Biology - Chapter 31: Excretion

functions of kidney

• excretion (removal of metabolic wastes such as urea and salts)

• osmoregulation (regulating water balance)

• maintains pH balance of blood

homeostatsis

the ability of an organism to maintain a constant internal environment

filtration process (1)

1. blood enters the nephron through the afferent arteriole contains waste products. Remember that efferent arteriole is narrower than afferent arteriole.

2. Filtration takes place in glomerulus. Small molecules like glucose, amino acids, vitamins, some hormones, urea, salts and water are forced out of plasma and squeezed into Bowman’s capsule. This creates glomerular filtrate.

3. Structure of glomerulus helps filtration in 3 ways:

   • pressure in glomerulus is greater than normal blood pressure due to already higher pressure of afferent arteriole being increased due to efferent arteriole being narrower. Filtration in Bowman’s capsule is called ultra-filtration

   • large surface area in glomerulus for filtration

   • walls of glomerular capillaries more porous & wall of Bowman’s capsule is 1 cell thick

4. larger substances don’t enter glomerular filtrate, including WBCs, RBCs, platelets, antibodies, clotting proteins and some hormones

reabsorption (2)

glomerular filtrate contains water, most salts & useful substances that are reabsorbed into the blood

1. Proximal convoluted tube:

   • 90% water reabsorbed through osmosis

   • useful molecules like glucose, amino acids and vitamins reabsorbed by combination of diffusion and active transport, as well as salts

   Proximal tubule:

   • one-cell thick

   • long

   • microvilli (infoldings) in its cells

   • high concentration of mitochondria to provide energy for active transport

2. Loop of Henle - descending limb of loop of henle is permeable to water reabsorbed

3. Ascending limb of Loop of Henle is permeable to salts - salts move out of nephron into fluid of the medulla. Initially salts are moved through diffusion, then at top of ascending limb it’s pumped out by active transport

4. Distal convoluted tube - controls water, salt and pH values of blood. Some water and salts reabsorbed if ADH is present

5. Collecting duct is permeable to water. Responds to ADH

6. Urine moves to pelvis, ureters and on to bladders for storage

secretion (3)

some substances pass from blood into nephron

• blood secretes K+ and H+ ions into distal convoluted tubules to maintain pH of blood

ADH - antidiuretic hormone

• affects DCT and collecting ducts

• if you are dehydrated → pituitary gland releases ADH

• DCT and collecting ducts are more porous

• more water reabsorbed into bloodstream

• lower volume of urine released

Control: dehydration

1. Receptors in hypothalamus of brain detects a decrease in water content

2. Pituitary gland receives information and coordinates a response by releasing more ADH

3. ADH travels to kidneys in the bloodstream, where in the kidneys, ADH causes walls of distal tubule and collecting ducts become more permeable

4. Kidneys reabsorb more water from the nephron, hence urine is more concentrated (reduction in salt concentration of plasma & volume of urine produced)

Control: hydration

1. Receptors in hypothalamus of brain detects a decrease in water content

2. Pituitary gland receives information and coordinates a response by releasing less ADH or even none

3. Distal tubules and collecting ducts become relatively impermeable to water

4. Very little water reabsorbed from distal tubules and collecting ducts, hence urine is more diluted (concentration of plasma remain unchanged and production of large volume of urine)

glomerular filtrate vs urine

• GF has more water than urine

• contains many useful molecules such as glucose & amino acids

formation of urea

• formed as byproduct of breakdown of amino acids in liver (deamination)

• urea is transported in blood to kidneys, where it’s removed and secreted in the urine

functions of the skin

• epidermis protects body from damage and acts as a barrier to prevent the loss of water and entry of pathogens

• dermis protects internal organs from damage due to bumps and bangs

• melanin protects the skin from ultraviolet radiation]sebum produced by sebaceous glands keeps hair moist, flexible and also prevents the skin from drying up and becoming cracked

leaves as excretory organs

• plants lose oxygen and water vapour during the day through the stomata & lenticels

• at night, plants also lose CO2

main processes in the kidneys

• filtration → takes place in the cortex

• reabsorption → takes place in cortex and medulla

• secretion → takes place in cortex