Excretion & Osmoregulation

Question 1

Define excretion

Answer 1

Expulsion from the body of the waste products of metabolism eg CO2, urea, uric acid, ammonia, excess water, excess mineral salts, bile pigments, oxygen (plants) etc.

Question 2

Define osmoregulation

Answer 2

Control of water and salt balance so that the concentration of dissolved substances in the body fluids remains constant

Osmotic conditions, especially concentration of various ions e.g. Na2+, K+, Cl- and water content.

Question 3

Define secretion

Answer 3

The production of substances useful to the body by cells.

Question 4

Define egestion

Answer 4

The removal from the body of undigested food and other substances, which have never been involved in the metabolic activities of cells.

Question 5

What is the significance/importance of excretion?

Answer 5

• Enables removal of unwanted by-products of metabolic pathways to prevent unbalancing the chemical equilibria of reactions.
• Removes toxic wastes that if accumulated would affect the metabolic activities of organisms e.g. may act as enzyme inhibitors.

Question 6

What is the importance of osmoregulation/osmotic control?

Answer 6

• It regulates ionic concentration of body fluids to facilitate efficiency of cell activities e.g. nervous coordination, protein synthesis, hormone production, muscle contraction, enzyme activity etc.
• It regulates the water content of body fluids.
• Enables regulation of ions that have a major influence on PH of body fluids e.g. H+ and HCO3-
• Enables removal of excess nutrients that are taken in that if allowed to accumulate would interfere with cell activities.
• Gives increased environmental independence

Question 7

What are the major problems with sea water environment?

Answer 7

• osmotic water loss
• salt gain by diffusion

Question 8

What are the major problems with fresh water environment?

Answer 8

• osmotic water gain
• salt loss by diffusion

Question 9

What is the major problem with terrestrial environment?

Answer 9

Water loss by evaporation

Question 10

Account for the absence of complex/elaborate excretory systems in plants as those in animals

Answer 10

• Toxic wastes do not accumulate because they are utilized by the plant e.g. CO2 and water are raw materials for photosynthesis while oxygen participates in respiration
• Extra gaseous wastes are removed from plant bodies by simple diffusion through the stomata and lenticels
• Most of the organic waste substances formed in plants are non harmful and can be stored in the plant tissues which are removed periodically e.g. leaves and bark
• Some plants store other wastes such as resins in organs that later fall off e.g. leaves
• Excess water and dissolved gases are removed by transpiration through the stomata
• Some plants remove waste products by exudation. e.g. gums, resins, latex and rubber
• In some plants, guttation occurs i.e. excess water with dissolved salts ooze out through hydathodes at leaf surfaces
• Organic acids which would be harmful to plants often combine with excess cations and precipitate as insoluble crystals which can be safely stored in plant cells. E.g. excess Ca2+ combines with oxalic and pectic acids to form the non-toxic calcium oxalate and calcium pectate
• Plants synthesize all their Organic requirements according to demand, leaving no excess of protein hence very little excretion of nitrogenous waste substances occurs
• The rate and amount of catabolicm is much slower and much less than that of animals of similar weight, and as a result the waste products accumulate more slowly.

Question 11

What are the Excretory products in plants?

Answer 11

• Carbondioxide, Water and Oxygen from respiration and photosynthesis respecti vely.
• Anthocyanins stored in petals, leaves, fruits, barks.
• Tannins deposited in dead tree tissues like wood and barks
• Calcium oxalates,calcium carbonates and Latex(rubber)
• Alkaloids like quinine, cannabis, cocaine, caffeine, morphine etc.

Question 12

What are Hydrophytes?

Answer 12

These are plants living completely or partially submerged in fresh water.
e.g. water lilies, water hyacinth, water lettuce, etc.

Question 13

What are Mesophytes?

Answer 13

These are plants inhabiting normal well-watered soils.

Question 14

What are Xerophytes?

Answer 14

These are plants inhabiting dry areas e.g. desert.

Question 15

What are Halophytes?

Answer 15

These are plants inhabiting areas of high salinity e.g. estuaries, salt marshes.

Question 16

State structural adaptations of xerophytes for surviving water balance (more loss than uptake from soil).

