excretion [2015]

EXCRETION &

OSMOREGULATION

OVERVIEW

A) DEFINITIONS & IMPORTANCE OF EXCRETION AND OSMOREGULATION

B) OSMOREGULATION IN A TERRESTRIAL INSECT

C) OSMOREGULATION IN BONY FISH (TELEOSTS)

D) THE HUMAN KIDNEY

Definitions

Excretion the elimination of waste metabolic substances

from the body which if permitted to accumulate would prevent the maintenance of a steady state

CO2

urea

Definitions

Egestion the elimination of waste substances, mainly

undigested food, which have never been involved in the metabolic activities of cells

Definitions

Secretion :

- the process involved in producing and releasing a substance which is useful, from the cell e.g.

digestive juices

hormones

sweat

milk

Definitions

Osmoregulation :

the maintenance of constant osmotic conditions in the body

the control of the gain and loss of:

water

solutes

Importance of excretion and osmoregulation

1) Removal of unwanted by-products of metabolic pathways

is important to prevent unbalancing chemical equilibria of reactions

e.g. A + B → C + D

- constant synthesis of C requires constant removal of D


3) Regulation of ionic concentration of body fluids e.g. Na+, Ca2+

An albatross possesses nasal salt glands that

can secrete excess salt through ducts and out of

the nostrils.


4) Regulation of water content of body fluids

California 2007 A woman who competed in a radio station’s contest to see how much water she could drink without going to the bathroom died of water intoxication, the coroner’s office said Saturday.

5) Regulation of the pH of the body fluids

Nitrogenous excretory products and environment

nitrogenous waste products are produced by the breakdown of:

proteins

nucleic acids

excess amino acids

A variety of animals also excrete small quantities of: creatine creatinine

Ammonia:

is the immediate nitrogenous waste of deamination

amino acid NH3

ammonia

Ammonia may be :

excreted immediately

converted into: urea or uric acid

The exact nature of the excretory product is determined by:

1. the availability of water to the organism

2. the extent to which the organism controls water loss

The relationship between excretory products and habitat of representative animal groups

Animal Excretory product Habitat Protozoan Ammonia Freshwater Insect Uric acid Very dry terrestrial Carp Ammonia Freshwater Cod Urea,

trimethylamine oxide

Marine

Bird Uric acid Terrestrial Dog Urea Dry terrestrial

Correlation with habitat

Ammonia: Freshwater

Uric acid: Terrestrial

Urea: Marine/Terrestrial

Ammonia:

• extremely soluble

• highly diffusible through water

• highly toxic

• cannot be stored in the body

• no energy is needed in its formation

• large volumes of water are needed for dilution to be excreted

Urea is less toxic & soluble than ammonia

Urea is about 100,000 times less toxic than ammonia.

Urea forms

during the Ornithine Cycle

Bird droppings =

faeces + nitrogenous waste

Car covered in bird droppings.

Uric acid

Why is uric acid an ideal excretory product for terrestrial organisms

(e.g. insects, reptiles & birds) which produce shelled eggs?

Uric acid can be stored in cells without producing toxic or harmful osmoregulatory

effects.

Uric acid is stored in the allantois.

Uric acid: is largely insoluble

in water can be excreted as

a paste with little water loss

Energy required for production: Amount of water required for excretion: Toxicity of waste :

Ammonia Urea Uric acid

None Moderate High

High Moderate Low

High Low Low

Question: DEC, 1987

The earthworm, although terrestrial, excretes mainly ammonia. How does this affect the

worm’s habitat preferences?

Must inhabit moist environments.

Question: DEC, 1987

Marine turtles have the ability to excrete all three nitrogenous waste products, yet they excrete mainly

ammonia. What are the advantages and disadvantages of excreting ammonia?

Advantages: no energy is needed to produce ammonia

Disadvantages: animal loses a lot of water to eliminate ammonia, risking dehydration

Percentage of waste nitrogen excreted as:

Ammonia Urea Uric acid

51 12 6

AMMONIA Tadpoles excrete:

UREA Frogs excrete:

Question: [MAY, 2010]

1. What are nitrogenous wastes? Name a biochemical process that produces nitrogenous waste. (2)

Nitrogenous wastes are substances produced as a result of metabolism that are not required by the body and contain nitrogen.

