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Define Excretion

Excretion is the removal from the body of the waste products of metabolic processes


Outline the function of the Afferent Arteriole

Brings blood to the nephron to be filtered


Outline the function of the Efferent Arteriole

Removes filtered blood from nephron


Outline the function of the Glomerulus

A group of branching capillaries where filtration occurs


Outline the function of the Bowman's Capsule

First part of the nephron where filtration occurs and the filtrate is collected


Outline the function of the Proximal Convoluted Tubule

Where selective reabsorption occurs


Outline the function of the Loop of Henle

Important for establishing a salt gradient in the medulla


Outline the function of the Distal Convoluted Tubule

Final site of selective reabsorption


Outline the function of the Collecting Duct

Leftover filtrate goes to the ureter; where osmoregulation occurs


Outline the function of the Vasa Recta

Blood network which reabsorbs components from the filtrate


Explain the process of ultrafiltration

• Ultrafiltration occurs in the Bowman's capsule in the cortex of the kidney
• Blood enters the afferent arteriole and leaves the efferent arteriole
• The afferent arteriole is larger than the efferent arteriole, causing high pressure in the glomerulus
• Water, glucose, amino acids and solutes (WAGS) are forced out of blood through the fenestrated capillaries and the basement membrane
• Podocyte cells act as a filter
• Plasma proteins, platelets and red/white blood cells are large, so remain in blood
• Glomerular filtrate is carried through the nephron, where selective reabsorption takes place in the proximal convoluted tubule


Define osmoregulation

The control of the water balance of the blood, tissue or cytoplasm of a living organism


Explain selective reabsorbtion in the kidney

• Water, salts, amino acids and glucose in glomerular filtrate need to be reabsorbed
• Selective reabsorption occurs in proximal convoluted tubule (PCT) of nephron
• Convolution and microvilli increase surface area for reabsorption
• Large numbers mitochondria present in PCT cells generate ATP
• Active transport pumps Na+ ions from PCT cells into capillaries, generating concentration gradient between PCT cells and filtrate in lumen
• Na+ ions move from lumen to PCT cells by facilitated diffusion
• Glucose and amino acids follow by cotransport
• Solute concentration increases in PCT cells, leading to osmosis of water from lumen into PCT cells
• Facilitated diffusion carries glucose and amino acids into blood
• Flow of blood maintains concentration gradient
• ~80% of water is reabsorbed by osmosis
• 100% glucose and amino acids recovered
• Active transport used for the transport of glucose and amino acids, so there are large number of mitochondria to provide the required ATP
•Some urea also diffuses back into the blood


State the two locations in the kidney where osmoregulation occurs

The Loop of Henle and the Collecting Duct


Explain the role of the Loop of Henle in maintaining the water balance of the blood

• Descending limb is permeable to water but not to Na+
• Ascending limb is permeable to NA+, but not to water
• Na+ is pumped out of the ascending limb into the medulla, generating osmotic potential between the nephron and medulla
• Some water leaves descending loop by osmosis
• Output is reduced volume, reduced salt concentration


Explain the role of the Collecting Duct in maintaining the water balance of the blood

• Filtrate enters collecting duct from distal convoluted tubule
• Countercurrent flow of blood in capillaries and filtrate in duct maintains concentration gradient, allowing for osmosis of water into blood
• Dehydration (detected by hypothalamus) leads to an increased level of ADH (released by pituitary)
• ADH opens aquaporins (water channels) in walls of duct
• Increased transfer of water into blood, therefore hypertonic urine
• Excess water in blood leads to break down of aquaporins and hypotonic urine


Outline the function of the kidneys

The two kidneys act as filters for the blood, removing harmful toxins