Chapter 15 - Homeostasis Flashcards
What is the definition of homeostasis?
The maintainence of a dynamic equilibrium inside of the body
Why do our bodies maintain a dynamic equilibrium as opposed to a static equilibrium?
Because it would be impossible to maintain an entirely stable state, as minute changes happen constantly
2 examples of positive feedback in the body
Platelet activation in blood clotting
Oxytocin levels during labour
3 examples of negative feedback in the body
Blood glucose control
Osmoregulation
Internal temperature control
What are the 5 stages of the feedback loop?
Input Receptor Coordination Effector Output
What are the two main processes the kidneys are involved in?
Excretion and osmoregulation
What are the kidneys made up of?
Millions of filtering units called nephrons
What are the 3 main areas of the kidneys?
The cortex
The medulla
The pelvis
What is the appearance of the cortex?
The dark outer layer of the kidneys
What is the appearance of the medulla?
The lighter in colour, inner section of the kidneys
What occurs in the cortex?
The filtering of the blood
What is the glomerulus?
A tangle of capillaries, in which ultrafiltration takes place
Where is the glomerulus found?
Within the Bowman’s capsule
What is the proximal convoluted tubule?
The first coiled region of the tubule after the Bowman’s capsule
What area of the kidneys is the proximal convoluted tubule found?
The cortex
What is the Bowman’s capsule?
The cup-shaped structure that contains the glomerulus, into which blood enters during ultrafiltration
What primarily happens in the proximal convoluted tubule?
Many of the substances needed by the body are reabsorbed into the blood
What is the name of the region between the proximal and distal convoluted tubule?
The loop of Henle
What is the distal convoluted tubule?
The second coiled region of the tubule
What primarily occurs in the distal convoluted tubule?
The fine-tuning of the water balance
What does the permeability of the walls of the distal convoluted tubule depend on?
The levels of antidiuretic hormone (ADH)
What is the collecting duct?
Where urine passes down, through the medulla to the pelvis
What is the difference in the levels of urea as blood enters and leaves the kidneys?
Blood leaving the kidneys has greatly reduced levels of urea
What arteriole does blood enter the glomerulus through?
The afferent arteriole
What arteriole does blood leave the glomerulus through?
The efferent arteriole
What is the difference between the afferent and efferent arteriole, and what is the effect of this?
The efferent arteriole is much narrower, which means there is considerable pressure in the capillaries of the glomerulus
What is the effect of the different lumen sizes between the afferent and efferent arterioles?
It creates a filtration pressure in the glomerulus, which forces blood out of the capillaries into the Bowman’s capsule
What makes up the first sieve/filter of ultrafiltration?
Gaps in the capillary endothelium and the basement membrane
How do gaps in the capillary endothelium and the basement membrane act as a sieve?
Substances in the blood that are too large to fit through will remain in the glomerulus and leave out of the efferent arteriole
What makes up the second sieve/filter of ultrafiltration?
Podocytes
What are podocytes?
Specialised cells found on the wall of the Bowman’s capsule that act as an additional filter in ultrafiltration
How do podocytes act as an additional filter?
They have extensions called pedicels that wrap around the capillaries forming small slits, ensuring that anything that has made it past the first sieve that shouldn’t have, does not get through to the Bowman’ capsule
What elements of the blood can’t make it through ultrafiltration?
Red blood cells
White blood cells
Platelets
Large plasma proteins
What substances do make it through ultrafiltration?
Urea Glucose Salt ions Water Amino acids
What does the word ‘ultrafiltrate’ refer to?
The blood and substances that enters the Bowman’s capsule through ultrafiltration
What is the purpose of reabsorption?
To return the necessary substances (such as glucose) which have passed through ultrafiltration back into the bloodstream
What substances are completely reabsorbed in the proximal convoluted tubule and how?
All of the glucose, amino acids, vitamins and hormones are reabsorbed by active transport
What other substances are reabsorbed in the proximal convoluted tubule?
85% of Na+ Cl- ions and water
How are Na+ removed in the proximal convoluted tubule and what is the effect of this?
Na+ ions are removed by active transport, which increase the water potential inside the tubule, causing water to leave by osmosis.
This loss of water increases solute potential causing Cl- ions to also leave by diffusion
Where do substances go once they have left the tubule in reabsorption?
They go into the tissue fluid, and then diffuse down steep concentration gradients into the extensive capillary network which surrounds the tubules
How are steep concentration gradients maintained in the capillary network surrounding the tubules in the nephron?
The concentration gradients are maintained by the constant supply of fresh blood through the capillaries
What adaptions does the proximal convoluted tubule have and how do they help?
- Covered with microvilli, which increase the SA over which substances can be reabsorbed
- Many mitochondria which provide ATP needed for active transport of substances out of the tubule
What leads on from the proximal convoluted tubule?
The descending loop of Henle
What is the descending loop of Henle permeable and impermeable to, and what is the effect of this?
It is permeable to water, and impermeable to salt ions. This results in water leaving the descending loop, which means the ion concentration increases as you go down the descending loop
What is the hypertonic point of the loop of Henle?
The bottom of the loop, in which the salt ion concentration is at its highest, and the water potential is at its lowest
What happens to the salt ion concentration as you go up the ascending loop of Henle?
Salt ion concentration will decrease, as salt ions diffuse out of the ascending loop down the concentration gradient
In the ascending loop of Henle, what happens when salt ion concentration is isotonic with salt ion concentration in the tissue fluid?
The ions are actively pumped out into the medulla tissue
Why does water not also leave the ascending limb as ion concentration decreases (hence water potential increases)?
As the ascending limb of the loop of Henle is impermeable to water
Simplistically, how does the loop of Henle have a countercurrent multiplier system?
It works like a positive feedback system.
The pumping of salt ions out of the ascending loop causes the medulla tissue fluid to have a very low water potential.
This creates a steeper concentration gradient for water to leave the descending loop, meaning more water leaves.
This means the salt ion concentration in the tubule as you go down and around the loop of Henle will be even higher.
This increased salt ion concentration will mean even more salt ions leave the ascending loop, creating an even steeper water concentration gradient between the medulla tissue fluid and the descending limb.
And so on…
What is the result of the countercurrent multiplier system?
High salt concentration in the medulla tissue fluid, and a low salt concentration in the blood entering the distal convoluted tubule
Where does the balancing of water levels happen in the kidney?
The distal convoluted tubule and the collecting duct
How are salt levels balanced in the distal convoluted tubule?
If more salt is needed by the body, Na+ ions will be actively pumped out, and Cl- ions will follow by diffusion down the electrochemical gradient
How are water levels balanced in the distal convoluted tubule and collecting duct?
ADH will control the amount of aquaporins on the tubule, which will control the amount of water that is reabsorbed into the blood
What are aquaporins?
Channel proteins that facilitate the transport of water out of the tubule, hence making the tubule more or less permeable to water
Where is ADH secreted from?
The pituitary gland