Deamination / break down of nucleic acids. 2. Name ONE organism that excretes nitrogenous wastes

as urea and ONE organism that excretes nitrogenous wastes as ammonia: (2) Urea: mammal / marine bony fish / frog Ammonia: protozoans / Amoeba / freshwater bony fish / tadpole

OVERVIEW




D) THE HUMAN KIDNEY

Malpighian Tubules in insects:

Function: excrete uric acid

Location:

- lie in the abdomen

- open into the hindgut at its junction with the midgut

Malpighian Tubules are:

blind-ending tubules of the hindgut of insects

Tubules vary in :

Shape:

long & slender or

short and compact

Number:

a pair - several hundred

Figure 44.12

Upper segment: Absorbs fluid from blood

Lower segment: Cells have microvilli

Rectal glands: Reabsorb water

Haemolymph

Mechanism

Faeces & uric acid

removed as semi-solid wastes

The excretory product is semi-solid

cockroach droppings

OVERVIEW




D) THE HUMAN KIDNEY

Fish are osmoregulators:

control concentration of body fluids

use energy to regulate

Osmoconformer

e.g. marine invertebrates osmoregulator

Hypertonic solution

Hypotonic solution

Isotonic solution

osmoregulator

Bony Fish can be:

FRESHWATER

eliminate ammonia

MARINE

eliminate urea &

trimethylamine oxide

Trimethylamine oxide = fish odour

Why is the nitrogenous waste product different in the two groups of fish?

FRESHWATER fish

afford to lose water

MARINE fish

cannot afford to lose water

Gills are permeable to:

1. Water

2. Ions

Freshwater Bony Fish:

Hypertonic body fluids

Not salty

Gills of freshwater bony fish:


GAIN water

But this disturbs body fluid concentration!!

LOSE salts

gills



Water flows by osmosis through the gill surfaces

Water must be removed otherwise fish is no longer hypertonic.

How?



The fish discharges copious quantities of very dilute urine, few salts lost

Nitrogenous waste: Ammonia

No drinking

Kidneys of freshwater bony fish:

contain many large Malpighian bodies, with large

glomeruli

high rate of filtration produces a large

volume of glomerular filtrate

How does the fish remain hypertonic?

GAINS salts:

Selective reabsorption in kidney


Selective uptake of Cl- at gills

In food

Marine Fish:

Hypotonic body fluids

SALTY

Marine Fish:


Loss of water by osmosis

Compensatory mechanisms to

avoid dehydration MUST be present

At gills:

Gain of ions by diffusion

Marine Fish DRINK sea water:


To replace water lost

But this means SALTS are gained too. What happens to salts?

Excess ions are:

Nitrogenous waste: Urea + Trimethylamine oxide


actively secreted by special excretory cells in the gills

Some marine bony fish have NO glomeruli at all :

and so do not filter their blood urine is isotonic with the body fluids

Question: [SEP, 2010]

Fish do not need to convert ammonia into urea. Suggest ONE reason for this. (1)

Freshwater bony fish eliminate ammonia by adding large quantities of water to make it less toxic. As they can afford to lose a lot of water, they eliminate their nitrogenous waste in the form of ammonia rather than urea.

OVERVIEW




D) THE HUMAN KIDNEY

The kidneys contribute to homeostasis

Let us see how:

Functions of the Human Kidney:

1. Removal of metabolic waste products

2. Regulation of the water content

3. Regulation of the pH of body fluids

4. Regulation of the chemical composition of body fluids by removal of substances which are in excess of immediate requirements

Position and structure of kidneys

External structure of a Pig Kidney

Kidneys are surrounded by a fibrous capsule:

LS through human kidney

medulla

cortex

LS through human kidney

the apex of each pyramid

papilla

The renal artery branches inside kidney

Renal artery

Ureter

Renal vein

Each capillary supplies blood to hundreds of thousands of tiny filtration units called nephrons

Detail of a nephron

Two types of nephron:

