ascending it starts to wiggle again as it becomes the distal tubule and finally joins with the collecting duct
- Many collecting ducts join together into the calyces
- Many calyces join together to drain into the ureter
18
Q
Describe the structure of the blood vessel component.
A
- The renal artery enters the kidney and is subdivided into many arteries until each nephron is supplied by an afferent arteriole
- The afferent arteriole enters the interstitial space within the glomerulus and forms a dense capillary network
- The glomerulus is drained away by another arteriole called the efferent arteriole
- These efferent arterioles further subdivides into two capillary networks: the peritubular capillary (which surrounds the proximal and distal tubules) and the vasa recta (which surrounds the loop of Henle)
- These networks then drain into the venules which all come together to drain into the renal vein
19
Q
What processes take place in the kidneys?
A
- Glomerular filtration: Water and solute from the glomerulus is filtered out into interstitial space and crosses into the lumen of the Bowman’s capsule
- Tubular reabsorption: Most of the salt and water are reabsorbed back into the peritubular capillaries
- Tubular secretion: Some substances are secreted from the peritubular capillaries into the kidney of the kidney tubule
- Excretion of water and solutes in urine: What is leftover after all these processes is the urine that is to be excreted
20
Q
What is glomerular filtration?
A
- Relies on Starling forces (Hydrostatic pressure and oncotic pressure) to drive water and solutes across the capillary membrane
- Small molecules pass readily whereas large molecules are unable to pass
- This leads to the formation of plasma ultrafiltrate in the Bowman’s capsule
- The rate at which this occurs is called the Glomerular Filtration Rate (GFR) and is the amount of filtrate produced by both kidneys per minute
- GFR is an important clinical indicator as it is reduced in renal failure
21
Q
What is tubular reabsorption?
A
- Most of the filtered water and salt is reabsorbed from the proximal tubule.
- There is further reabsorption at the loop of Henle however it does not absorb as much as the proximal tubule.
- Finally there is a very tiny fraction of reabsorption at the distal tubule and collecting duct
- The amount of reabsorption (in the distal tubule) can be adjusted to maintain salt and water balance
22
Q
What is tubular secretion?
A
- It is important for disposing substances that have reached beyond their level of filtration and in eliminating toxins and metabolic by-products
- Furthermore it also helps with potassium balance (as excess K+ is secreted in the distal tubules and collecting duct)
- They also maintain acid-base balance which relies on H+ secretion by distal tubule and collecting duct
23
Q
What is excretion of water and solutes in urine?
A
- The fluid left in the tubules after filtration, reabsorption and secretion are excreted as urine
- It can be used to estimate GFR
24
Q
How is osmotic equilibrium maintained?
A
- When we drink water the ECF becomes diluted.
- This sets up an osmotic gradient and so causes water do diffuse into the ICF component
- It does this in such a way that allows the ICF and ECF to be expanded and have the same concentrations
- This causes the kidneys to increase renal water excretion which will reduce the ECF volume of water
- This sets up another osmotic gradient causing the water to shift from the ICF into the ECF
- This results in an overall decrease in ECF and ICF water concentartion resulting in osmotic equilibrium.
25
How does osmoregulation occur?
- By increasing or decreasing the amount of solute-free water excreted by the kidneys
- This process depends on monitoring the osmolarity of the ECF so that appropriate responses can be made
- Change in fluid intake is counteracted by change in fluid output/excretion
26
What is the mechanism of osmoregulation?
- A change in body fluid osmolarity is picked up my receptors/sensors (osmoreceptors in the anterior hypothalamus).
- These sensors sends a signal to an effector (which elicit a hormonal response from the posterior pituitary) causing a change in the renal water excretion
27
What is water balance?
- What water balance is 0 then Input of water = Output of water
- When water balance is negative then input of water < output of water (i.e. we are loosing more water)
- When water balance is positive then input of water > output of water (i.e. we are gaining more water)
28
What dos a change in water balance cause?
- A change in body fluid osmolality
| - Shift of water between ICFV and ECFV
29
What are the physiological changes to water loss?
- Increased thirst
- Increased secretion of ADH (which decreases urine volume and increases urine osmolality)
- Therefore a smaller volume of concentrated urine is released
30
What are the physiological changes in response to water intake?
- Decreased thirst
- Reduced secretion of ADH (which increases urine volume and decreases urine osmolality)
- Therefore a large volume of dilute urine is excreted
31
How does volume regulation occur?
- ECF volume is determined by the amount of Na+ in the compartment
- Na+ intake and excretion must be balanced to maintain constant ECF volume
- A fall in blood volume is opposed by hormonal signalling promoting Na+ retention. As a result water follows osmotically which restores volume
- Main volume sensors are in the CVS through stretch and baroreceptors
32
How is volume regulated?
- When we add salt and water to the ECF the concentration stays the same (as the solution is isotonic) however the volume within the ECF increases/expands
- When expansion occurs the receptors will detect this and so the body will reduce retention of Na+ and as a result will reduce the retention for water
- This will bring the ECF volume back down
33
What is the mechanism of volume regulation?
- There is a change in ECF volume will detect this (via baroreceptors and pressure receptors in the CVS)
- It then sends an impulse to the effectors (which elicit a hormonal response)
- This changes renal sodium and water excretion
34
What are the hormonal systems involved in balance?
- Renin-angiotensin-aldosterone system (RAAS) which increases renal Na+ absorption and increase ECF volume
- Cardiac natriuretic peptides (ANP) which decreases renal Na+ absorption and decreases ECF volume
