Excretion Bio (Pt2)
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What is the role of the kidney?
To remove waste products from the blood and to produce urine.
The kidney is supplied with blood from the ___ ___ and is drained by the ___ ___.
List the general areas of the kidney.
- Surrounded by capsule
- Outer region is cortex
- Inner region is the medulla
- In the centre is the pelvis which leads into the ureter.
What is a nephron?
The functional unit of the kidney. Microscopic tubule that receives fluid from capillaries in the cortex and converts this to urine, which drains into the ureter.
Briefly describe the structure of the nephron and its different parts.
- Starts in the cortex, where capillaries form knot called glomerulus, surrounded by Bowman's capsule. Fluid in blood pushed into capsule by ultrafiltration.
- Capsule leads into 4 parts of nephron - proximal convoluted tubule, distal convoluted tubule, loop of Henle, collecting duct.
The vessel that takes blood into each glomerulus is called the ____ ____, and the vessel that takes the filtered blood away from it is called the ___ ___.
- afferent arteriole
- efferent arteriole
- (the efferent arteriole is narrower - ensuring that capillaries of glomerulus is under increased pressure. This pressure is higher than in Bowman's capsule, so this tends to push fluid from blood into capsule.)
What are the capillaries that go across the ascending limb?
List the 3 layers that make up the barrier between the blood in capillary and the lumen of Bowman's capsule, and explain how each is adapted to allow ultrafiltration.
- Endothelium of capillaries: has narrow gaps/pores between cells allowing plasma and substances dissolved in it, to pass through.
- Basement membrane: consists of fine mesh of collagen fibres and glycoproteins - acting as filter to prevent passage of molecules with relative molecular mass of more than 69000. Most proteins and all blood cells can't get through. (MOST OF ULTRAFILTRATION)
- Epithelial cells of capsule: called padocytes have very specialised shape - having many finger-like projections called major processes - ensure that there are gaps between cells through which fluid can pass through to lumen.
What is filtered out of the blood in ultrafiltration? (5)
- Amino acids
- Inorganic ions (sodium, chloride, potassium)
Substances are removed from the fluid in nephronand reabsorbed into the blood. Most reabsorption occurs from ___ ___ ___, where about ___ of filtrate is reabsorbed. List the substances that are reabsorbed.
- proximal convoluted tubule
- All of glucose and amino acids
- Some salts
- Some water
How are the epithelial cells (cells lining PCT) are specialised to achieve reabsorption. (4)
- Plasma membrane in contact with tubule fluid is highly folded to form microvilli. Increase surface area for reapsorption.
- This membrane also contains special co-transporter proteins that transport glucose or amino acids, in association with Na+, from tubule into cell. Facilitated diffusion.
- Opposite membrane also is folded to increase SA. Also, has sodium-potassium pumps that pum Na+ out and K+ into cell.
- Cell cytoplasm has many mitochondria.
What mechanism is in place so that reabsorption occurs?
- 1. Sodium-potassium pumps remove Na+ from epithelial cells and reduce conc of Na+.
- 2. Na+ is transported into cell along with glucose or amino acids by facilitated diffusion.
- 3. Glucose and amino acids conc rise and so diffuse into blood stream and carried away. May also be enhanced by active removal of these.
- 4. Reabsorption of salts, glucose and amino acids increase w.p in tubule fluid, so water will enter cells and be reabsorbed into blood by osmosis.
- 5. Larger molecules (eg. small proteins) that were able to enter tubule, will be reabsorbed by endocytosis.
Explain what is meant by ultrafiltration.
Filtration at a molecular level. Small molecules pass through the basement membrane, which acts as a filter, while larger molecules are held in the blood.
ONTO MORE ABOUT
WATER CONTROL AND LOOP OF HENLE STUFF
What is the role of the loop of Henle?
- To create a low water potential in the tissue of the medulla, by transferring salts from the ascending limb to the sescending limb.
- This ensures that even more water can be reabsorbed from the fluid in the collecting duct.
How is a low w.p in medulla achieved by the loop of Henle?
- 1. Near top of ascending limb, salts (Na+ & Cl-) are actively puped out into the medulla. This part of ascending limb is impearmeable to water so creates a low w.p in medulla with high conc of ions.
- 2. Because of low w.p of medulla, water moves out of descending limb into medulla by osmosis. Makes filtrate more concentrated, (also cos descending limb isn't peameable to ions -check-). Water in medulla also reabsorbed into blood through capillary network.
- 3. Near bottom of ascending limb, ions diffuse out into medulla, further lowering w.p of medulla deep inside.
- [A great increase in ion conc in medulla, which lowers water potential. This causes water to move out of collecting duct by osmosis. Also water in medulla is reabsorbed into blood through capillary network.]
What is this mechanism called? And what does it mean?
- Hairpin countercurrent multiplier
- Arrangement of a tubule in a sharp hairpin so that one part of the tubule passes close to another part of the tubule with the fluid flowing in opposite directions. This allows exchange between contents and can be used to create very high conc of solutes in medulla.
How would the length of loop of Henle be different in an animal that lives in a dry habitat and why?
- Longer loop of Henle
- The longer it is, the more water they can reabsorb from the filtrate. More ions are actively pumped out by longer ascending limb, which creates very low w.p in medulla.
- More water moves out of nephron and collecting duct into the capillaries, giving very concentrated urine.
What is the role of the distal convoluted tubule?
Here, active transport is used to adjust the concentrations of various salts in fluid.