Juxtamedullary nephron

Cortical nephron

Juxta = close to

CORTEX

MEDULLA

Loop of Henle

Collecting duct

Nephron is the basic structural & functional unit of the kidney

Deal with the control of blood volume under

normal conditions of water

availability

Increase water retention when

water is in short supply

Cortical nephron Juxtamedullary nephron

Nephron Structure

vasa recta

Slow blood flow:

important to produce a

concentrated urine

The nephron 1.5 million per kidney

collecting duct

Bowman’s capsule

distal tubule

loop of Henle

proximal tubule

The nephron blood supply

peritubular capillaries

Vasa Recta

glomerulus

branch of renal artery

afferent arterioles

efferent arterioles

branch of renal vein

The glomerular capillaries drain into efferent arterioles not venules. ‘Portal System’

Three key process in urine formation:

Ultrafiltration

Excretion =

ultrafiltration – reabsorption + secretion

Selective reabsorption

Ultrafiltration

takes place in the renal capsule

is filtration under pressure

pressure comes from blood pressure (hydrostatic pressure)

http://en.wikipedia.org/wiki/File:Gray1130.svg

Glomerular Filtrate (GF): is the filtered fluid

chemical composition is similar to blood plasma, containing:-

Glucose

Amino acids

Vitamins

Ions

Nitrogenous waste

Some hormones

Water

Glomerular filtrate


Key Words!!

Nephron: structure in the kidney that acts as a microscopic filtration unit

Glomerulus:

dense mass of very fine blood capillaries at the nephron that act as a filter

Key Words!!

Bowman’s capsule: cup-shaped part of the nephron that holds a glomerulus and collects the products of filtration from it

Glomerular filtrate:

liquid removed from the blood by filtration in the kidney

Explain why proteins & RBC are not found in urine.

Too large to be filtered.

But can blood ever be detected in urine?

YES. But, this shows that

something is wrong .

Ultrafiltration takes place through three layers:

1) Endothelium of the blood capillary

2) Basement membrane of the blood capillaries

3) Epithelium of the renal capsule


Cells lining the Bowman’s capsule:

Podocyte

Squamous epithelium

Podocytes (modified squamous epithelial

cells):

highly modified for filtration


Podocytes:

each podocyte has many foot-like extensions projecting from its surface

the projections interlink with extensions from neighbouring cells

they fit together loosely, leaving slits called SLIT PORES or FILTRATION SLITS

The basement membrane is

the main filtration barrier

endothelial fenestration


Some types of capillaries have ‘fenestrations’

Fenestrated capillary has holes to facilitate filtration Continuous capillary

Filtrate passes through the

basement membrane & not across cells

Filtration through podocytes

Basement membrane

Fenestrated capillaries

(capillaries with windows) Permeable to substances < 100 nm

endothelial cell

fenestration

nucleus

Filtration Barrier

mesangial cells

podocyte

slit pore

glucose amino acids

(basement membrane)

podocyte slit pore

Na+

- -

-

-

-

- - -

- -

-

-

-

-

-

-

- -

- -

- -

-

-

- -

- -

-

Limited permeability to molecules between

7000 > mwt > 70000 Da 4 nm > diameter > 8 nm

Freely permeable to small molecules mwt < 7000 Da

diameter < 4 nm

Not permeable to large molecules mwt > 70000 Da diameter > 8 nm

Water Permeable albumin 60000 Da

completely excluded…

because of –ve charge

endothelial cell fenestration

Bowman’s Capsule

Bowman’s Space

Proximal Tubule

[email protected]

The net filtration pressure =

1.3 kPa

8 kPa

2.7 kPa 4 kPa

Arterial pressure 8 kPa

Plasma osmotic pressure 4 kPa

Glomerular capsule pressure 2.7 kPa 6.7 kPa

Arterial pressure - Plasma osmotic pressure

Glomerular capsule pressure +

Net outward pressure = 8 – (4 + 2.7) = 1.3 kPa

Three ways to increase the filtration rate:

1. Raising blood pressure

2. Dilating the afferent arterioles (to decrease the resistance to the flow of blood into the glomerulus)

3. Constricting the efferent arterioles

Efferent arteriole

Afferent arteriole

Filtration pressure

GFR maintained

Afferent arteriole narrow

LOW pressure

HIGH pressure

Efferent arteriole

wide

Dilating the afferent arterioles &

Constricting the efferent arterioles

BUT when arterial pressure falls too low,

however, the kidney fails to produce urine

Arterial pressure 8 kPa

Plasma osmotic pressure 4 kPa

Glomerular capsule pressure 2.7 kPa 6.7 kPa

Net outward pressure = 8 – (4 + 2.7) = 1.3 kPa

The Proximal Convoluted Tubule

longest (14 mm) and widest (60 m) part of the nephron

carries filtrate from Bowman’s capsule to loop of Henle

CORTEX

MEDULLA

Function of the nephron is to :

actively secrete

waste substances from the blood capillaries to

the tubules

selectively reabsorb

substances useful to the body

Proximal Convoluted Tubule is composed of:

a single layer of cuboidal epithelial cells with extensive microvilli forming a ‘brush border’ on the inside surface of the tubule

Figure 44.9

Proximal Convoluted Tubule is adapted for reabsorption in three ways:

1. large surface area due to:

Figure 44.9

Microvilli

Basal channels

BLOOD FILTRATE

Tight junction

Epithelial cell

Proximal Convoluted Tubule is adapted for reabsorption:

Figure 44.9

2. numerous mitochondria (M)

Proximal Convoluted Tubule is adapted for reabsorption:

Figure 44.9

3.closeness of blood capillaries

blood capillary Glomerular filtrate

Microvilli Cuboidal epithelium

Over 80% of filtrate is reabsorbed in the proximal tubule

REABSORBED

all the glucose, amino acids, vitamins, hormones

about 80% water

about 80% sodium

about 80% chloride

about 80% potassium

about 40-50% urea

MECHANISM

diffusion + active transport

osmosis

diffusion

+ active transport

diffusion

Fig. 15 Selective reabsorption of sodium in the proximal convoluted tubule

Figure 44.9

1

Selective reabsorption of glucose in the proximal convoluted tubule

Figure 44.9

Secondary Active

Transport

Na+

glucose Na+

ATP ADP

Blood

Urine

Proximal tubule epithelial cell

[email protected]

Selective reabsorption in the proximal convoluted tubule

In humans:

Glomerular filtrate production: 125 cm3 min-1

Urine production: 1 cm3 min-1

24 cm3

100 cm3

Urine 1 cm3

125 cm3

Question: MAY, 2012

Briefly describe the following processes in the context of urine formation in humans.

a) Ultrafiltration. (2)

Filtration of blood occurs under high pressure provided by the heart. Small molecules which can cross the glomerular lining, end up as glomerular filtrate inside the Bowman’s capsule.

b) Selective reabsorption of glucose. (3)

Occurs in the proximal convoluted tubule. All glucose is reabsorbed in a normal person but appears in urine in a diabetic one. Secondary active transport is involved in the reabsorption of glucose. A symport binds sodium ions and glucose to transport them from the lumen of the proximal convoluted tubule into the epithelial cells. Glucose leaves the cell by facilitated diffusion through a carrier protein. Glucose diffuses into the blood capillary.

THE LOOP OF HENLE

Function: to conserve water

the concentration of urine produced is directly related to the:

length of the loop of Henle

thickness of the medulla relative to the cortex

The longer the loop of Henle, the more concentrated the urine that can be produced

BEAVER (abundant water)

RABBIT (moderate water)

SAND RAT (scarce water)


Use your knowledge of biology to describe the selective advantage of the following adaptation.

Desert rats have a long loop of Henle. (5) The loop of Henle acts as a counter-current multiplier.

Fluid moves in opposite directions in the descending and ascending limbs. The ascending limb is permeable to salts which contribute towards a concentrated medulla. As water moves down the descending limb, it moves out into the vasa recta.

Desert rats need to conserve water. Thus having a long loop of Henle enables them to extract as much water as possible out of the glomerular filtrate as there is more time for reabsorption.


The table below gives the thickness of the medulla in relation to the rest of the kidney in a number of mammals. The maximum urine concentration for each mammal is also given. The data suggest that maximum urine concentration increases with relative thickness of the medulla.

Mammal Relative thickness of medulla

Maximum urine concentration in arbitrary units

Beaver 1.0 52

Human 2.6 140

Kangaroo rat

7.8 550

Species X 9.8 940

a) Why is such a relation between urine concentration and the relative thickness of the medulla observed?

