Excretion and osmoregulation Flashcards
(25 cards)
Blood supply in the kidneys
2 capillary beds connected by arteries and veins
Glomerulus and peritubular capillaries
Maintains constant blood flow through and around the nephron despite fluctuations in systemic blood pressure
Filtration
Ultrafiltrate forms in the Bowman’s capsule
Fenestrated capillaries allow most molecules passage but not cells or large proteins
Both cell layers contain negatively charged glycoproteins
Podocytes (modified epithelial cell) integrate forming a filter bed
Passes into the convoluted tubule
Secretion occurs across the nephron with the majority happening in the distil convoluted tubule
Relationship between pressure and filtration
No pressure = no filtration
If blood pressure rises the afferent arteriole restricts in order to protect the filtration blood
A drop in systemic blood pressure results in less glomerulus filtration and vasodilation of the smooth muscle cells in the afferent arteriole
Efferent and afferent arterioles
Afferent arteriole is narrower than the afferent
Afferent arterioles bring blood into the glomerulus while efferent arterioles carry blood away
Reabsorption
Once inside the lumen of the nephron, small molecules like ions, glucose and amino acids are reabsorbed from the filtrate back into circulation
Utilises co-transporters, active transport, osmosis, solvent drag and passive diffusion
90% of glucose is reabsorbed at the proximal convoluted tubule, it binds to SGLT-2
No glucose in the loop of henley, DCT or collecting duct
Factors affecting reabsorption
Rate of flow of the filtrate
The concentration of small molecules in the filtrate
There are a fixed number of transports in the nephrons
Counter-current multiplier system
Thin descending limb which is permeable to water and impermeable to salt
Thick ascending limb which is impermeable to water and actively transports salt
Renal physiology - how are ions and water regulated?
Central and autonomic nerves
Cardiac output
Blood vessel tone
Renal function
ADH
Catecholamines (a group of chemicals that act as hormones and neurotransmitters eg dopamine)
Renin-angitensin-aldosterone system (a hormonal system that regulates blood pressure, fluid and electrolyte balance and vascular resistance)
Secretion into the lumen of the tubule
Ammonia is secreted from plasma to the proximal convoluted tubule lumen
Ammonium is secreted from the PCT cytosol into the PCT lumen
There are specific transporters involved in this process
Renin-angiotensin-aldosterone signalling
Activated by dehydration, sodium deficiency
Renin acts to increase blood pressure
Increase in angiotensin I
Angiotensin II has multiple sites of action
Angiotensin is secreted from the liver and levels are increased by plasma corticosteroid, oestrogen, thyroid hormone and angiotensin II levels
It aids the secretion of aldosterone
Causes constriction of systemic arterioles
Increased sodium and water reabsorption and increased secretion of K+ and H+ into urine
This increases blood volume and blood pressure will increase until it returns to normal
Causes release of ADH
How does the kidney correct pH?
The kidney must reabsorb the filtered bicarbonate and excrete the daily acid load
It achieves this by hydrogen carbonate reabsorption, H+ secretion and ammonium secretion
Factors affecting blood pH
Diets rich in meat make the blood more acidic
Diets rich in fruit and veg are rich in bicarbonates
Exercising muscles produce lactic acid that must be eliminated from the body or metabolised
Respiration variations in gaseous exchange can effect blood pH
Acid-base balance
In humans the pH of extracellular fluid is 7.35-7.45
If plasma levels fall below 7.35 it results in acidaemia
If they rise above 7.45 it results in alkalemia
Can result in coma, cardiac failure and circulatory collapse
In biological terms pH is a mixture of H+ ions and sodium bicarbonate which acts as a buffer
Marine sharks
Their body fluid has a slightly higher osmotic concentration than the environment
NaCl concentration higher in the seawater than in the shark
Body fluid contains urea
If the environment is isosmotic or slightly hypoosmotic there’s some passive water gain
They will gain salt from the outside environment due to the concentration gradient
Nitrogen excretion is urea
Amphibians
Mostly freshwater
Live in a hypoosmotic environment
Passive water gain and loss of salt
Active salt uptake via skin
Produce a large amount of dilute urine
Bladder can be used as a storage organ for water
Nitrogen excretion in aquatic organisms like tadpoles is ammonia, adults secrete urea
Reptiles
No loop of Henle
Marine reptiles obtain a high salt load through the environment and food, salt gland in head
Nitrogen excretion in terrestrial reptiles is uric acid, in turtles urea and in marine reptiles ammonia
Urine formation in birds
Kidneys have a mixture of looped and loop-less nephrons
Marine birds have a salt gland they can use to get rid of excess salt
Factors affecting the mechanisms of excretion and osmoregulation
Phylogenetic group characteristics like organ systems, air or water breather and skin permeability
The environment (salinity, temperature, water availability)
Salt and water content of food
Isosmotic environment
Equal concentrations inside and outside of the cell
Hyperosmotic environment
Higher salt concentration in the environment than inside the cell
Loss of water
Hypoosmotic environment
Lower salt concentration in the environment than inside the cell
Excretion in vertebrates
All vertebrate groups have kidneys
Primary urine - ultrafiltration and secretion
Definite urine - reabsorption of water, salts and nutrients, secretion of waster products and toxins
Concentration of urine vs blood plasma (U:P ratio)
1 - urine and plasma have the same concentration
<1 Urine has a lower concentration than plasma, it’s dilute
>1 urine has a higher concentration than plasma, it’s concentrated
Only mammals and birds have a Loop of Henle, allowing them to produce concentrated urine
Teleost fish (freshwater)
Hypoosmotic environment
Freshwater fish will passively lose salt and gain water
To get rid of excess water they produce large amounts of dilute urine
Water enters osmotically though the gills + active absorption of salt
Ammonia is main nitrogen excretion product
