3.7 Homeostasis And The Kidney Flashcards
What is homeostasis
Maintenance of a constant internal environment
Why is homeostasis so important
Cells can function efficiently, independently of the fluctuations in the conditions of the external environment
Some examples of homeostasis
Core body temperature
Glucose levels
Solute potential
How does negative feedback work
Corrects deviations from the set point
Examples of negative feedback
Insulin
Antidiuretic hormone
Sequence of events that go on for a negative/positive feedback to happen
Input is detected by a receptor which monitors the condition and provides input to the control centre which then evaluates information and provides output to an effector which will initiate a response designed to take away the deviation and restore the norm
How does positive feedback work
By enhancing the size of the stimulus
Examples of positive feedback
Oxytocin during childbirth
What parts of the nephron are in the cortex
Bowmans capsule
Proximal convoluted tubule
Distal convoluted tubule
What parts of the nephron are in the medulla
Descending limb
Loop of henle
Ascending limb
Collecting duct
Simply, what happens during ultrafiltration
Urea is removed from the blood as the glomerulus and Bowmans capsule work together to filter the blood under high pressure
How is there high pressure during ultrafiltration
The Blood supply to the glomerulus enters through the afferent arteriole and leaves through the efferent arteriol
The efferent is narrower than the afferent which increases the hydrostatic pressure of the blood plasma
3 part filter
• endothelium of capillaries of Glomerulus has fenestrations between the cells to increase filtration
• podocytes of Bowman’s Capsule have finger like projections called pedicels which wrap around the capillaries, the filtration slits between pedicels allow free passage of the filtrate from blood in the lumen of the Bowman’s capsule but not proteins and cells as they’re mostly too large
• basement membrane of capillaries and podocytes act as a selective barrier which allow water and small molecules to pass from the blood into the nephron
How is the efficiency of ultrafiltration increased
• pedicels increase surface area
• there is a short diffusion distance between podoctyes and the capillary
• filtration slits between pedicels increase the concentration gradient between the tissue fluid
What is the mechanism of ultrafiltration
• the large hydrostatic pressure causes small soluble molecules such as glucose, amino acids, salt, water and urea to be forced out of the plasma and they enter the channels between the pedicels and the capillary walls. The concentration then increases in the channels do they enter the Bowman’s capsule and form the glomerular filtrate
• the basement membrane of the capillary acts as a selective barrier between the blood and the nephron and allows small sized particles through whilst plasma proteins and cells remain in the blood due to their larger size
Net filtration pressure of ultrafiltration
The hydrostatic pressure is greater than the combined effect of the osmotic pressure of the plasma and fluid pressure of the filtrate so water and small molecules pass out of the plasma
What is osmoregulation
Control of the water content and solute composition of bodily fluids such as blood, tissue fluid and lymph
What is excretion
Elimination of waste products of metabolism from an organism
What biological molecule is used in excretion and why
Amino acids cannot be stored so excess amino acids are deaminated in the liver so the amino group forms ammonia which is converted to urea and transported into the blood plasma
Where does selective reabsorption occur
Proxima convoluted tubule
What is reabsorbed during selective reabsorption
All glucose
All amino acids
Most water
Most salts
Adaptations for selective reabsorption
• cells lining the tubule have a large surface area due to the microvilli and basal channels and also due to the length and large numbers of nephrons per kidney
• cells contain many mitochondria which provide ATP for active transport
• the close association with capillaries provides a short diffusion distance between cells and peritubular capillaries
• tight junctions between adjacent cells prevent the seepage of reabsorbed materials back into the filtrate
How are mineral ions and salts reabsorbed?
Facilitated diffusion and active transport into the epithelial cells
How are glucose and amino acids reabsorbed?
Secondary active transport using a co-transport mechanism with Na+. Glucose is co-transported with 2 Na+ by facilitated diffusion into the cell. Na+ and glucose move separately into capillaries
How is 90% of water reabsorbed?