(1)

The thicker the medulla, the higher the urine concentration produced due to more chance for water reabsorption.



Beaver 1.0 52

Human 2.6 140

Kangaroo rat 7.8 550

Species X 9.8 940

b) What habitat is species X likely to inhabit? (1)

Desert / dry habitat



Beaver 1.0 52

Human 2.6 140

Kangaroo rat

7.8 550

Species X 9.8 940

Birds & Mammals are the only vertebrates:

which can produce a urine which is more concentrated than the blood

[hypertonic]

with loops of Henle

Loop of Henle

The loop of Henle creates a concentration gradient

humans can produce urine that is 4x more

concentrated than their blood plasma

11200/ 300 = 4

Pelvis Medulla

Cortex

a countercurrent multiplier mechanism made possible by the anatomical arrangement of the loops of Henle

The concentrating ability of the mammalian kidney arises from

hairpin turn of the loop of Henle

countercurrent refers to the opposing directions in which the tubule fluid in the descending and the ascending limbs flows

multiplier refers to the ability of this system to create a solute concentration gradient in the renal medulla

Medulla

Cortex

The loops of Henle do not themselves produce concentrated urine;

rather they increase the osmolarity of the extracellular fluid in the medulla in a graduated way:

from 300 to 1,200 mosm/l

Osmolarity is :

a measure of solute concentration

the osmolarity of a solution is the number of moles of active solutes per litre of solvent

osmole [Osm or osmol]

[For your information only]

The loops produce this effect as explained below

Three distinct regions in the loop of Henle

Thin ascending limb

Descending limb

Thick ascending limb

Thin walls

Thick walls

Permeability of the loop of Henle to water:

Highly permeable

Descending limb

Almost

totally impermeable

to water Thin ascending limb


Permeability of the loop of Henle to Na+ & Cl-ions:

Not very permeable

Descending limb

Thin ascending limb


Permeable

Active secretion

What happens to the concentration of the fluid in the ascending limb as it reaches the

distal convoluted tubule?

The fluid becomes very dilute

Distil convoluted tubule

Reason: IONS are lost

WHY it is vital for ions to move out of the tubule?

ions

To create an Osmotic Gradient From Cortex to Medulla

Pelvis Medulla

Cortex

The outer layer of the kidney is isotonic with the blood:

~300 milliosmoles/liter

The innermost layer (medulla) is very hypertonic: ~1200 milliosmoles/liter

The concentration gradient allows:

water to move out by

osmosis from the

descending loop of Henle

Vasa recta as countercurrent exchangers

• the countercurrent exchange of salt occurs in the vasa recta

1. Blood flowing into the medulla in the descending limb picks up salt from the hypertonic medulla.

2. As the surrounding medullary fluid becomes more salty toward the papilla, more salt is picked up by the descending vasa recta limb.

Vasa recta as countercurrent exchangers

3. But as the blood heads back up to the cortex in the ascending limb of the vasa recta, the interstitial fluid becomes less and less salty

4. This causes the gradient to reverse and salt diffuses back out of the vasa recta into the medulla

What is the importance of the vasa recta as an exchanger of salts?

1. to help conserve salt

2. keep the medulla hypertonic

Countercurrents exist when :

fluids flow in opposite directions in parallel and adjacent tubes

Fig. 20 Three Countercurrents:

1. the two limbs of the Henle's loop



2. the two limbs of the vasa recta



2. the two limbs of the vasa recta

3. the descending limb of Henle with the ascending limb of the vasa recta;

the ascending limb of Henle and the descending vasa recta

Question: [SEP, 2009]

Briefly describe the role of each of the following in osmoregulation in humans:

i) The descending limb of the Loop of Henle; (2)

Is permeable to water. Functions towards water conservation.

ii) The ascending limb of the Loop of Henle; (2)

Is relatively impermeable to water but permeable to salts. The tissue fluid inside the medulla becomes concentrated as salts move out of the ascending limb. This causes water to be drawn out of the descending limb.

