Homeostasis (II) Excretion and Osmoregulation P.1
Homeostasis (II)Excretion and Osmoregulation
E
xcretion, the elimination of metabolic w_____ products, is a vital homeostatic process. The main excretory materials in animals are carbon dioxide and water from respiration, and nit______-containing wastes derived from the breakdown of surplus protein. In certain circumstances these materials may perform useful functions; for example, an animal living in hot dry conditions normally uses m______water to maintain its body's water balance.
I)Nitrogen Excretion
In mammals, amino acids derived from body pro____breakdown / tissue replacement, together with those taken in surplus, cannot be stored and so are transported to the l____. There, the nitrogen-containing a_____ group is removed (deamination), and the carbon-containing residue, a keto acid, is oxidized or undergoes conversion into a storage material by entering theres _ pathway at an appropriate point. Deamination can be summarised as follows:
R.CH.NH2COOH + 1/2 O2 R.CO.COOH + NH3
amino acid keto acid ammonia
Ammonia is highly t____ and must either be removed from the body rapidly or converted into a less toxic compound, usuallyurea or uric acid. All three compounds are normally present in the wastes of animals but their proportions vary according to the type of animal groups and habitat preferences.
Q. Why do you think fish excrete nitrogen waste in the form of ammonia, while insects and most terrestrial reptile usually do so in the form of uric acid?
II)The Mammalian kidney
Each kidney consists of an outer c_____ and an inner m_____. Kidneys receive blood from the r____ artery which branches off from the aorta; blood leaves the kidney via the r____ vein.
Each human kidney is estimated to contain around one million nephrons. Each nephron consists of:- Malpighian body
- proximal convoluted tubule,
- Loop of Henle
- distal convoluted tubule,
Major Processes involved in kidney functioning :
1.Ultrafiltration -- The high hy pressure of the blood in the gl forces water, ions and sm____ molecules such as glucose, amino acids and urea, through the wall of the capillary into the Bowman's capsule.
2.Reabsorption -- As the filtrate passes along the tubule many of the use___ constituents are reabsorbed into the c______network that surrounds the tubule.
- Glu_____ and a_____ acids are reabsorbed in the pro_____ convoluted tubule.
- Mineral s___ are reabsorbed in bothcon_____ convoluted tubules;
- W____ is reabsorbed in both regions.
b. Active Transport:Itis involved in the reabsorption(glucose and many ions)and active secretion(wastes such as urea and some ions).
The rateof active transport can be reduced by resp______-inhibitor such as cyanide. The presence of numerous m______in the epithelial cells of the tubule provide energy for the transport process. /
3. Active secretion--remaining wastes in the plasma are act _secreted from the capillary into the d______convoluted region of the tubule.
A)Ultrafiltration in the Bowman's Capsule
a)Ultrafiltration
This is thefilt of blood from the glomerular c______into the space of the B______’s capaule under hydrostatic pressure. There are two factors for ultrafiltration to functions efficiently :- an effective barrier (filter) between the blood and the capsular space that retain substances of large molecular mass yet permitting small molecules to pass through.
- a very high hydrostatic pressure that forces fluid through the filter.
There arethree layers of structures in between the blood within the glomerulus and the capsular space of the Bowman’s capsule:
1) The singlecell layer - endo______of the capillary,
2) The b______membrane of the glomerulus,
3) The wall of the Bow____’s capsule– note the large gap between the cell (podocytes) /
Q.All three layers are porous. Which one of these has the smallest pore-size ?
Which layer is mostly likely the effective barrier for ultrafiltration ? /
ii)a High Blood Pressure at the Glomerulus
This high pressure is maintained because in each Bowman's capsule the a arteriole has alarger diameter than the efferent a .
The diameter of the afferent and efferent arterioles are undern and h control. Further cons______ of the efferent arteriole will raise the hydrostatic pressure in the glomerulus and speed up the formation of glomerular f______.
