5. Excretion

Question 1

Excretion

Definition

Answer 1

Excretion is the removal of metabolic waste products and toxic materials from organisms.

Question 2

Importance of Excretion

Answer 2

Excretion is important as metabolic waste products can be harmful and prevent the maintenance of a steady state in the body if allowed to accumulate to high concentrations.

Question 3

Examples of Excretion

Answer 3

Examples of excretion in humans:

1. Lungs excrete CO₂ during expiration
2. Kidneys and skin excrete urea (deamination of amino acids), uric acid (breakdown of nucleic acid), and creatinine (breakdown of muscle tissue) during urination and sweating
3. Lungs, kidney, and skin excrete excess water during expiration, urination, and sweating
4. Liver excretes bile pigment (breakdown of haemoglobin) which is in faeces

Unicellular organisms use diffusion for excretion, multicellular organisms need organs.

Question 4

Kidney

Structure & Function

Answer 4

Structure:

• Cortex: the outer dark red region
• Medulla: the inner pale red regions consisting of renal pyramids
• Renal pyramid: conical structures consisting of nephrons
• Nephrons (kidney tubules)
• Renal pelvis: enlarged portion of the ureter inside the kidneys

Function:

• Formation of urine for excretion: contain numerous nephrons which remove urea, excess water, and heat from blood to form urine
• Osmoregulation: the process of maintaining a constant water potential of body fluids

Question 5

Ureter

Structure & Function

Answer 5

Structure:

• A tube that connects the kidney to the bladder

Function:

• Carries urine from the kidneys to the bladder

Question 6

Bladder

Structure & Function

Answer 6

Structure:

• A muscular bag

Function:

• Stores urine temporarily

Question 7

Urethra

Structure & Function

Answer 7

Structure:

• A muscular tube from the bladder to the exterior

Function:

• Carries urine out of the body

Question 8

Nephron (Kidney Tubule)

Structure & Function

Answer 8

Structure:

• Bowman’s capsule: a cup-like structure found at the beginning of the nephron
• Proximal convoluted tubule: a short, coiled tubule that straightens out as it enters the medulla
• Loop of Henle: the U-shaped portion of the tubule in the medulla
• Distal convoluted tubule: as the tubule re-enters the cortex, it coils again to form the distal convoluted tubule
• Collecting duct: the tubule combines with other tubules to form the collecting duct

Function:

• Formation of urine through 2 main processes: ultrafiltration of small molecules and selective reabsorption of useful substances

Question 9

Renal Artery & Renal Vein

Function

Answer 9

Renal artery brings oxygenated blood to kidney

• Blood has high concentration of nitrogenous waste products (urea, uric acid, creatinine)

Renal vein carries deoxygenated blood away from kidney

• Blood has low concentration of nitrogenous waste products (urea, uric acid, creatinine)

Question 9

Ultrafiltration

Definition

Answer 9

Ultrafiltration is the non-selective filtering process where most of the blood plasma and dissolved substances are forced out of the glomerulus into the Bowman’s capsule by high blood pressure.

Question 9

Proximal Convoluted Tubule

Structural Adaptations

Answer 9

1. Dense network of blood capillaries around the PCT:
   - The continuous transport of useful substances away from the PCT maintains a steep concentration gratident, for faster selective reabsorption of useful subtances by facilitated diffusion and active transport and water by osmosis
2. Long and highly coiled:
   - To increase surface area to volume ratio, for faster selective reabsorption of useful subtances by facilitated diffusion and active transport and water by osmosis
   - To slow down the movement of filtrate to allow more time for selective reabsorption of useful substances back into the blood stream
3. The epithelial cells lining the tubules have numerous microvilli:
   - To increase surface area for faster selective reabsorption of the useful substances into the blood stream.

Question 9

Ultrafiltration

Process

Answer 9

1. Blood enters the glomerulus through the afferent arteriole and leaves through the efferent arteriole.
2. The lumen of the afferent arteriole is wider than the lumen of the efferent arteriole. Blood enters the glomerulus more readily through the wider afferent arteriole than it can leave through the narrower efferent arteriole. As a result, blood dams up in the glomerulus, creating a high blood pressure.
3. This high pressure forces blood plasma containing water, glucose, amino acids, mineral salts, and nitrogenous products out of the glomerular capillaries, into the Bowman’s capsule, forming glomerular filtrate. (NOT DIFFUSION)
4. Blood cells, platelets and large molecules such as blood proteins and fats cannot pass through walls of the glomerular capillaries and the basement membrane that lines the glomerular capillaries and stays in the blood, which leaves the glomerulus via the efferent arteriole.

Question 9

Selective Reabsorption

Definition

Answer 9

Selective reabsorption is the process where useful substances are transported from the filtrate back into the bloodstream.

Useful solutes are selectively reabsorbed back by active transport and facilitated diffusion.
Water is reabsorbed by osmosis.
Most unwanted substances remain in the glomerular filtrate.

Question 10

Selective Reabsorption

Process

Answer 10

Selective reabsorption occurs at the proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.