The w.p of medulla gets ___ as you go deeper into the medulla. So as collectingduct moves down medulla to the pelvis, __ water moves out by osmosis, to the tissue and into the capillaries.
Amount of water reabsorbed from the collecting duct depends on..?
the permeability of the walls of the collecting duct.
What is osmoregulation and why is it important?
- The control of water levels and salt levels in the body.
- Important to conserve water and not get dehydrated, but also to maintain the correct water balance between cells and the surrounding fluids, in order to prevent problems with osmosis.
How is the permeability of the collecting duct to water altered?
- Walls of collecting duct respond to level of antidiuretic hormone (ADH) in the blood - cells in wall have receptors for this hormone.
- ADH binds to these receptors and initiate a chain of enzyme-controlled reactions inside cell.
- End result is to insert vesicles containing water-pearmeable channels (aquaporins) into plasma membrane. Walls more permeable to water. More ADH = more aquaporins = more concentrated urine.
- If less ADH in blood, plasma membrane folds inwards to create new vesicles that remove aquaporins from membrane.
Where and how is low water levels in body detected?
- Water potential of blood is monitored by osmoreceptors in the hypothalamus of brain.
- Osmoreceptors probably detect low w.p because of effects of osmosis. When w.p is low, osmoreceptors lose water by osmosis and shrinks, and this stimulates neurosecretory cells in hypothalamus.
How does this neurosecretory cell adjust ADH level in blood?
- These are specialised neurones that produce and release ADH.
- ADH manufactured in cell body, which lies in hypothalamus. ADH flows fown axon to terminal bulb in posterior pituitary gland. Stored there until needed.
- When neurosecretary cells are stimulated, they send an action potential down axon and cause release of ADH.
- ADH enter blood capillaries running through posterior pirtuitary gland - transported around body and acts on target cells in collecting duct.
ADH is slowly broken down. It has a half-life of about ___ mins. What is half-life?
- Half-life of substance is time taken for its conc to drop to half its original value.
What are some of the common causes of kidney failure? (3)
- Diabetes mellitus (both type 1 & 2)
- Hypertension (very high blood pressure) - can damage glomeruli as they are already under high pressure and capillaries get damaged. Proteins and other large molecules may be in urine.
- Infection - inflammation damaging cells
List some of the problems created by kidney failure. (4)
- Waste products that kidneys usually remove (eg urea) start to build up in blood.
- Fluid start to accumulate in tissue because kidneys can't remove excess water - body parts swell.
- Unbalanced amount of ions in body. May become too acidic, an imbalance of calcium and phosphate can lead to brittle bones. Salt build up cause water retention.
- Long-term kidney failure causes anaemia - lack of haemoglobin.
What are the 2 main treatments for kidney failure?
- Renal dialysis
- Kidney transplant
Describe the idea behind the most common treatment - renal dialysis.
- It removes wastes, excess fluid and salt from blood by passing the blood over a dialysis membrane - a partially permeable membrane, allowing exchange substances between blood and dialysis fluid.
- This fluid contains the correct conc of salts, urea, water and other substances in blood plasma.
- So, any that are low in conc will diffuse into the blood, and any substances in excess will diffuse across membrane into dialysis fluid.
- Must be combined with a carefully monitored diet.
Describe the 2 types of dialysis.
- Haemodialysis: blood passed into machine that contains artificial dialysis membrane, then retuerned via vein. Hepartin used to avoid clotting Usually performed in hospital for 3 sessions a week, in 3-5hr sessions. Quite expensive and inconvinient. Also, patient usually feel increasingly unwell between dialysis.
- Peritoneal dialysis (PD): Body's own abdominal membrane is used as filter. Called ambulatory PD because patient can have dialysis daily while walking around.
Give advantages and disadvantages of dialysis.
- Ad: Much less risk than transplant. Can keep patient alive until transplant.
- Disad: Have to visit hospital regularly for sessions that last several hours. Also expensive. Patient also feel unwell between dialysis. Diet restricted.
Describe kidney transplant.
A major surgery is undertaken under anaesthesia, in which surgeon implants new kidney (from donor with same blood and tissue type) into lower abdomen and attach to blood supply.
Give advantages and disadvantages of kidney transplant.
- Ad: More convenient for person
- Cheaper than to keep them on dialysis for a long time
- Don't have problem of feeling unwell between dialysis
- Disad: Major operation which is risky
- Need immunosuppressants to supress immune system rejecting kidney (more susceptable to infections)
- Frequent checks for signs of organ rejection
Describe how urine samples can be used to test for pregnancy.
- Human embryo starts secreting hormone called human chorionic gonadotrophin (hCG) - a small glycoprotein with mass 36700 (lower than 69000).
- Monoclonal antibodies, tagged with blue beads, will bind to hCG.
- The hCG-antibody complex move up strip until it sticks to band of immobilised antibodies and all are held in one place, forming a blue line.
Describe how urine samples can be used to detect misuse of anabolic steroids.
- Anabolic steroids increase protein synthesis in cells, and banned from major sporting bodies. (also have dangerous side effects)
- They have half-life of about 16hrs so stay in blood for several days.
- Analyse urine sample using gas chromatography or mass spectrometry.
- Sample is vaporised and each substance moves at different speeds along column with liquid agent. Can compare time taken for substances in sample to pass through sample with time taken for steroid to pass through.
- Also comparison of where the drug marks on the chromatogram.
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