Answer 16

• Possession of extremely deep roots so as to obtain water from deep
• Shallow root system for absorbing moisture even after slight showering e.g. cactus
• Possession of fleshy succulent stems and leaves that store water in large parenchyma cells e.g. bryophylum and cactus.
• Reducpion in stomata number To reduce on transpiration.
• Possession of stomata sunken and hairy leaf surface to trap air and reduce on transpiration.
• Rolling / curling / folding of leaves to reduce Transpiration e.g. marram grass (Ammophila)
• Hairy epidermis for reflecting solar radiation and trapping humid air next to leaf surface and reduce transpiration.
• Possession of thick cuticle, which is impermeable to water e.g. prickly pear (Opuntia).
• Reduction of surface area over which transpiration has to occur by having small leaves.

Question 17

State physiological adaptations of xerophytes for surviving unfavourable water balance (more loss than uptake from soil).

Answer 17

• Reversal of the normal stomatal rhythm in some
• Increased levels of abscisic acid, which induces stomatal closure so as to reduce water loss.
• Possession of tissues tolerant to desiccation e.g. low solute potential of cytoplasm and production of resistant enzymes.
• Leaf fall in deciduous trees so as to cut down transpiration
• Survival of drought as seeds or spores that are highly dehydrated and protected within a hard coat

Question 18

Into what two main categories with regard to their osmoregulation are animals placed?

Answer 18

• Osmotic conformers
• Osmotic regulators

Question 19

What are Osmotic conformers (Osmo conformers)?

Answer 19

Animals whose Osmotic concentration of body fluids fluctuates according to that of the environment. E.g. fresh water lower animals.

Question 20

What are Euryhaline animals?

Answer 20

These are those that tolerate wide variations in salt concentration of water.
They usually live in brackish water

Question 21

What are Stenohaline animals?

Answer 21

These are those with narrow tolerance to environmental variation of salt concentration in water e.g. Maia, Arenicola.

Question 22

What are Euryhaline Osmotic conformers (tissue tolerant species)?

Answer 22

These are species that tolerate wide external and therefore internal osmotic fluctuactions.

Question 23

What are Stenohaline osmotic conformers?

Answer 23

These are species that tolerate only limited external and therefore internal osmotic fluctuactions.
Such organisms’ habitats are limited to environments of constant concentration e.g. the hagfish is strictly marine and stenohaline, its body fluids are iso-osmotic (have same concentrations as sea water)

Question 24

What are Osmotic regulators (Osmo regulators)?

Answer 24

These are animals that maintain or regulate within narrow limits the internal body osmolarity despite environmental changes. E.g. Most marine vertebrates, higher fresh water animals (they remain hyperosmotic)

Question 25

What are Euryhaline Osmotic regulators?

Answer 25

These are species that maintain within narrow limits the internal body osmolarity over a wide range of environmental changes. E.g. migratory fish like eel (Anguilla bengalensis) which migrate from fresh wter to sea water, Salmon (Salmo fario) which migrate from sea to fresh water for spawning,

Question 26

What are Stenohaline osmotic conformers?

Answer 26

These are species that regulate the internal body osmolarity over a narrow range of external environmental changes.

Question 27

What does Ammonia require for its excretion?

Answer 27

A lot of water for its dilution as it is highly toxic. Being highly soluble and readily difusable, it is excreted by fresh water bony fish, protozoa, porifera, Cnidarians wh ich live in abundance of water. Such animals are said to be ammoniotelic.

Question 28

What does urea require for its excretion?

Answer 28

It is relatively toxic and very soluble hence can be easily diluted before elimination. It is excreted byterrestrial animals like mammals and marine ones whose body fluids are hypotonic to seawater. Animals that excrete mainly urea are said to be ureotelic

Question 29

What does uric acid require for its excretion?

Answer 29

• Uric acid is almost non-toxic and highly insoluble, requiring very little water for its elimination so it is excreted by animals living in very arid conditions e.g. birds, insects and reptiles, which live in water shortage.
• These animals are said to be uricotelic

Question 30

What does trimethylamineoxide require for its excretion?

Answer 30

Trimethylamineoxide is soluble but non-toxic, requiring relatively less water for its elimination. It is excreted by marine bony fishes suffering from water shortage.

