Excretion

Question 1

What is anabolic vs catabolic

Answer 1

Anabolic: Building up of complex molecules from simpler ones
Catabolic: Complex molecules being broken down in to simpler ones

Question 2

What is metabolism?

Answer 2

Metabolism refers to the sum of all the anabolic and catabolic reactions that take place in the body of an organism.

Question 3

Define excretion

Answer 3

The process by which metabolic products and toxic substances are removed from the body of an organism

Question 4

Why is excretion important?

Answer 4

To remove waste products that are produced from metabolic reactions that may harm the body if accumulated.

Question 5

What is the function of the nephron?

Answer 5

It is the site of urine formation

Question 6

Describe the process of ultrafiltration.

Answer 6

Ultrafiltration occurs at the renal corpuscle (Bowman’s capsule and glomerulus).
High (hydrostatic) blood pressure forces most of the blood plasma out of the glomerular blood capillaries and into the Bowman’s capsule.
The partially permeable basement that wraps around the glomerular blood capillaries acts as a filter to allow small molecules such as water, glucose, amino acids, mineral salts and nitrogenous waste products to pass through. (Large molecules such as plasma proteins, RBCs, WBCs, platelets don’t pass through)

Question 7

What are the requirements/conditions for ultrafiltration?

Answer 7

• High (hydrostatic) blood pressure at the glomerulus
• A filter (the basement membrane around the glomerular blood capillaries)

Question 8

What are the 2 main processes for urine formation?

Answer 8

Ultrafiltration and selective reabsorption

Question 9

What is selective reabsorption?

Answer 9

It is the reabsorption of useful substances from the filtrate as the filtrate passes through the kidney tubule (nephron).

Question 10

Describe the process of selective reabsorption.

Answer 10

At the proximal convoluted tubule, most mineral salts and, in a healthy person, all of the glucose and amino acids are reabsorbed through the walls of the tubule into the surrounding blood capillaries.
These solutes are reabsorbed via diffusion and active transport. Most of the water in the filtrate is reabsorbed by osmosis here.
At the loop of Henle, some water is reabsorbed.
At the distal convoluted tubule, some water and mineral salts are reabsorbed.
At the collecting duct, some water is reabsorbed.
The remaining fluid that passes out of the collecting duct into the renal pelvis forms urine.
Urine contains the excess water, excess salts and metabolic waste products such as urea, uric acid and creatinine.

Question 11

What does urine contain / What is the composition of urine?

Answer 11

Urine contains excess water, excess salts and metabolic waste products such as urea, uric acid and creatinine.
For diabetic patients, glucose too.

Question 12

Describe the pathway of blood into urine.

Answer 12

Blood enters the kidney via the renal artery, which branches into many arterioles.
Each arteriole further branches into a mass of blood capillaries in the Bowman’s capsule called the glomerulus.
At the renal corpuscle, water, glucose, amino acids, mineral salts and nitrogenous waste products are forced and filtered into the Bowman’s capsule in a process known as ULTRAFILTRATION. (+ explain ultrafiltration)
The filtrate passes into the proximal convoluted tubule, where most of the mineral salts and, in a healthy person, all of the glucose and amino acids are reabsorbed through the walls of the tubule into the surrounding blood capillaries via diffusion and active transport.
Most of the water is also reabsorbed by osmosis here. (PCT)
At the loop of Henle, some water is absorbed.
At the DCT, some water and mineral salts are reabsorbed.
At the collecting duct, some water is reabsorbed.
The remaining fluid (consisting of excess water, excess salts and metabolic waste products like urea, uric acid and creatinine) forms urine and is passed out of the collecting duct via the ureters into the urinary bladder / renal pelvis.

( Blood leaves the glomerulus and enters blood capillaries surrounding the kidney tubule. These blood capillaries then unite to form venules, which in turn join to form a branch of the renal vein. Blood exits the kidney via the renal vein. )

Question 13

How does a protein-rich diet affect the composition of urine?

Answer 13

It will result in more urea being present in the urine, as urea is formed when excess amino acids are deaminated in the liver.

Question 14

How does taking in more liquids / water-rich foods affect the composition of urine?