Question: MAY, 2012

The diagram below shows the simplified structure of a human nephron. the loop of Henle

Substance Quantity passing

through P

Quantity passing

through Q

%

reabsorbed

Water 180 dm3 1.5 dm3 99.17%

Glucose 180 g 0 g 100%

Urea 53 g 25 g 52.8%

The table below represents the quantities of water, glucose and urea passing through P and Q over a period of time, while the last column shows the percentage reabsorption during the same period of time.

Question: MAY, 2012

a) Relate the role of structure R to the filtrate composition as it passes through Q. (5)

Structure Q is permeable to water. Water is reabsorbed by the vasa recta as fluid passes through Q. This is possible because the ascending limb creates the ideal concentration gradient within the medulla by losing ions. The thin ascending limb of Structure R is permeable to ions but impermeable to water. The thick ascending limb of Structure R allows ions to move actively out of it and is also impermeable to water. Loss of ions from the whole ascending limb, creates an ever increasing salt concentration on moving deeper into the medulla.

Question: MAY, 2012

Substance Quantity passing

through P

Quantity passing

through Q

%

reabsorbed

Water 180 dm3 1.5 dm3 99.17%

Glucose 180 g 0 g 100%

Urea 53 g 25 g 52.8%

b) Explain the biological significance of the percentage reabsorption of water and urea. (3)

Most of the water is reabsorbed to avoid dehydration.

Only half of the urea is reabsorbed so that it contributes to the concentration of solutes in the medulla. A high solute concentration is needed to ensure reabsorption of water from the loop of Henle.

THE DISTAL CONVOLUTED TUBULE AND COLLECTING DUCT

Functions:

1. fine tuning of the body fluid composition

2. control blood pH

Cell structure of the distal tubule:

similar structure to those of the proximal tubule, with:

microvilli

mitochondria

REGULATION OF KIDNEY FUNCTIONS

Several regulatory mechanisms act on the kidneys to maintain:

blood pressure

blood osmolarity

blood composition

Glomerular filtration rate is regulated

if the kidneys stop filtering blood, they cannot accomplish any of their functions

the maintenance of a constant GLOMERULAR FILTRATION RATE (GFR) depends on:

an adequate blood supply to the kidneys

at an adequate blood pressure

High pressure results from TWO ways:

1. renal arteries that deliver blood to the kidneys at high pressure because they are early branches off the aorta

2. AUTOREGULATORY mechanisms ensure adequate: blood supply blood pressure

regardless of what is happening elsewhere

in the body

Autoregulatory mechanisms involve:

The dilation of the afferent renal

arterioles when blood pressure falls

The release of the enzyme RENIN from the kidney into the blood, if arteriole filtration

does not keep the GFR from falling

1

2

Renin is released:

From: a group of secretory cells,

the JUXTAGLOMERULAR COMPLEX situated between the: distal convoluted afferent arteriole

When: the blood pressure volume

decrease

Role of renin:

angiotensinogen

[made by liver]

Renin

converts a circulating protein produced by the liver, ANGIOTENSINOGEN, into ANGIOTENSIN I

angiotensin I

angiotensin II or angiotensin

enzyme

1. It constricts the efferent arterioles.

2. It constricts peripheral blood vessels all over the body.

3. It stimulates the adrenal cortex to release the hormone ALDOSTERONE.

Efferent arteriole

Afferent arteriole

Filtration pressure

GFR maintained

Angiotensin has several effects that help restore the GFR to normal:

4. It acts on the brain to stimulate thirst.

Effect of aldosterone on the distal convoluted tubule:

Stimulates the Na+/K+ pumps in the cells of the tubule

Decrease in Na+

Results in a low blood volume & pressure (as less water enters by osmosis)

Activates angiotensinogen to become ANGIOTENSIN I

RENIN is released

ALDOSTERONE is released

Causes Na/K pump in distal tubule to take up Na+ into the blood

Water enters the blood

ANGIOTENSIN I changes into ANGIOTENSIN II

OSMOREGULATION, ADH & URINE FORMATION

In this topic we mention TWO hormones that affect the kidneys:

Urine

ADH (antidiuretic

hormone)

Posterior pituitary

Adrenal cortex

Aldosterone

ADH brings about the precise control of

solute potential in TWO ways:

1. increasing the permeability of the distal convoluted tubule and collecting duct to water

