11.3 The Kidney and Osmoregulation

Question 1

Excretion

Answer 1

Removal from the body of waste products of metabolic pathways and other non-useful materials and the removal of water to maintain osmoregulation
defacation is not considered excretion as faeces is undigested food not metabolic waste

Question 2

Osmoregulation

Answer 2

control of water balance of the blood, tissue or cytoplasm of a living organism
Osmoregulation occurs in the medulla of the kidney and involves two key events:
The loop of Henle establishes a salt gradient (hypertonicity) in the medulla
Anti-diuretic hormone (ADH) regulates the level of water reabsorption in the collecting duct

Question 3

Excretion in single-celled organisms

Answer 3

waste products are lost directly through the surface of the cell and they osmoregulate using contractile vacuoles

Question 4

Removal of nitrogenous waste

Answer 4

Nitrogenous wastes are produced from the breakdown of nitrogen-containing compounds like amino acids and nucleotides

Nitrogenous wastes are toxic to the organism and hence excess levels must be eliminated from the body
The type of nitrogenous waste in animals is correlated with the evolutionary history of the animal and the habitat

Question 5

Aquatic animals - nitrogenous waste

Answer 5

Most aquatic animals eliminate their nitrogenous wastes as ammonia (NH3)

Ammonia is highly toxic but also very water soluble and hence can be effectively flushed by animals in aquatic habitats

Question 6

Mammals - nitrogenous waste

Answer 6

eliminate their nitrogenous wastes as urea, which is less toxic and hence can be stored at higher concentrations

Question 7

Reptiles and birds - nitrogenous waste

Answer 7

eliminate wastes as uric acid, which requires more energy to make but is relatively non-toxic and requires even less water to flush (it is eliminated as a semi-solid paste)

Question 8

Osmoconformers

Answer 8

maintain internal conditions that are equal to the osmolarity of their environment
osmoconformers minimise water movement in and out of cells
Less energy is used to maintain internal osmotic conditions within an osmoconformer

Question 9

Osmoregulators

Answer 9

keep their body’s osmolarity constant, regardless of environmental conditions
While osmoregulation is a more energy-intensive process, it ensures internal osmotic conditions are always tightly controlled
Osmoregulators can maintain optimal internal conditions whereas osmoconformers are affected by environmental conditions

Question 10

Malpighian Tubules

Answer 10

In insects, the excretory system (Malpighian tubules) connects to the digestive system of the animal
Insects have a circulating fluid system called hemolymph that is analogous to the blood system in mammals
Malpighian tubules branch off from the intestinal tract and actively uptake nitrogenous wastes and water from the hemolymph
The tubules pass these materials into the gut to combine with the digested food products
Solutes, water and salts are reabsorbed into the hemolymph at the hindgut, whereas nitrogenous wastes (as uric acid) and undigested food materials are excreted via the anus

Question 11

Structure/functioning of a kidney

Answer 11

Blood enters the kidneys via the renal artery and exits the kidneys via the renal vein
Blood is filtered by specialised structures called nephrons which produce urine
These nephrons are located in the cortex and the medulla
The urine is transported from the kidneys via the ureter, where it is stored by the bladder prior to excretion

Question 12

Composition of blood in the renal vein vs renal artery

Answer 12

Blood in the renal vein (i.e. after the kidney) will have:

Less urea (large amounts of urea is removed via the nephrons to form urine)
Less water and solutes / ions (amount removed will depend on the hydration status of the individual)
Less glucose and oxygen (not eliminated, but used by the kidney to generate energy and fuel metabolic reactions)
More carbon dioxide (produced by the kidneys as a by-product of metabolic reactions)

Question 13

Units of a nephron

Answer 13

Bowman’s capsule
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule

Question 14

Bowman’s capsule

Answer 14

first part of the nephron where blood is initially filtered (to form filtrate)
Within the Bowman’s capsule, the blood is filtered at a capillary tuft called the glomerulus

Question 15

Proximal convoluted tubule

Answer 15

folded structure connected to the Bowman’s capsule where selective reabsorption occurs
these folds create a greater surface area
here glucose, amino acids, and Na ions are reabsorbed by active transport and water by osmosis across microvilli into the blood

Question 16

Loop of Henle

Answer 16

a selectively permeable loop that descends into the medulla and establishes a salt gradient
further reabsorption into the blood occurs here
salt by active transport and water by osmosis