The hydrostatic pressure of the blood is opposed by two forces :- The colloidal solute / osmotic potential exerted by the plasma proteins of blood
- the hydrostatic pressure of the glomerular filtrate
Hydrostatic Blood pressure -
(colloidal solute potential + glomerular filtrate hydrostatic pressure)
= KPa - ( + ) KPa
= KPa
As a result of this effective net pressure, substances are forced through the endo______pores of the capillary, across the b______membrane and into the B______'s capsule by ultrafiltration. /
The glomerular filtrate contains substances with a relative molecular mass less than 68,000. e.g. g______, a_____ a_____, v______, some h______, u____, i____ and w_____.
Remaining in the blood, along with some w_____, are r___ and w______corpuscles, pl______and p______p______which are too large to pass the filter provided by the basement membrane.
Ul______is a p_____ process and selection of substances into glomerular filtrate is made entirely according to relative molecular m___. In the later stages of re______and active s______, both passive and active processes are involved.
B)Reabsorption and Active Secretion in the Proximal Convoluted Tubule
a)REABSORPTION
It is the movementof substances out of the renal t______ and into the surrounding / peri______capillaries. Many substances are wholly or partially absorbed depending on body needs.
i)Adaptations of the Proximal Convoluted Tubule (pct) for Reabsorption
The ep______cells of the tubule are specially adapted for reabsorption.
Over80% of the glomerular filtrate is reabsorbed here, including all the f____ substances(glucose and amino acids) and most of the s______chloride and w_____[1].
- It is the longest region of the nephron. It is also highly con______so that filtrate flows sl_____ allowing more time for reabsorption.
- The blood p______in the peritubular capillary is low, this favours reabsorption.
- It comprises a s_____ layer of epithelial cells, with numerous m______forming a brush border. The base of each cell is con______ where it is adjacent to a blood capillary and there are numerous inter______spaces.
- Large number of m______provide the ATP necessary for a______transport.
ii)Mechanisms of Reabsorption
Reabsorption occurs as a result of d______, o______, a_____ and p_____ transport.1)Actively reabsorption of : Glucose, amino acids, vitamins, sodium, calcium and potassium
- Amino acids, glucose and ions diffuse into the cells
- they are then a transported into the intercellular spaces
- from there theydiffuse into the surrounding capillaries.
- The constant removal of these substances from the cells of the convoluted tubule create a g______for dfffusion that causes them to enter from the lumen of the tubule into the epithelial cells.
2)Passive reabsorption of Water -- by Osmosis
- The active uptake of Na+(accompanied by anion, eg. Cl-), lower the o______p______in the cells and water enters them by osmosis.
- The pct is always p to water, along with 90% of Na+ reabsorption about 80% of the water is reabsorbed here by o .
- This ‘obligatory reabsorption’ of water occurs regardless of the degree of hydration of the body.
- About half the u present in the tubular filtrate also returns to the blood by diffusion. Proteins of small molecular mass which may have been forced out of the blood in the Bowman's capsule are taken up at the base of the microvilli by p .
b)ACTIVE SECRETION
This is the a_____, sel______movement of a substanceout of the p______capillaries and into the renal t_____. This process allows the kidney to increase its efficiency in clearing certain substances from blood plasma. e.g. H+, K+, urea, creatinine and ammonia.
Besides removing undesirable and excessive substances, this process is important in regulating blood p . e.g. when plasma pH decreases, more H+ ions are actively secreted by the tubular cells and more bicarbonate and K+ ions are reabsorbed.
Homeostasis (II) Excretion and Osmoregulation P.1
C)The Loop of Henle and the Production of Hypertonic Urine
TheLoop of Henle enables mammals to produce urine which could be hy______to blood.In mammals, the concentrationof urine isfound to be directly related to the l______of the Loop of Henle.
The Loop of Henle is s____ in semi-aquatic mammals which have a correspondingly narrow medulla, and extremely l___ in desert dwelling mammals such as the desert rat. /
Desert rat produces a small v______of urine, ten times more c______ than that produced in large volumes by a beaver.
In between theloops is the interstitial region of the medulla which contains b_____ vessels and connective tissues wrapping around the limbs of the loop.
The countercurrent multiplier hypothesis: /
The d______limb of the Loop of Henle is permeable to w but relatively impermeable to salt.
The wall of the as______limb is impermeable to water. Along the length of the a limb,s____ are removed from the filtrate by a __ transport.
