HL Human Physiology: 11.3 Kidneys

Question 1

Outline the key function of the excretory system in removing nitrogenous waste from the body (why must it be removed, how it is produced in different organisms)

Answer 1

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 2

What kind of nitrogenous waste is eliminated by aquatic animals?

Answer 2

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 3

Explain types of nitrogenous wastes produced by terrestrial animals

Answer 3

Terrestrial animals have less access to water and hence must package nitrogenous waste in less toxic forms

Mammals eliminate their nitrogenous wastes as urea, which is less toxic and hence can be stored at higher concentrations
Reptiles and birds 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 4

Outline the need for the excretory system to removes excess water to maintain a suitable osmolarity within the tissues and cells as its function?

Answer 4

Water levels within an organism are constantly changing as a result of metabolic activity

Water is produced via condensation reactions (anabolism) and is consumed during hydrolysis reactions (catabolism)
The concentration of water within cells (osmolarity) will impact tissue viability (i.e. governs osmotic pressure within cells)

Question 5

Animals may be either osmoconformers or osmoregulators: Define them

Answer 5

Animals may be either osmoconformers or osmoregulators according to how they manage their internal osmotic conditions:

Osmoconformers maintain internal conditions that are equal to the osmolarity of their environment
Osmoregulators keep their body’s osmolarity constant, regardless of environmental conditions

Question 6

Outline how osmoconformers work

Answer 6

By matching internal osmotic conditions to the environment, osmoconformers minimise water movement in and out of cells

Less energy is used to maintain internal osmotic conditions within an osmoconformer

Question 7

Outline how osmoregulators work (the kind of process, benefit)

Answer 7

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 8

Discuss the type of excretory system animals and insects have

Answer 8

All animals possess a specialised excretory system for osmoregulation and the removal of nitrogenous wastes

In mammals, the excretory system (kidneys) is separate from the digestive system of the animal
In insects, the excretory system (Malpighian tubules) connects to the digestive system of the animal

Question 9

Describe the excretory system of an insect (9 marks)

Answer 9

Insects have a circulating fluid system called haemolymph that is analogous to the blood system in mammals
Malpighian tubules are a series of small tubes that extend from the body cavity and drain into the insect’s digestive system

The tubules are lined with cells that actively transport ions such as sodium (Na+) and potassium (K+) from the haemolymph into the tubule lumen, raising the osmolarity and altering the charge of the lumen contents
Water moves into the lumen from the haemolymph by osmosis
Nitrogenous waste enters the tubules from the haemolymph along an electrical gradient
The ions, water, and nitrogenous waste drain from the malpighian tubules into the digestive system
Nitrogenous waste is converted into uric acid
Useful salts and water are reabsorbed from the hindgut into the haemolymph
Uric acid remains in the digestive system, from which it later leaves the body along with faeces through the anus

Question 10

List the structures of the human excretory system with their functions

Answer 10

Humans have two kidneys covered by a fibrous capsule, which remove waste products from the blood and maintain the blood’s balance of water and solutes
The renal artery supplies oxygenated blood to the kidneys, while the renal vein carries deoxygenated blood away
The filtrate produced by the kidneys forms urine which is transferred to the bladder via a tube called the ureter

Question 11

Define nephron

Answer 11

Nephrons are the functional unit of the kidney and are responsible for the formation of urine

Question 12

Different parts of the nephron are found in different regions of the kidney

Answer 12

The cortex
Location of the glomerulus, Bowman’s capsule, proximal convoluted tubule, and distal convoluted tubule
The medulla
Location of the loop of Henle and collecting duct
The renal pelvis
All kidney nephrons drain into this structure, which connects to the ureter

Question 13

Why blood composition differs between renal artery and renal vein?

Answer 13

The kidney contains specialised structures called nephrons which function to filter the blood and eliminate wastes

Consequently, the composition of blood entering the kidney (via renal artery) differs to that exiting the kidney (via renal vein)

Question 14

Outline how blood in the renal vein (i.e. after the kidney) will have a different composition

Answer 14

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 15

Define each of the component structures in a nephron

Answer 15

Bowman’s capsule – first part of the nephron where blood is initially filtered (to form filtrate)
Proximal convoluted tubule – folded structure connected to the Bowman’s capsule where selective reabsorption occurs
Loop of Henle – a selectively permeable loop that descends into the medulla and establishes a salt gradient
Distal convoluted tubule – a folded structure connected to the loop of Henle where further selective reabsorption occurs

Question 16

Discuss further details/structures of the Bowman’s capsule

Answer 16

The blood to be filtered enters the Bowman’s capsule via an afferent arteriole and leaves the capsule via an efferent arteriole

Within the Bowman’s capsule, the blood is filtered at a capillary tuft called the glomerulus
The efferent arteriole forms a blood network called the vasa recta that reabsorbs components of the filtrate from the nephron

Question 17

Outline collecting duct in nephrons

Answer 17

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 18

Discuss what stages of the process are there for nephron’s function

Answer 18

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 19

Define Ultrafiltration

Answer 19

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 20

Describe the structure of bowman capsule in relation to ultrafiltration

Answer 20

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 21

Explain the base membrane as a structure responsible for the first process

Answer 21

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 22

Explain a principle phenomenon responsible for the optimisation of ultrafiltration

Answer 22

Ultrafiltration involves blood being forced at high pressure against the basement membrane, optimising filtration

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 23

What is selective reabsorption and where it takes place

Answer 23

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

Question 24

Discuss the characteristic features or adaptations of the structures involved in selective reabsorption

Answer 24

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

Question 25

Discuss what is responsible selective reabsorption

Answer 25

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)

Question 26

Discuss the products of reabsorption and how they are reabsorbed

Answer 26

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 27

What is osmoregulation and what two key events are involved

Answer 27

Osmoregulation is the third of three processes by which blood is filtered and urine is formed

Osmoregulation is the control of the 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 28

Explain the first key event of osmoregulation

Answer 28

Establishing a Salt Gradient

The function of the loop of Henle is to create a high solute (hypertonic) concentration in the tissue fluid of the medulla
Sodium and chloride ions are pumped out of the filtrate in the ascending limb of the loop of Henle into the surrounding medulla region, raising its osmolarity
The ascending limb of the loop of Henle is impermeable to water, so water is unable to leave the loop here by osmosis
The osmolarity of the ascending limb decreases as it rises back into the cortex due to the removal of solutes and retention of water, therefore resulting urine is slightly dilute
Additionally, the vasa recta blood network that surrounds the loop of Henle flows in the opposite direction (counter-current)
The neighbouring descending limb is permeable to water, thus water moves out of the descending limb by osmosis due to the high osmolarity of the medulla created by the ascending limb
The descending limb has few transport proteins in the membranes of its cells, so has low permeability to ions
The osmolarity of the filtrate increases as the descending limb moves down into the medulla due to the loss of water and retention of ions
The water and ions that leave the loop of Henle for the medulla make their way into nearby capillaries (vasa recta)

Question 29

Explain the second key event of osmoregulation

Answer 29

Water Reabsorption

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 30

What is dehydration and its consequences

Answer 30

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 31

What is over-hydration and its consequences

Answer 31

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 32

Explain how the habitat or water conservation needs affects length of Loop of Henle in animals

Answer 32

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

Question 33

Outline the difference in the length of loop of henle amongst moist and arid living animals

Answer 33

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 34

What is the significance of kidney disease

Answer 34

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

Question 35

Hence the presence of these materials in urine can be used as an indicator of disease:

Answer 35

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

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

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

Drugs / toxins: Many drugs pass through the body into urine and can be detected (e.g. performance enhancing drugs)