Question 17

Distal convoluted tubule

Answer 17

a folded structure connected to the loop of Henle where further selective reabsorption occurs
salts are reabsorbed by active transport and water by osmosis
K ions and ammonia salts are secreted from the blood into the distal tubule

Question 18

Collecting duct

Answer 18

Each nephron connects to a collecting duct (via the distal convoluted tubule), which feed into the renal pelvis

The collecting ducts are shared by nephrons and hence are not technically considered to be part of a single nephron

Question 19

Nephron Function

Answer 19

Nephrons filter blood and then reabsorb useful materials from the filtrate before eliminating the remainder as urine

This process occurs over three key stages:

Ultrafiltration – Blood is filtered out of the glomerulus at the Bowman’s capsule to form filtrate
Selective reabsorption – Usable materials are reabsorbed in convoluted tubules (both proximal and distal)
Osmoregulation – The loop of Henle establishes a salt gradient, which draws water out of the collecting duct

Question 20

Ultrafiltration

Answer 20

Ultrafiltration is the first of three processes by which metabolic wastes are separated from the blood and urine is formed

It is the non-specific filtration of the blood under high pressure and occurs in the Bowman’s capsule of the nephron

Question 21

Structure of the Bowman’s capsule

Answer 21

As the blood moves into the kidney via afferent arterioles it enters a knot-like capillary tuft called a glomerulus
This glomerulus is encapsulated by the Bowman’s capsule, which is comprised of an inner surface of cells called podocytes
Podocytes have cellular extensions called pedicels that wrap around the blood vessels of the glomerulus
Between the podocytes and the glomerulus is a glycoprotein matrix called the basement membrane that filters the blood

Question 22

Basement membrane

Answer 22

Blood is filtered by a mesh called the basement membrane, which lies between the glomerulus and Bowman’s capsule

Glomerular blood vessels are fenestrated (have pores) which means blood can freely exit the glomerulus
The podocytes of the Bowman’s capsule have gaps between their pedicels, allowing for fluid to move freely into the nephron
Consequently, the basement membrane functions as the sole filtration barrier within the nephron

The basement membrane is size-selective and restricts the passage of blood cells and large proteins

Hence when the blood is filtered, the filtrate formed does not contain any blood cells, platelets or plasma proteins

Question 23

Hydrostatic pressure - ultrafiltration

Answer 23

This high hydrostatic pressure is created in the glomerulus by having a wide afferent arteriole and a narrow efferent arteriole
This means it is easy for blood to enter the glomerulus, but difficult for it to exit – increasing pressure within the glomerulus
Additionally, the glomerulus forms extensive narrow branches, which increases the surface area available for filtration
The net pressure gradient within the glomerulus forces blood to move into the capsule space (forming filtrate)

Question 24

Selective reabsorption

Answer 24

Selective reabsorption is the second of the three processes by which blood is filtered and urine is formed

It involves the reuptake of useful substances from the filtrate and occurs in the convoluted tubules (proximal and distal)
The majority of selective reabsorption occurs in the proximal convoluted tubule, which extends from the Bowman’s capsule

The proximal convoluted tubule has a microvilli cell lining to increase the surface area for material absorption from the filtrate

The tubule is a single cell thick and connected by tight junctions, which function to create a thin tubular surface with no gaps

There are also a large number of mitochondria within these tubule cells, as reabsorption involves active transport

Substances are actively transported across the apical membrane (membrane of tubule cells facing the tubular lumen)
Substances then passively diffuse across the basolateral membrane (membrane of tubule cells facing the blood)

The tubules reabsorb all glucose, amino acids, vitamins and hormones, along with most of the mineral ions (~80%) and water

Mineral ions and vitamins are actively transported by protein pumps and carrier proteins respectively
Glucose and amino acids are co-transported across the apical membrane with sodium (symport)
Water follows the movement of the mineral ions passively via osmosis

Question 25

Hormonal control of kidney function

Answer 25

Antidiuretic hormone (ADH) and aldosterone regulate the composition and volume of urine
ADH is secreted by the brain in response to falling water levels in the blood hence it stimulates water reabsorption
(alcohol stimulates decrease in ADH)

Question 26

Establishing a salt gradient in kidney

Answer 26

The function of the loop of Henle is to create a high solute (hypertonic) concentration in the tissue fluid of the medulla
The descending limb of the loop of Henle is permeable to water but not salts
The ascending limb of the loop of Henle is permeable to salts but not water
This means that as the loop descends into the medulla, the interstitial fluid becomes more salty and hypertonic
Additionally, the vasa recta blood network that surrounds the loop of Henle flows in the opposite direction (counter-current)
This means that salts released from the ascending limb are drawn down into the medulla, further establishing a salt gradient