Question 31

What does Guanine require for its excretion?

Answer 31

It is less soluble than uric acid and requires no water for its elimination, hence is excreted by terrestrial spiders that live in scarcity of water.

Question 32

Which nitrogenous base is also an excretory product?

Answer 32

Guanine

Question 33

Describe osmoregulation in sea water

Answer 33

• Antennal glands at the base of the antennae excrete excess water and nitrogenous wastes.
• Antennal glands are incapable of holding back salts (they eliminate salts and water alike), resulting into production of urine isotonic with blood.
• Gills absorb salts from the surrounding medium and secrete them into blood against a concentration gradient so as to maintain an internal osmotic pressure (opi) higher than external osmotic pressure (ope).

(In crayfish)
- Here antennal glands are capable of eliminating excess water but reabsorb salts, resulting into production of urine hypotonic with blood and an internal osmotic pressure (opi) higher than external osmotic pressure (ope).
- Reabsorption of salts occurs as the urine flows along the coiled tubule

Question 34

Describe osmoregulation in a terrestrial insect

Answer 34

• A terrestrial insect is liable to water loss, which is minimized by:
• An impermeable cuticular covering coated with wax
• Production of non-toxic and almost insoluble waste product, uric acid which requires little water for its elimination being almost no toxic.
• Reabsorption of water by malpighian tubules and rectal glands, resulting in very concentrated urine.
• Laying cleidoic eggs such that water loss is prevented during embryo development by a relatively impermeable shell.
• Possession of valve-like structures and hair in the spiracles to reduce on water loss

Question 35

Describe excretion in a terrestrial insect

Answer 35

• The peristaltic movements of malpighian tubules stir up the coelomic fluid (blood) enabling epithelial cells to absorb nitrogenous wastes like sodium and potassium urate.
• Within the tubule cells, Water and CO2 react with potassium urate to form potassium hydrogen carbonate and uric acid.
• Potassium hydrogen carbonate is absorbed back into blood while uric acid is deposited in the tubule lumen.
• As the uric acid moves from distal to proximal end of the malpighian tubule, water is vigorously back into blood while solid crystals of uric acid are deposited in the lumen and later rectum to be passed out.

Question 36

Describe osmoregulation in fresh water protozoa e.g. amoeba or paramecium

Answer 36

• Contractile vacuoles carry out smoregulation in fresh water protozoa.
• Since the cell contents are hypertonic to the surrounding, and the cell membrane is partially permeable, there is constant influx of water into the cytoplasm by osmosis.
• Small vesicles in the cytoplasm fill up with fluid from the cytoplasm and pump salts back into the cytoplasm by active transport, using energy provided by ATP from the numerous mitochondria surrounding the vesicles.
• The vesicles, now containing water fuse with the contractile vacuole which gradually expands.
• The impermeability of the vacuolar membrane to water prevents osmotic out flow of water.
• On reaching critical size, the contractile vacuole fuses with the cell surface membrane, contracts suddenly and releases its water.

Question 37

How are fresh water teleosts adapted to excretion and osmoregulation? (bony fish)
e.g. tilapia, stickle back, trout, etc adapted to excretion and osmoregulation

(Opi > Ope)

Answer 37

The excretory and osmoregulatory organs are the gills and kidneys.

Internal body fluids being hypertonic to the surrounding water, there is:
a) Osmotic influx of water across the gills, lining of mouth and pharynx.
- This is addressed by not drinking water and production of large volume of dilute (hypotonic) urine

b) Efflux of solutes (ions and ammonia) into water by diffusion.
- This is addressed by reabsorbing ions across the nephron tubules, from the glomerular filtrate back into blood.
- The high glomerular filtration rate is enabled by numerous large glomeruli in the kidneys.
- In addition, there is active uptake of salts from water by chloride secretory cells in the gills.

Question 38

How are Marine teleosts adapted to excretion and osmoregulation? (Opi < Ope)

Answer 38

The excretory and osmoregulatory organs in marine teleosts are the gills and kidneys.

Internal body fluids being hypotonic to the surrounding water, there is osmotic extraction of water from the body leading to dehydration of the tissues, a situation described as ‘physiological drought’.