The salts pass into theinterstitial region of m (with blood vessels called vasa recta) between the two limbs, making the i______regionvery con______, especially towards the apex of the Loop.
The salt build up causesw____ to be drawnout of the d______limb by o______.
The difference in the osmotic concentration between the ascending and descending limbs at any one level is small but over the whole length of the Loop these have a cum effect.The longer the Loop, the greater the diff______in concentration. The fact that the fluid in the two limbs flows in opposite directions, and the effect is cumulative, gives rise to the termcountercurrent multiplier.
The following features enable the Loop of Henle to functions as a countercurrent multiplier:
- The ___-shaped arrangement of the descending and ascending limb,
- The descending limb being permeable to w______but not to salt,
- The active s___ removal mechanism of the ascending limb
As a result, salt becomes concentrated in the i medulla with i______concentration near the a___ of the loop.
The Loop of Henle does NOT just directly reabsorb water from the filtrate into the blood stream,it causes a build-up of s______chloride in the m______and this results in the movement of ______out of the c______ducts by o______.
- How is the osmotic concentration of the filtrate compared to the blood before it enters the Loop of Henle?
Q.Despite the fact that water is being removed from the the Loop of Henle, the Loopshould not be taken simply as a region for the reabsorption of water in the process of making hypertonic urine. Thefiltrate after passing through the Loop of Henle actually is hypotonic to blood.’ Explain how the filtrate actually became hypotonic after passing through the Loop of Henle.
Thus the production of Hypertonic urine depends on
- A countercurrent multiplier system exists in the l____ of Henle which con______salt and raisesosmolarity of tissue fluid in the renalm______; the region around the bottom of the Loop of Henle contains the h______concentration of salt.
- Osmotic uptake occurs in which the surroundinghypertonic tissue extracts water from the f______moving down the l_____ofHenle and the c______duct.
In addition, it is now believed that urea also contributes significantly to the hypertonicity of the medulla tissue. By the time the filtrate reaches the collecting duct, the urea concentration is already quite high--largely due to the continued loss of water from the tubule. The lower portion of the collecting duct is permeable to urea so it diffuses from the collecting duct into the interstitial region. The increased osmolarity around the surrounding tissue helps to draw water out of the descending limb of the loop of Henle.
Events occurring in different parts of the renal tubule and the formation of hypertonic urine:
Region / Permeability to water / Permeability to salt (NaCl) / ConsequencesDescending limb of the loop of Henle / Permeable to Water / Relatively impermeable to salt / Removal of water from tubule by osmosis
Filtrate becomes increasingly concentrated
Bottom of loop / / / / / Highest concentration of salt in filtrate is reached
Ascending limb of the loop of Henle / Impermeable to water / Active transport of salt out of the ascending limb / Progressive decrease in osmolarity in filtrate
Countercurrent multiplier effect : the longer the loop, the greater the osmolarity build up at the bottom of the loop.
Urea is concentrated due to loss of water in the pct.
Distal tubule and Collecting duct / Permeability to water controlled by ADH / Urea diffuses out of collecting duct helps to promote hypertonicity of tissue which draws water out of the descending limb of the loop of Henle.
Osmotic uptake : Water removed from the renal fluid in distal tubule and collecting duct as it passes through increasingly hypertonic tissue fluid in the medulla
Amount of water reabsorbed is promoted by ADH which increase permeability of tubule wall to water
D)Distal Convoluted Tubule
The cells in this region are very similar to those of the pct, having a b______border and numerous m______. The permeability of their membranes is affected by hormones and so precise control of the salt and water balance of the blood is possible.
The distal convoluted tubule alsocontrols the pH of the blood, maintaining it at 7.4 by excreting h______ions and retaining hydrogen carbonate ions if the pH falls, and the reverse if it rises. As a result the pH of the urine may vary between 4.5 and 8.5.
E)Collecting Duct
The p of the walls of the collecting duct, like those of the distal convoluted tubule, are affected by hormones. This hormonal effect, together with the hypertonic interstitial fluids built up by the loop of Henle in the medulla, determine whether h______or h urine is released from the kidney. If the walls of the collecting duct are water-p______, water leaves the ducts to pass into the hyperosmotic surroundings and c______urine is produced. If the ducts are impermeable to water the final urine will be less concentrated.