2. increasing the permeability of the collecting duct to urea

1. Urea moves into

medulla

2. Medulla becomes concentrated

RESULT: 3. Water moves out of

descending limb

ADH is released when osmoreceptors: detect a low level of water in blood

kidney

Water Salts

Fig. 23 The effect of ADH on the permeability of the distal convoluted tubule and collecting duct to water

Blood too concentrated

ADH level high

Blood too dilute

ADH level low

Dilute

urine

Urine concentrated

Water Salts

Fig. 22 Aquaporins

H2O

H2O

H2O

Release of ADH from the posterior pituitary is inhibited by drinking

alcohol & caffeine.

How would this affect urination?

Increases

ADH

Failure to release sufficient ADH leads to a condition: DIABETES INSIPIDUS

large quantities of dilute urine are produced

Water level regulation by negative feedback control

Water content of the blood normal

Water content of the blood HIGH

Water content of the blood LOW

Too much water drunk

Too much salt or sweating

Brain produces More ADH

Urine output LOW

Brain produces Less ADH

Urine output HIGH

High volume of water reabsorbed by kidney

Low volume of water reabsorbed by kidney

(small volume of Concentrated urine)

(large volume of dilute urine)


1. The human kidney, in association with various hormones, plays a central role in the regulation of the chemical and physical characteristics of blood.

a) List THREE ways through which the human kidney may affect

the chemical composition of blood. (3) 1) Through aldosterone, the kidney determines the amount of

sodium and potassium in the blood. 2) Through ADH, the kidney plays a role in the amount of

water in the blood. 3) The kidney helps to keep the blood pH constant by secreting

H+ or OH-.

b) Complete the table below by filling in the empty spaces with the appropriate answers: (3)

Hormone Site of

production

Effect

Antidiuretic

hormone

Adrenal

cortex

Stimulates excretion of

potassium ions and

reabsorption of sodium

ions in the nephron

b) Complete the table below by filling in the empty spaces with the appropriate answers: (3)

Hormone Site of

production

Effect

Antidiuretic

hormone

Hypothalamus

Stimulates distal

convoluted tubule and

collecting duct to

reabsorb water

Aldosterone

Adrenal

cortex

Stimulates excretion of

potassium ions and

reabsorption of sodium

ions in the nephron


c) Briefly describe how vasoconstriction and vasodilation of blood vessels may affect blood pressure. (4)

When blood vessels dilate, the blood pressure is lowered as there is less resistance to blood flow. When blood vessels constrict, the blood pressure becomes higher as cross-sectional area decreases.

Control of Blood pH Blood pH:

maintained at 7.4

Urine pH varies:

4.5 - 8.2

Abrupt changes in blood pH

are prevented by

Plasma proteins

Phosphate

Hydrogen carbonate

buffers

Longer-term adjustments in the ion balance of the blood

are made in the distal convoluted

tubule

Falls below 7.4:

distal tubule cells secrete H+ into the

urine

Rises:

distal tubule cells secrete OH- & HCO3

- into the urine

H+ HCO3

-

OH-

If the pH:

Essay Titles

1. Give an overview of the role of the mammalian kidney in excretion and osmoregulation.

[SEP, 2000]

2. Evaluate the role of the human kidney in excretion and osmoregulation. [SEP, 2002]

3. The mammalian kidney is a homeostatic organ. Discuss. [SEP, 2004]

Essay Titles

4. Write an account on biological countercurrent systems. [SEP, 2013]

Gills in bony fish – blood & seawater flow

Thermoregulation – blood flow in artery & vein in a limb

Excretion – loop of Henle; vasa recta

Pregnant female - blood of embryo & uterus in certain mammals like rabbits, but not humans

Erythropoietin [EPO] :

Made by: Kidneys

Released when:

O2 levels in blood are low

Causes:

RBC formation

Negative Feedback Control

EPO is abused by certain athletes. What is the benefit?

Blood carries more oxygen.

Manneken Piss [Brussels, Belgium]

excretion [2015]

Education

ammonia urea

excretion osmoregulation

terrestrial urea

habitat ammonia

definitions osmoregulation

uric acid ammonia protein

definitions excretion

freshwater uric acid