This is overcome by:
-Drinking large amounts of seawater and having a kidney with low filtration rate enabled by few small sized glomeruli.
- The ions Ca2+, Mg2+ and SO42- (divalent ions) in the seawater a marine fish drinks are eliminated through the anus while K+, Na+, and Cl- (monovalent ions) are absorbed into blood and are actively transported out of blood across the gills, reverse to the direction in fresh water fish.. The divalent ions that enter blood are secreted into the nephron tubules and excreted in urine.
-Excreting trimethylamine oxide, which is soluble but non-toxic requiring little water for elimination

Question 39

How are Marine elasmobranchs e.g. (cartilaginous fish) e.g. dog fish, sharks, rays adapted to excretion and osmoregulation?

Answer 39

Their tissue fluid is slightly hypertonic to seawater, causing slight influx of water, which is readily expelled by the kidneys.

Hypertonic tissue fluid results from urea retention, which is facilitated by:
- Impermeability of gills to urea.
- Urea reabsorption from the nephron tubules, maintaining its concentration at over 100 times higher than that in mammals.
- Tolerance of tissues and enzymes to high urea concentration.
- The highly toxic urea is detoxified by TriMethylamine Oxide (TMAO)

Question 40

How are Migratory fishes eg salmon and eels adapted to excretion and osmoregulation?

Answer 40

These are fish that keep moving from one extreme osmotic environment (sea) to another (fresh water) during lifetime.

This is achieved through adjustments like:
- Changes in kidney filtration rate.
- Reversal of the direction in which the chloride secretory cells transfer salt i.e. in fresh water they take in salt and may move them outwards in seawater.

Question 41

How do terrestrial animals gain water?

Answer 41

(1) By drinking directly
(2) taken along with food
(3) from metabolic by-product e.g. respiratory product

Question 42

How do terrestrial animals lose water?

Answer 42

(1) In urine
(2) Faeces
(3) Sweating
(4) Evaporation from lungs
(5) External secretions e.g. tears

Question 43

What are Physiological adaptations against water loss in terrestrial vertebrates?

Answer 43

• Reduction in glomerular filtration rate e.g. the desert frog, chiroleptes has few and smaller glomeruli than its relatives living in moist temperate regions.
• Production of non-toxic nitrogenous waste e.g. the insoluble uric acid (reptiles, birds and insects) and the relatively less toxic urea (mammals and amphibians) that require little water for removal.
• Extensive water reabsorption from glomerular filtrate (mammals and birds) and rectum (insects). E.g. kangaroo rat has an extra long loop of Henle enabling it to produce hypertonic urine.
• Use of metabolic water from fat through respiration. This explains why desert animals like kangaroo rat (Dipodomys) mostly metabolise fat, which yields more water (1 gm yields 1.1gm of water) on oxidation than carbohydrate (1 gm yields 0.6 gm of water) and protein (1 gm yields 0.3 gm of water). Dipodomys may spend its entire life without drinking water.
• Possession of tissues tolerant to dehydration. E.g. a Camel can survive for a week without drinking water, but can gulp 80 litres in 10 minutes.
• Ability to sweat at abnormally higher temperature e.g. a camel begins sweating at 410C from its normal body temperature of 340C
• Ability to reduce the need for nitrogenous excretion e.g. a camel secretes urea into the lumen of alimentary canal where bacteria convert it to protein, which is then utilized as food.

Question 44

What are the morphological/structural adaptations of terrestrial vertebrates to reducing water loss?

Answer 44

Possession of waterproof integuments, which include the keratinous scales of reptiles, cornified epithelium of mammals and the waxy cuticle of insects.

Question 45

What are the behavioral adaptations of terrestrial vertebrates to reducing water loss?

Answer 45

• Change of habitat depending on the weather conditions.
• Some animals e.g. African lungfish aestivate.

Question 46

Define Aestivation

Answer 46

This is seasonal response by animals to drought or excessive heat during which they become dormant, body temperature and metabolic rate fall to the minimum required for maintaining the vital activities of the body.