Answer 14

Taking in more liquids / water-rich foods will increase the water potential of the blood. Hence, the volume of urine excreted is larger as more excess water is present.

Question 15

How does cold weather affect the composition of urine?

Answer 15

Sweat secretion is reduced, increasing the water potential of the blood. Hence, the volume of urine excreted is larger as more excess water is present.

Question 16

How does a high intake of salty foods affect the composition of urine?

Answer 16

There will be more excess salts in the blood, hence more excess salts will be secreted in the urine.

Question 17

Explain why a person with diabetes mellitus will have glucose in their urine.

Answer 17

A diabetic is unable to store up excess glucose as glycogen due to the lack of insulin in their body or their body cells’ resistance to insulin.
Hence, a person with diabetes has a high concentration of glucose in their blood.
This glucose is filtered out of the glomerulus to form part of the glomerular filtrate during ultrafiltration.
However, the nephrons are unable to reabsorb all the glucose fast enough during selective reabsorption, and therefore a significant amount of glucose passes out in the urine.

Question 18

What is osmoregulation?

Answer 18

Osmoregulation is the control of water and solute concentrations in the blood to maintain constant water potential in the body.

Question 19

Why is osmoregulation important?

Answer 19

The water potential of blood plasma needs to be kept relatively constant. If there are fluctuations in the water potential, numerous problems can occur, such as RBCs lysing or crenating.

Question 20

How does increased anti-diuretic hormone release decrease the volume of urine created?

Answer 20

Increased ADH release increases the permeability of cells of collecting ducts to water, hence increasing water reabsorption at the collecting duct.

Question 21

Describe the mechanism of osmoregulation when blood water potential decreases.

Answer 21

When blood water potential (stimulus) decreases, e.g. due to loss of water through sweating, the hypothalamus (receptor) is stimulated.
This causes the pituitary gland to release more ADH into the bloodstream as a corrective mechanism. The increased ADH release increases the permeability of cells in the walls of the collecting duct to water, allowing more water to be reabsorbed back into blood capillaries. Hence, a smaller volume of urine is produced / the urine produced is less diluted / more concentrated.
This results in the blood water potential increasing (condition increases) back to normal blood water potential.

Question 22

How does water potential of blood relate to blood pressure?

Answer 22

The amount of water reabsorbed affects blood volume. As the blood volume increases, the blood pressure increases.
As a result, kidneys also play a vital role in controlling blood volume and pressure. (HBP can cause the blood vessels in the brain to burst, resulting in stroke.)

Question 23

Why are kidneys important?

Answer 23

Kidneys are excretory organs, to excrete metabolic waste products such as urea, excess water and excess mineral salts, in the form of urine.
Kidneys are osmoregulators, because they regulate the water and solute concentrations in the blood to maintain a constant water potential in the blood.

Question 24

Outline the mechanism of dialysis in the case of kidney failure.

Answer 24

A dialysis machine mimics the function of a kidney.
Blood is drawn from the vein in the patient’s arm and is pumped through the tubing in the dialysis machine.
The tubing is bathed in specially-controlled dialysis fluid, and the walls of the tubing are partially permeable.
Small molecules, like urea and other metabolic waste products, diffuse out of the tubing into the dialysis fluid. Blood cells, platelets and large molecules remain in the fluid.
The filtered blood is then returned to a vein in the patient’s arm.

Question 25

What are the features of a dialysis machine?

Answer 25

The dialysis fluid contains the same concentration of essential substances as healthy blood. This ensures that essential substances like glucose, amino acids and mineral salts do not diffuse out of the blood and into the dialysis fluid. And, if the patient’s blood lacks these essential substances, these substances will diffuse from the dialysis fluid into the blood.

The dialysis fluid does not contain metabolic waste products. This sets up a concentration gradient that allows waste products like urea, uric acid, creatinine and excess water and mineral salts to diffuse out of the tubing into the dialysis fluid. The waste products are removed from the blood, which maintains the correct solute composition and water potential of the blood.

The direction of the blood flow is opposite to the flow of the dialysis fluid. This maintains the concentration gradient for the removal of waste products.

The tubing in the machine is narrow, long and coiled. This increases the surface area to volume ratio which helps to speed up the rate of exchange of substances between the patient’s blood and the dialysis fluid.