Question 27

Water reabsorption

Answer 27

As the collecting duct passes through the medulla, the hypertonic conditions of the medulla will draw water out by osmosis
The amount of water released from the collecting ducts to be retained by the body is controlled by anti-diuretic hormone (ADH)
ADH is released from the posterior pituitary in response to dehydration (detected by osmoreceptors in the hypothalamus)
ADH increases the permeability of the collecting duct to water, by upregulating production of aquaporins (water channels)
This means less water remains in the filtrate, urine becomes concentrated and the individual urinates less (i.e. anti-diuresis)
When an individual is suitably hydrated, ADH levels decrease and less water is reabsorbed (resulting in more dilute urine)
Remember: ADH is produced when you Are DeHydrated

Question 28

Dehydration

Answer 28

Dehydration is a loss of water from the body such that body fluids become hypertonic
Individuals will experience thirst and excrete small quantities of heavily concentrated urine (to minimise water loss)
Blood pressure will drop (less water in plasma) and the heart rate will increase to compensate for this
The individual will become lethargic and experience an inability to lower body temperature (due to lack of sweat)
Severe cases of dehydration may cause seizures, brain damage and eventual death

Question 29

Overhydration

Answer 29

Overhydration is a less common occurrence that results when an over-consumption of water makes body fluids hypotonic
Individuals will produce excessive quantities of clear urine in an effort to remove water from the body
The hypotonic body fluids will cause cells to swell (due to osmotic movement), which can lead to cell lysis and tissue damage
Overhydration can lead to headaches and disrupted nerve functions in mild cases (due to swelling of cells)
In severe cases, overhydration may lead to blurred vision, delirium, seizures, coma and eventual death

Question 30

Water conservation and the loop of Henle

Answer 30

All animals need to maintain an appropriate water balance, however the need for water conservation will depend on habitat

Animals in arid, desert environments will need more efficient water conservation than animals in moist, mesic environments

Water conservation can be improved by having a longer loop of Henle, which increase the salt gradient in the medulla

A greater the salt gradient in the medulla means more water is reabsorbed by the collecting ducts and urine is concentrated

Hence, the length of the loop of Henle is positively correlated with the degree of water conservation in animals

Animals living in moist environments have short loops of Henle that don’t descend deeply into the medulla (cortical nephrons)
Animals living in arid environments have long loops of Henle that descend deeply into the medulla (juxtamedullary nephrons)

Question 31

What does presence of glucose in urine mean?

Answer 31

The presence of glucose in urine is a common indicator of diabetes (high blood glucose = incomplete reabsorption)

Question 32

What does presence of protein in urine mean?

Answer 32

High quantities of protein in urine may indicate disease (e.g. PKU) or hormonal conditions (e.g. hCG = pregnancy)

Question 33

What does presence of blood cells in urine mean?

Answer 33

The presence of blood in urine can indicate a variety of diseases, including certain infections and cancer

Question 34

Hemodialysis

Answer 34

Kidney dialysis involves the external filtering of blood in order to remove metabolic wastes in patients with kidney failure

Blood is removed and pumped through a dialyzer, which has two key functions that are common to biological membranes:

It contains a porous membrane that is semi-permeable (restricts passage of certain materials)
It introduces fresh dialysis fluid and removes wastes to maintain an appropriate concentration gradient

Kidney dialysis treatments typically last about 4 hours and occur 3 times a week – these treatments can be effective for years

Question 35

Kidney transplant

Answer 35

Hemodialysis ensures continued blood filtering, but does not address the underlying issue affecting kidney function

The best long-term treatment for kidney failure is a kidney transplant:

The transplanted kidney is grafted into the abdomen, with arteries, veins and ureter connected to the recipient’s vessels
Donors must typically be a close genetic match in order to minimise the potential for graft rejection
Donors can survive with one kidney and so may commonly donate the second to relative suffering kidney failure

Question 36

Kidney diseases

Answer 36

Kidney diseases are conditions which incapacitate the kidney’s ability to filter waste products from the blood

Individuals with kidney diseases will demonstrate a reduced glomerular filtration rate (GFR)
If untreated, kidney diseases can lead to kidney failure – which is life threatening