It is an adaptation for temperature regulation as well as water conservation.
During aestivation, the African lungfish burrows down and encases in a cocoon of hard mud lined with mucus

Question 47

Describe osmoregulation in amphibians

Answer 47

(1) Body fluids of amphibian are hypertonic to fresh water resulting in;
(i) osmotic influx of water which is readily lost by the kidneys expelling large volumes of urine
(ii) salt loss by diffusion which are replaced actively across the skin

(2) During aestivation, amphibia instead of the usual ammonia form urea, which is less toxic and therefore can be retained until water is available for excretion

(3) Amphibia never drink water hence water gain is osmotic via the skin or in food consumed.

Question 48

How do reptiles minimize water loss?

Answer 48

• laying cleidoic eggs with waterproof embryonic membranes and supporting shell
• possession of waterproof keratinized skin and scales
• possession of kidneys with reduced glomeruli hence low rate of glomerular filtration
• production of insoluble uric acid which is almost non-toxic and therefore requires little water for elimination
• absorption of water by the cloaca from faeces and nitrogenous wastes.

Question 49

Give the functions of the kidneys in the body

Answer 49

• Excretion of metabolic waste products such as urea, excess water, uric acid, ammonia, creatine etc
• Regulation of water and solute content of blood (osmoregulation)
• Maintenance of PH of body fluids at 7.4 (acid-base balance by removing or neutralizing excess acidity / alkalinity
• Regulation of blood levels of ions such as Na+, K+, Cl-, Ca2+
• Secretion of the hormone erythropoietin, which stimulates red blood cell production for transporting oxygen
• Retention of important nutrients such as glucose and amino acids through reabsorption from glomerular filtrate into blood.

Question 50

How does the kidney maintain pH of body fluids at 7.4?

Answer 50

• The body produces more acids than bases, causing the blood PH usually to lower (become acidic) from the normal PH of 7.4 due to high concentration of H+ ions that result from metabolic processes.
• In the cells of distal convoluted tubules, the CO2 from aerobic respiration, catalysed by carbonic anhydrase enzyme reacts with water to form carbonic acid, which dissociates into H+ and HCO3-ions.
• The H+ ions are pumped into the lumen where they are buffered by hydrogen phosphate (HPO42-) as it takes up H+ ions to form sodium dihydrogen phosphate (NaH2PO4), which is excreted in urine while the HCO3- ions are absorbed and retained in blood.
• Exceptional lowering of PH causes the cells lining the distal tubule to deaminate glutamine amino acid to form ammonia, which on combining with H+ ions forms ammonium ions, which are excreted.
• Blood pH rises (becomes less acidic) due to absorption of HCO3-that result from dissociation of carbonic acid.
• In order to control PH, the HCO3- are excreted while H+ ions are retained.

Question 51

How does the body maintain constant pH?

Answer 51

• Lungs expelling CO2,which would accumulate and react with water to form carbonic acid
• the buffering mechanism involving plasma protein in blood
• the kidneys expelling H+ and retaining HCO3-

Therefore PH (acid-base balance) is controlled by the lungs, blood and kidneys

Question 52

In what three ways is the concentration of any particular type of ion in blood and tissue fluid regulated?

Answer 52

i) Hormones control the uptake of the ions into bloodstream from the gut.
ii) Hormones control the removal of ions from the blood by kidneys and elimination in the urine.
iii) Hormones control the release of ions into the bloodstream from reservoirs like organs / tissues e.g. bones in which they are at high concentrations.

Question 53

Describe the role of the kidney in regulation of calcium ions in blood

Answer 53

• Low blood calcium level stimulates the parathyroid glands (surrounding the thyroid gland) to secrete the parathormone (parathyroid) hormone which increases the calcium level and decreases the hydrogen phosphate (HPO42-) level through promoting:
• Bone breakdown by osteoclasts
• Calcium retention by kidneys
• Excretion of hydrogen phosphate (HPO42-) in urine by kidneys
• Activation of vitamin D, which in turn stimulates the absorption of calcium from the gut.
• High blood calcium level stimulates the thyroid gland to secrete calcitonin hormone, which increases bone buildup by osteoblasts so as to reduce calcium level.

Question 54

What role does calcium play in the body?

Answer 54

Has a role in nervous conduction, muscle contraction and blood clotting.

Question 55

What is the result of deficiency of parathyroid hormone?

