Excretion (kidneys) Flashcards
What are the functions of the kidney?
- Excretion: Removal of nitrogenous waste.
- Osmoregulation: Control of blood/body water potential.
- Regulation of blood pH.
What is the structure of the kidney?
- Outside of kidney surrounded by fibrous layer called the capsule.
- The outermost layer is the cortex and is where the renal corpuscules, PCTs (Proximal Convoluted Tubules) and DCTs (Distal Convoluted Tubules) are located.
- The second layer is the medulla and is where the Loop of Henle and collecting ducts are located.
- The innermost layer is the pelvis, which is where the renal artery and vein connects. It leads to the ureter.
What is the structure of a nephron?
- A nephron starts with the Bowman’s capsule, which contains the glomerulus and is where the glomerular filtrate is formed.
- Filtrate then enters the PCT, where selective reabsorption occurs and most solutes are reabsorbed.
- Filtrate then flows into the descending limb of of the loop of Henle, where water is removed by osmosis into surroundings.
- Filtrate flows into ascending limb which is split into the thin and thick sections. The thin section is permeable to water but the thick is not, and is where active transport of NaCl happens.
- Filtrate then flows into DCT where more adjustments are made to the salt concentrations.
- Filtrate flows into the collecting ducts which reabsorbs more water (as needed).
- Collecting ducts join at pelvis which leads into ureter.
What is the structure of blood vessels in the kidneys?
- Blood enters kidneys through renal artery.
- Renal artery branches off into smaller arterioles (afferent arterioles).
- Afferent arterioles branch further to form a network of fine capillaries called the glomerulus, which sits inside Bowman’s capsule.
- Capillaries recombine to form efferent arterioles which leads away from Bowman’s capsule.
- Blood then joins network of capillaries called the peritubular capillaries which wrap around the nephron and reabsorb substances.
- Capillaries join together to form venules, which combine to form renal vein; taking blood away from kidneys.
How does filtrate content change throughout nephron?
- 85% of the filtrate is reabsorbed along the PCT (isotonic)
- Water potential decreased along descending limb as salts diffuse in and water diffuses out.
- Water potential increases again along ascending limbs as water is prevented from leaving and NaCl is removed (hypotonic).
- Water potential decreases agains as water leaves the collecting ducts by osmosis.
- Concentrated (hypertonic) urine is formed
How is high pressure generated in the glomerulus?
The afferent arteriole is much wider than the efferent arteriole, which creates a bottleneck effect in the glomerulus. As blood enters at a higher rate than can be removed, pressure increases beyond pressure inside Bowman’s capsule, which creates pressure gradient for ultrafiltration.
What are the filtration barriers during ultrafiltration?
- Endothelium of capillaries consist of gaps which fluid is able to pass through. Offers no real barrier as blood cells can easily squeeze past.
- Basement membrane (made from fine collagen fibres) holding together endothelium is only real filtration barrier. Only molecules small enough are able to fit through gaps. Red blood cells and large plasma proteins cannot fit through and remain in capillaries.
- Podocytes make up epithelium of Bowman’s capsule. They have finger-like projections called major processes. These ensure there are gaps between the cells which allow glomerular filtrate to freely enter Bowman’s capsule.
What is the content of glomerular filtrate?
- Water.
- Glucose.
- Amino acids.
- Vitamins.
- Hormones.
- Urea.
- Inorganic ions (salts).
- Small proteins.
How much of each substance is reabsorbed into the blood from PCT?
Glucose - All Amino acids - All Water - Most Urea - Some Inorganic ions - some Small proteins - All Vitamins - All Hormones - Some
How are epithelial cells lining the PCT adapted for selective reabsorption?
- Microvilli increases the surface area which results in larger exchange surface and greater rate of diffusion.
- Plasma membrane contains lots of co-transporter proteins used to reabsorb glucose and salts.
- Contains lots of mitochondria to produce energy as ATP for use in active transport.
- One layer thick to minimise diffusion distance.
- Nuclei contain instructions to manufacture transport membranes.
- Basal membranes on capillary side of cells also folded to increase surface area.
- Tight junction between cells to force filtrate through epithelial cells to control absorption rather than leaking into capillaries directly.
How is water reabsorbed in the PCT?
Due to ultrafiltration, there is a very low water potential in the capillaries next to epithelial cells, much lower than the glomerular filtrate (especially due to Na+ and glucose being removed). Water potential gradient set up across the epithelial cells between capillaries and PCT which results in water moving out of PCT, across epithelial cells and into blood by osmosis.
How is glucose and Na+ ions reabsorbed?
- Sodium-potassium pump in basal membrane pumps Na+ ions out of epithelial cells by active transport, which maintains low Na+ concentration in cell.
- Na+ ions diffuse into epithelial cells through co-transporter proteins, down concentration gradient.
- Movement of Na+ ions provides energy for glucose to be pumped into epithelial cells, even against concentration gradient (indirect active transport).
- Concentration of glucose increases in cell so it diffuses out and into tissue fluid. It then diffuses into blood and is carried away.
- This process may be accelerated by active transport.
How are amino acids reabsorbed?
Amino acids also enter the cells through co-transporter proteins with Na+ ions and then diffuse into blood down concentration gradient. This process can also be accelerated through active transport.
How are larger molecules like small proteins reabsorbed?
Endocytosis.
What is the headpin countercurrent multiplier effect?
The arrangement of tubules in a tight hairpin shape. This means that fluid is always flowing in the opposite direction to each other and there is always a concentration gradient between the fluid in the two limbs of the loop of Henle. This makes it much easier for salts to be transferred from one limb to the other, and thus lowering the water potential of the medulla.