1. Proximal convoluted tubule:
   - All glucose, amino acids, and vitamins are selectively reabsorbed through the walls of the nephron into the surrounding capillaries.
   - 85% of water and mineral salts (Na⁺, Cl^-) are reabsorbed. The active uptake of ions into the blood increases the water potential of the filtrate in the nephron, and this causes the water to leave the nephron into the capillaries by osmosis.
2. Loop of Henle:
   - Some water and mineral salts (Na⁺, Cl^-) are reabsorbed.
3. Distal convoluted tubule:
   - Smaller concentration of water and mineral salts (Na⁺, Cl^-) are reabsorbed.
4. Collecting duct:
   - Remaining water as required by the body is reabsorbed.

Question 11

5 Factors Affecting the Composition of Urine

Answer 11

1. A protein-rich diet will result in more urea being present in the urine, as more excess amino acids are deaminated in the liver
2. Taking in more liquids or water-rich food result in a larger volume of urine, as the water potential of blood increases
3. Cold weather will result in a larger volume of urine, as sweat production is reduced
4. Taking in lots of salty food will result in more mineral salts being present in urine, as excess salts are excreted in the urine
5. A patient with diabetes mellitus will result in glucose in urine, as the nephrons are unable to selectively reabsorb the high concentration of glucose fast enough

Question 12

Describe the homeostatic process that occurs when blood water potential is too high.

Answer 12

Osmoregulation:

1. When water potential of blood increases above the normal conditions (stimulus),
2. osmoreceptors in the hypothalamus detect the stimulus, and send nerve impulses to the hypothalamus, the control centre.
3. The hypothalamus sends nerve impulses to the effectors: the pituitary glands.
4. The pituitary glands release less ADH into the bloodstream (corrective mechanism).
   - ADH is transported to kidneys by the blood.
   - The walls of the distal convoluted tubule and collecting duct become less permeable to water.
   - Less water is reabsorbed back into the blood by osmosis.
   - Urine is more diluted (less concentrated), and there is a larger volume of urine.
5. Once water potential of blood decreases back to normal, osmoreceptors in the hypothalamus detects this, and sends a negative feedback to the hypothalamus to
6. stop further corrective action. ADH secretion increases back to normal.

Question 13

Describe the homeostatic process that occurs when blood water potential is too low.

Answer 13

Osmoregulation:

1. When water potential of blood decreases below the normal conditions (stimulus),
2. osmoreceptors in the hypothalamus detect the stimulus, and send nerve impulses to the hypothalamus, the control centre.
3. The hypothalamus sends nerve impulses to the effectors: the pituitary glands.
4. The pituitary glands release more ADH into the bloodstream (corrective mechanism).
   - ADH is transported to kidneys by the blood.
   - The walls of the distal convoluted tubule and collecting duct become more permeable to water.
   - More water is reabsorbed back into the blood by osmosis.
   - Urine is less diluted (more concentrated), and there is a smaller volume of urine.
5. Once water potential of blood increases back to normal, osmoreceptors in the hypothalamus detects this, and send a negative feedback to the hypothalamus to
6. stop further corrective action. ADH secretion decreases back to normal.

Question 14

4 Common Causes of Kidney Failure

Answer 14

1. High blood pressure
2. Diabetes mellitus
3. Alcohol abuse
4. Accidents or infections to the kidney

Question 15

Hemodialysis

What it is + Process + Why drawn from vein

Answer 15

Hemodialysis is a treatment for patients with kidney failure:

1. Blood is drawn from the fistula (a vein) in the patient’s arm and pumped through a dialysis machine.
2. In the dialysis machine, blood is passed through a dialyser.
3. There are many tubings in the dialyser, which have partially permeable walls, where diffusion of waste materials from the blood into the dialysis fluid/dialysate occurs.
4. Small molecules such as urea and other metabolic waste products diffuse out of the tubing into the dialysis fluid.
5. Blood cells, platelets and large molecules such as blood proteins and fats cannot pass through walls of tubing and remain in the blood in the tubing.
6. Cleaned blood is then returned through the fistula in the patient’s arm.

Veins are safer as they are closer to the surface unlike arteries which are deeper in the arm. Veins also have low pressure making it easier to stop the bleeding after dialysis.

Question 16

Features of the Dialysis Fluid & Dialyser

Answer 16

Z: Zero waste products in fluid:

• Dialysis fluid contains no metabolic waste products (urea, uric acid, creatinine, excess salts), which sets up a suitable concentration gradient for metabolic waste products in blood to diffuse out of the tubing into the dialysis fluid

E: Equal concentration of useful substances in fluid

• Dialysis fluid contains equal concentration of useful substances (glucose, amino acids, mineral salts) as healthy blood so that useful substances will not diffuse out of the blood into the dialysis fluid
• If the patient is lacking such useful substances, these substances in the dialysis fluid can diffuse into the blood

R: Tubing has high surface area to volume Ratio

• The tubing long and narrow, to increase the surface area to volume ratio, to speed up the rate of diffusion between blood and dialysis fluid

O: blood and dialysate flow in Opposite directions

• This maintains the concentration gradient along the entire length of the tubing for the removal of waste products by diffusion

Fresh dialysis fluid in contact with tubing where blood is about to enter the patient

• This ensures no waste product enters the patient