Answer 55

It results in tetany (shaking of body due to continuous muscle contraction caused by increased excitability of the nerves, which fire spontaneously and without rest)

Question 56

Describe the role of the kidney in regulation of sodium ions (Na+) in blood.

Answer 56

• A decrease in blood sodium leads to decreased blood volume and reduced blood pressure because less water is drawn into blood by osmosis.
• Low levels of sodium in blood are detected by the hypothalamus, which stimulates the anterior pituitary gland to secrete the hormone adrenocorticotropic hormone (ACTH).
• This stimulates the juxtaglomerular complex (situated between the distal convoluted tubule and afferent arteriole) to release the enzyme Renin (don’t confuse it with the digestive enzyme rennin).
• Renin catalyses the conversion of angiotensinogen, a plasma protein into a hormone angiotensin which stimulates the adrenal cortex to secrete the hormone Aldosterone
• Aldosterone has the following effects:
• Stimulates the active uptake of sodium ions from the glomerular filtrate into the plasma of capillaries surrounding the nephron. This induces osmotic uptake of water into blood thus increasing both the blood volume and sodium level back to the norm, accompanied by loss of potassium ions.
• Stimulates sodium absorption in the gut and decreases loss of sodium in sweat so as to raise sodium levels to cause an osmotic inflow of water thus increasing the blood volume and pressure
• Stimulates the brain to increase the sensation of thirst.
• Increased sodium level in blood causes increased blood volume and pressure, less production of renin and angiotensin
• This results in less secretion of aldosterone by the adrenal cortex hence less uptake of sodium from the glomerular filtrate occurs, restoring sodium level to the norm.

Question 57

Describe the role of the kidneys in regulation of water and solute content of blood (Osmotic regulation)

Answer 57

• Increased concentration of solutes in blood (little water relative to salts) is detected by osmoreceptors in the hypothalamus which stimulate the posterior pituitary gland to secrete antidiuretic hormone (ADH)/vasopressin and at the same time triggering the sensation of thirst resulting in drinking of water.
• ADH increases the permeability of distal convoluted tubule and collecting duct to water, allowing the osmotic flow of water from the glomerular filtrate into the cortex and medulla hence reducing the osmotic pressure of blood but increasing that of urine.
• ADH also increases the permeability of the collecting duct to urea, enabling its diffusion from urine into the medulla tissue fluid where it increases the osmotic pressure resulting in osmotic extraction of water from the descending limb.
• Low solute concentration in blood (too much water relative to salts) inhibits ADH release
• tubule walls and collecting duct become impermeable to water
• less water is reabsorbed from glomerular filtrate into blood and large volume of dilute urine is passed out hence raising the osmotic pressure of blood.

Question 58

What is Diuresis?

Answer 58

This is the production of copious dilute urine, antidiuresis being the opposite.

Question 59

What is the result of Insufficient production of ADH?

Answer 59

It leads to a condition known as diabetes inspidus, characterised by frequent copious urination.

Question 60

What causes an increase in blood osmotic pressure (BOP)?

Answer 60

• ingestion of little water
• much sweating
• ingestion of large amount of salt
• while a decrease in BOP may be due to little sweating, ingestion of large volume of water and low salt intake.

Question 61

What processes occur in the nephron enabling it to accomplish its excretory function?

Answer 61

(i) Ultra filtration (pressure filtration) at the glomerulus of bowman’s capsule
(ii) Selective reabsorption in the tubules
(iii) Tubular secretion at the proximal and distal convolulated tubule
(iv) Counter current multiplier effect in the loop of Henle
(v) Water reabsorption in the distal convoluted tubule and collecting duct.

Question 62

Describe the process of ultrafiltration (pressure filtration) in the bowman’s capsule

Answer 62

• This is the first stage of urine formation at the glomerular capillary wall of kidney nephrons
• Hydrostatic pressure forces small molecules in blood of glomerular capillaries to pass across the basement membrane into the capsular space but large molecules are held back.
• The substances that are forced by pressure to pass passively across the fine basement membrane filter include small molecules like water, glucose, amino acids, vitamins, urea, uric acid, ions, creatine, and some hormones
• The large substances retained in blood include red blood cells, platelets, white blood cells and large sized plasma proteins.
• Although filtration occurs through three layers of glomerular capillary, the endothelium is a coarse screen retaining only blood cells
• the negatively charged basement membrane retains negatively charged large sized protein, while the selective filtration occurs at the diaphragms of slit pores formed by foot-like projections of supporting cells called podocytes

Question 63

Where does the high hydrostatic pressure of blood in the glomerulus which facilitates ultrafiltration arise from?