Osmosis
How are some filtered proteins and some urea reabsorbed
Diffusion
What is the concentration of the filtrate at the end of the PCT, relative to the blood plasma
Isotonic
Where does osmoregulation occur
Loop of Henle
Permeability of ascending and descending limbs to water
Ascending - impermeable
Descending - permeable
Counter-current multiplier
- Na+ and Cl- are actively pumped out of the filtrate in the ascending limb into the tissue fluid causing a low water potential in the tissue fluid
- This causes water to leave the descending limb by osmosis and is carried away by the vasa recta
- Contents of the descending limb become more concentrated as they reach the tip of the Loop of Henle due to the loss of water
- As the filtrate passes up the ascending limb it becomes more dilute due to loss of ions
Simply, the role of antidiuretic hormone
Role in negative feedback restoring the normal osmotic concentration in the blood
How is antidiuretic hormone released
It is secreted by the hypothalamus and stored in the posterior lobe of the pituitary
What happens in terms of ADH when someone is dehydrtated
• it is detected by the osmoreceptors in the hypothalamus which triggers the posterior pituitary gland to release more ADH
This increases the permeability of the distal convoluted tubule and collecting duct so that more water is reabsorbed into the blood and more concentrated urine is produced
Causes of kidney failure
Diabetes
High blood pressure
Auto-immune disease
Infection
Crushing injuries
In general, what do all kidney failure treatments try to achieve
To balance the fluids in the blood
What are some treatments for kidney failure
Medication to control blood K and Na levels
Low protein diet
Drugs to lower blood pressure
Dialysis
Kidney transplant
Why must the medication to control blood K and Na levels be controlled
Can lead to heart disease and kidney stones
How is a low protein diet a treatment for kidney failure
It reduces the concentration of excess amino acids hence the concentration of the yrea
Examples of drugs to lower blood pressure
• beta blockers which reduce the effect of adrenaline
• calcium channel blockers which dilate blood vessels which lowers the blood pressure
• ACE inhibitors which reduce the effect of angiotensin which cause the blood to restrict
What are the two kinds of dialysis
Haemodialysis
Peritoneal dialysis
Haemodialysis
• a dialysis machine removes excess water, urea and ions from the plasma as fresh dialysate is constantly moving in opposite direction to that of the blood flow to create a counter-current system so equilibrium is not reached
• Hepain is used to prevent blood clotting
• this takes several hours and is repeated several times each week
Pertioneal dialysis
• dialysis fluid is passed into the peritoneum through a catheter. The peritoneum contains numerous capillaries which exchange materials with dialysis fluid which is changed after about 40 minutes
• this is repeated several times a day
• the patient can move around
• this is less effective than haemodialysis and can cause fluid retention
Kidney transplant as treatment
This is a final resort
The transplanting one kidney from closely matched donor
Immunosuppressants are used afterwards to prevent rejection
What is the excretory product of freshwater fish and why?
Ammonia
This is highly soluble and toxic so cannot be stored
This is excreted immediately using large volumes of water to dilute it
The large volumes of water is freely available to freshwater fish
What is the excretory product of Birds. Reptiles and insects and why?
Uric acid
This is virtually non-toxic
Not much water is needed to dilute it - advantageous to birds as a reduced weight in flight
The conversion of ammonia to uric acid requires much energy but allows these to survive in arid environments
What is the excretory product of mammals and why?
Urea
Much less toxic than ammonia so requires less water to dilute it and can be stored
It required energy to convert ammonia to urea but is an adaptation to life on land as it helps to prevent dehydration because less water is needed to excrete it
What does the length of Loop of Henle correspond to?
The typical availability of water
Beavers length of Loop of Henle and effect of this
Short loop of Henle
Live in freshwater and so water is plentiful
Large volumes of dilute urine produced
Kangaroo rat length of Loop of Henle and effect of this
Long Loop of Henle
Live in arid environment
Small volumes of highly concentrated urine due to higher ion concentration in medulla is created by counter-current multiplier
Higher proportion of juxtamedullary nephrons where the Bowman’s capsule is nearer to the medulla and the Loop of Henle penetrates deep into the medula
Camel water supply
Metabolic water
They largely respire fat stored in its hump
Some behavioural adaptations to minimise waterloss
Nocturnal
