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exam b ief Biology
Topic 3: Excretion
Vertical section of a kidney
A nephron and its blood supply
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exam b ief Biology
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The kidneys have 2 main functions:
Function 1 is ‘Excretion’: ‘the removal from the body of the waste products of
metabolism, as urine’.
Outline account on ‘urine formation’ in the kidney nephron: ( Higher level only )
Urine formation involves two stages: ‘ filtration and reabsorption’.
Stage 1: Filtration
• Occurs in the glomerulus, into the bowman’s capsule.
• Blood in the afferent arteriole is under high pressure, as it is arterial.
• Plasma is filtered into the Bowman’s capsule, forming filtrate.
• Molecules such as urea, glucose, water, salts and amino acids are filtered, as they
are small enough to pass through the glomerular pores.
• All the blood cells and plasma proteins are too large to pass through the glomerulus and will eventually leave the kidneys through the renal veins.
• Features of the nephron which aid filtration include: its large surface, efferent arteriole is narrower than the afferent and the lining is one cell thick.
Stage 2: Reabsorption
• The filtrate passes from the Bowman’s capsule, into the proximal convoluted tubule, where reabsorption of substances needed by the body, takes place.
• Substances reabsorbed, into the blood at the Proximal tubule include:
a. All the glucose and amino acids ---- ‘by active transport’
b. Most of the water --------------------- ‘by osmosis’
c. Most of the salts ---------------------- ‘by active transport’
• More water is reabsorbed at the Loop of Henle and at the Distal convoluted tubule.
• The remaining liquid is now called ‘urine’ and this passes into the common collecting duct → pelvis of kidney → ureters → bladder for storage.
• Features of the nephron which aid reabsorption include:
º Presence of mitochondria to provide energy for ‘active transport’
º The wall of the Proximal convoluted tubule is ‘one cell thick’ and has microvilli
to
increase the surface area for absorption.
Function 2 is ‘Osmoregulation’: ‘maintaining the correct balance of water and
dissolved solutes in the blood’ ---- under the control of the hormone ADH.
Mechanism of osmoregulation:
A: Bodily fluids are too concentrated --- ‘there is a drop in the water content of
the blood’
Caused by a combination of 3 factors:
• Much sweating.
• Low water intake.
• High salt intake.
exam b ief Biology
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Solution
º The raised blood concentration is registered by osmoreceptors in the hypothalamus
of the brain.
º The hypothalamus stimulates the pituitary gland to secrete more of the hormone
ADH ( = anti-diuretic hormone ).
º ADH causes the walls of the distal tubules and collecting ducts to become more
permeable to water; therefore, more water is reabsorbed from the urine as it flows
down the collecting duct.
º A smaller volume of more concentrated urine is excreted.
B: Bodily fluids are too dilute --- ‘there is rise in the water content of the blood’
Caused by a combination of 3 factors:
• Little sweating.
• Large intake of water.
• Low salt intake.
Solution
º The lower blood concentration inhibits the release of ADH.
º There is little or no reabsorption of water from the distal tubule and collecting duct
> therefore, little or no reabsorption of water from urine in the collecting duct occurs.
º A larger volume of more dilute urine is excreted.
2008 question 13
13. (a)
(i)
(ii)
What is meant by excretion?
Urea and carbon dioxide are excretory products of the human body.
In the case of each product name a substance from which it is
derived. (9)
(b) The diagram shows the structure of a nephron and its associated blood supply.
(i)
Name the parts A, B, C, D, E and F.
(ii)
From which blood vessel is A derived?
(iii)
Where in the kidney is B located?
(iv)
Give the part of the nephron in which each of the
following takes place:
1. filtration, 2. reabsorption of amino acids.
(v)
Give two features of the nephron that aid filtration.
(vi)
Name a group of biomolecules in the blood which
are too large to pass through the filtration system
of the nephron.
exam b ief Biology
(c) (i)
(ii)
(iii)
(iv)
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Suggest two situations which may result in a drop in the water content of
the blood.
When the water content of the blood drops a hormone is released.
Name this hormone and the endocrine gland from which it is secreted.
Give a precise target area for this hormone. How does the hormone
reach the target area?
Explain the role of the hormone at its target area, when the water con
tent of the blood is low. (24)
13
(a) (i) Removal from the body of the waste products of metabolism
(ii) Urea is derived from protein (or amino acid)
Carbon dioxide is derived from glucose (or carbohydrate)
(b) (i) A= afferent arteriole
B= Bowman’s capsule
C = Proximal tubule
D = Loop of Henle
E = Collecting duct
F = Distal tubule (will accept Loop of Henle)
(ii) Renal artery (or renal arteriole)
(iii) Cortex
(iv) 1. Filtration occurs in the Glomerulus (or Bowman’s capsule)
2. Amino acids are reabsorbed in the Proximal tubule.
(v) Two features which aid filtration are
1. efferent arteriole is narrower than the afferent arteriole
2. large surface area
(vi) Proteins
(c) (i) Two situations include:
a) exercise with sweating
b) low water intake
(ii) Hormone name = ADH
Secreted from the pituitary
(iii) Precise target area for ADH is the collecting duct (or distal tubule)
The hormone is carried in the bloodstream
(iv) makes the walls more permeable so there is more reabsorption of water
*Note* words underlined are compulsory and ‘all’ answer must be correct for full
marks