Answer 63

It results from the afferent arteriole having a larger diameter than the efferent arteriole.

Question 64

Describe selective reabsorption from the tubules

Answer 64

• Because particle size rather than their importance determines the substances to pass through the basement membrane during ultra filtration, useful substances such as glucose enter the capsular space to form glomerular filtrate and have to be reabsorbed later.
• As the glomerular filtrate (renal fluid) flows along the tubule of the nephron, all the glucose, 85% of the water, Na+, Cl-, amino acids, vitamins, hormones, 50% of urea are reabsorbed from the proximal convoluted tubule into the surrounding blood capillaries.
• Glucose, amino acids and Na+, H2PO4- and HCO3- diffuse into proximal tubule cells and then actively transported into the blood capillaries.
• The active uptake of Na+ followed by the passive uptake of Cl- raises the osmotic pressure in the cells enabling entry of water into capillaries by osmosis.
• 50% of urea is reabsorbed by diffusion but the small sized proteins in the renal filtrate are removed by pinocytosis.
• As a result of all this activity, the tubular filtrate is isotonic with blood in the surrounding capillaries

Question 65

Describe Tubular secretion at the proximal convolulated tubule

Answer 65

• Finally, active secretion of unwanted substances like creatine, some urea, ammonia, uric acid, H+, and K+ occurs from blood capillaries into the proximal tubule

Question 66

True or false Only birds and mammals have loops of Henle in their Kidneys

Answer 66

True
This enables production of hypertonic urine

Question 67

Describe Counter current multiplier effect in the loop of Henle

Answer 67

• A system of parallel and opposite flow of renal fluid in the descending and ascending limbs of the loop of Henle in the kidney with active salt concentration in the medullary interstitial tissue, an increase in salt concentration in the renal fluid of the descending limb and a decrease in salt concentration in the ascending limb to cause production of hypertonic urine
• The loop of Henle is the counter current multiplier and the vasa recta is the counter current exchanger. If the vasa recta did not exist, the high concentration of solutes in the medullary interstitium would be washed out.
• The ascending limb of the loop of Henle is relatively impermeable to water while the descending limb is freely permeable to water but relatively impermeable to salt and urea.
• Na+ and Cl- are actively pumped out of the upper part of ascending limb, but diffuse from the lower part, raising the solute concentration in the interstitial region and lowering the concentration in the ascending limb.
• Water is osmotically drawn from the descending limb and collecting duct and carried away by blood in the vasa recta, resulting into a slightly higher solute concentration in the descending limb than the adjacent ascending limb and hypertonic urine to form.
• The concentrating effect is multiplied such that the fluid in and around the loop of Henle becomes saltier with the saltiest region being the hairpin bend.
• The glomerular filtrate becomes less salty as it goes up the ascending limb

Question 68

Describe water reabsorption in the distal convoluted tubule and collecting duct

Answer 68

• This is under the influence of hormones
• As the fluid flows down the collecting duct, water is drawn out of it osmotically into the interstitium, resulting in hypertonic urine production

Question 69

How are the proximal convoluted tubule cells adapted for reabsorption?

Answer 69

• Bear numerous microvilli at the free end to increase the surface area for reabsorption of substances like glucose, amino acids, vitamins, NaCl, water.
• Contain numerous mitochondria to form ATP that provide energy required in active transport of glucose, amino acids, Na +, H2PO4- and HCO3- into the blood capillaries
• The cell surface membrane is indented to form a large area of intercellular spaces bathed with fluid.
• Contain numerous pinocytic vesicles, which enable the digestion of small protein molecules from the renal filtrate.
• Form a thin thickness of one cell layer to ease reabsorption of substances.

Question 70

What is renal-plasma ratio?

Answer 70

The ratio obtained after dividing the concentration of substances in renal fluid by the concentration of same substances in blood plasma.