F)Anti-diuretic Hormone (ADH) & Osmoregulation
The body maintains the osmotic potential of the blood at an approximately steady state by balancing the water uptake from the diet with water loss by our body. The precise control of osmotic potential, however, is achieved by effect of ADH on the p of the dct and the collecting duct.
When osmotic potential of blood falls, osmoreceptors in the hypo are stimulated, nerve impulses are sent to the p pituitary where ADH is released. ADH increases the permeability of the dct and the collecting duct to water. A larger p of water is reabsorbed into the peritubular capillaries of the medulla and a re ___ volume of h______urine is produced.
The mechanism by which urine is concentrated is illustrated below :
Q. ADH also increases the permeability of the collecting duct to urea. What would you suggest about the possible functional significance of this ?
Treatment of Kidney Diseases (reference notes)
Kidney failure will lead towaste products remain in the blood, causing chemical imbalance. This would cause the heart and other organs to dysfunction.
I)Dialysis
Dialysis involves the removal of waste products and excess water and minerals from the blood. The basic principles of dialysis involve a selectively permeable membrane (dialysis membrane) and a special solution called dialysate (dialysis solution).
The dialysis membrane acts as a filter. It keeps the blood and dialysate apart from each other. Its pores allow water molecules and certain dissolved substances in theplasma to pass through but not the blood cells and plasma proteins. The dialysate has a solute composition and concentration similar to the normal plasma,but without any waste products. Therefore, useful substances are retained in the blood.
There are two types of dialysis treatment currently inuse:peritoneal dialysis and haemodialysis.
A.Peritoneal dialysis
The peritoneum is a thin membrane that lines the inside of the abdominal walls and covers all the abdominal organs such as the stomach and liver.It has two layers, and the peritoneal cavity between the layers can be used as areservoir for dialysate.This treatment involves the infusion of dialysate into the peritonealcavity via a catheter.
The catheter is surgically inserted in the abdomen and stays there as long as the patient is using this form of dialysis.
The patient always has dialysate in the peritoneal cavity, so the blood is constantly being cleaned
. /
When blood flows through the blood vessels in the peritoneum, waste products and excess water and minerals pass from the blood across the membrane into the dialysate. In this way the peritoneum works as a "natural filter", which performs the same function as the "artificial filter" used in haemodialysis.
B.Haemodialysis
In haemodialysis, blood is drawn from the patient through a special internal graft in the arm to a dialysis machine (kidney machine). The blood then circulates to the dialyzer where dialysis takes place. The dialyzer consists of a bundle of fine fibres made of selectively permeable membranes in a tank of dialysate. The patient's blood and the dialysate flow in opposite directions that make the dialysis process more efficient.
Waste products and excess water and minerals diffuse out from the blood to the dialysate.The purifiedblood is returned to the patient. As only a very small amount of blood is in the dialyser at any given time, blood need to be circulated from patient to dialyzer for about 4 hours. Treatment is usually done 3 times per week.
II)Kidney Transplantation
The best treatment for patients with end-stage kidney failure is to transplant a healthy kidney into the patient. If the graftedkidney can survive successfully in the patient, it can take over the function ofthe two diseased kidneys. The patient can lead a normal and healthy life. However, rejection may occur from a few weeks to a few months after the operation has been performed. It because the immunesystem protects our body by attacking anything that is not recognized as foreign.
Permanent survival of the graftedkidney is more likely when the organ of the donormatches with the recipient through tissuetyping. Various drugs can be given to the patients to slow down or entirely stop the host from producingantibodies that kill the transplant. /After an organ (e.g. kidney) has been removed from a person, it is cut off the supply nutrients and oxygen. The cells will die and the organ will go rotten very soon. To prevent this, the donated organ must be kept under very low temperature to reduce the metabolic rate of the cells so that they can survive on the very small amount of nutrients and oxygen that still remain. Immersing in ice-cold water can make sure that all parts of the organ are kept under the same low temperature. However, immersing in distilled water may cause the cell to swell up by osmosis, leading to cell damage. In order to eliminate this effect, a salinesolution (0.9% sodium chloride solution), with concentration approximately equal to that of body fluids, is usually used instead of distilled water.