Question 71

What is the glomerular filtration rate?

Answer 71

Glomerular filtration rate (GFR) is the net rate of formation of filtrate by the two kidneys.

GFR is equal to the renal plasma flow (RPF) rate, the rate of plasma flow through the renal arteries, multiplied by the fraction of this plasma flow that is filtered (the filtered fraction, FF), so GFR = (RPF) x (FF)

Question 72

What 3 factors determine the filtered fraction?

Answer 72

(i) filtration pressure across the glomerular capillary walls
(ii) permeability of the renal filter to fluid
(iii) the total surface area available for filtration.

Question 73

When does GFR increase?

Answer 73

• if the mean glomerular capillary pressure rises as a result of either dilation of the afferent arterioles or constriction of efferent arterioles, or
• if the concentration of plasma proteins falls, because this reduces the force favouring reabsorption.

Question 74

When does GFR fall?

Answer 74

• If the hydrostatic pressure in bowman’s capsule rises (e.g. if the ureters are occluded) or
• plasma proteins escape into Bowman’s capsule, because protein in Bowman’s capsule makes net osmotic force across the glomerular wall smaller.

Question 75

What is the adaptive significance of formation of urine by filtration in the glomerulus followed by reabsorption and secretion in the later parts of the nephron?

Answer 75

It is to enable the kidney excrete soluble chemicals that might enter the body, e.g. drugs and bacterial toxins but for which there are no specific tubular reabsorption pathways.

Question 76

Why does urine production almost stop after serious bleeding?

Answer 76

The amount of urine produced is proportional to the amount of blood flowing through the kidneys.

The total blood volume in the body reduces if serious bleeding occurs, resulting into diversion of blood from other tissues (including the kidneys) to the brain to maintain life.

Therefore the volume of blood flowing through the kidneys reduces greatly to the extent that less ultrafiltration and hence formation urine occurs.

Question 77

How are the structures of the parts other than the tubules of the mammalian nephron related to the functions they perform?

Answer 77

• Afferent arterial entering the Bowman’s capsule has a wider lumen than that of efferent arterial leaving it, resulting into high hydrostatic pressure that causes ultrafiltration to occur.
• The glomerular capillaries are highly coiled to increase the surface area for ultrafiltration to occur.
• The structural arrangement of the three layers of glomerular capillary enables the diaphragms of slit pores formed by foot-like projections of podocytes to offer selective filtration while blood cells and the negatively charged large plasma protein are retained by endothelium and basement membrane respectively.
• The Bowman’s capsule is funnel-shaped to direct the renal filtrate into the proximal convoluted tubule.

Question 78

How are the structures of the tubules of the mammalian nephron related to the functions they perform?

Answer 78

The proximal convoluted tubule cells:
- Bear numerous microvilli at the free end to increase the surface area for reabsorption of substances like glucose, amino acids, vitamins, NaCl, water
- Contain numerous mitochondria to form ATP that provide energy required in active transport of glucose, amino acids, Na+,H2PO4- and HCO3- into the blood capillaries
- The cell surface membrane is indented to form a large area of intercellular spaces bathed with fluid
- Contain numerous pinocytic vesicles, which enable the digestion of small protein molecules from the renal filtrate.
- Form a thin thickness of one cell layer to ease reabsorption of substances.
- The loop of Henle is U-shaped with parallel, opposite flows of tubular fluid in its limbs to provide a multiplier effect that create a concentration gradient, which enables increased water reabsorption.
- The capillaries of vasa recta form loops that accompany the loops of Henle resulting into countercurrent exchange of solute and water between ascending and descending blood.
- The capillaries of vasa recta are in close proximity with tubules to increase the reabsorption of useful substances from the filtrate.
- The distal convoluted tubule is long and coiled to increase the surface area for reabsorption of water and mineral salts.
- The distal and proximal convoluted tubules are coiled to slow down the movement of renal filtrate to allow more time for efficient reabsorption of substances like water, mineral salts.

Question 79

Within plasma, which compounds act as pH buffers temporarily?

Answer 79

Hydrogen carbonate (HCO3-), protein and hydrogen phosphate (HPO42-)act as PH buffers by temporarily taking up any excess H+ ions and at the same time keeping the PH constant.