Elective 1.1: Regulation of Water Content

Question 1

List three main ways in which our bodies gain water.

Answer 1

Drinks (~1500mL), food (~700 mL), metabolism (eg. respiration, ~300mL)

Question 2

List 4 main ways in which our bodies lose water.

Answer 2

Urine (~1500mL), sweat (~500mL), exhaled air (~400mL), faeces (~100mL)

Question 3

Explain the possible consequences of having an imbalance between water gain and water loss in the body.

Answer 3

If there is an imbalance between water gain and water loss in the body, the water content of the blood and hence the water potential of the tissue fluid that surrounds the body cells will change. Water will therefore enter or leave the cells by osmosis. If too much water is gained or lost, the cells will not function properly and may even die.

Question 4

What is osmoregulation?

Answer 4

Osmoregulation is the maintenance of a stable water potential of body fluid in an organism.

Question 5

Name an organ and a system playing an important role in osmoregulation in humans.

Answer 5

Kidneys and the urinary system.

Question 6

Name the organs in the human urinary system.

Answer 6

2 kidneys, 2 ureters, 1 urinary bladder and 1 urethra.

Question 7

State the properties and functions of the kidneys.

Answer 7

They are a pair of bean-shaped organs each about the size of a fist. They are found along the posterior wall of the abdominal cavity. They are usually covered by visceral fat which protects them from mechanical shock.
The kidneys are supplied with blood from the renal artery and drained by renal veins. The kidneys filter the blood and form urine, removing metabolic wastes from the body and regulating water content of the body.

Question 8

State the properties and functions of ureters.

Answer 8

Ureters connect the kidney to the urinary bladder. They undergo peristalsis to carry urine from the kidneys to the urinary bladder.

Question 9

State the properties and functions of the urinary bladder.

Answer 9

It is hollow, muscular and elastic and stores urine temporarily.
There are two rings of sphincter muscles at the outlet of the urinary bladder.

Question 10

Describe the activity of the sphincter muscles when urination takes place.

Answer 10

Normally the sphincter muscles contract to close the exit from the bladder to prevent urine from flowing out.
During urination, the sphincter muscles relax and the muscles of the wall of the bladder contract, pushing urine out from the bladder into the urethra.

Question 11

State the properties and functions of the urethra.

Answer 11

It is a tube that carries urine from the urinary bladder to the outside of the body.
The female urethra is much shorter than the male urethra.

Question 12

Name the three main parts of the kidney.

Answer 12

The outer cortex, the inner medulla and the central cavity called the pelvis.

Question 13

State the properties and functions of the nephron.

Answer 13

It is the structural and functional unit of the kidney, which filters the blood and produces urine.
In each kidney, there are about one million nephrons. It is situated partly in the cortex and partly in the medulla.

Question 14

Describe the pathway of blood flow inside human kidneys.

Answer 14

Blood from the renal artery branch up and enters the afferent arteriole. Then, it enters the glomerulus and reaches the Bowman’s capsule. It leaves the Bowman’s capsule in the efferent arteriole and branch out in a network of capillaries. These capillaries eventually join to from the renal vein.

Question 15

Name the structures in one kidney tubule in order.

Answer 15

First coiled tubule / proximal convoluted tubule, loop of Henle, second coiled tubule / distal convoluted tubule.

Question 16

Name the main processes of urine formation.

Answer 16

Ultrafiltration and reabsorption

Question 17

Describe the process of ultrafiltration.

Answer 17

Blood flows into the kidney via the renal artery. When it reaches the glomerulus, it is kept at a high blood pressure due to the pumping action of the heart. The difference in pressure between the glomerulus and the capsular space in the Bowman’s capsule forces water and small soluble molecules (eg. glucose, amino acids, salts and urea) in the plasma through the thin and porous walls of the glomerulus and Bowman’s capsule into the capsular space.

Question 18

Explain whether ultrafiltration requires energy.

Answer 18

No, because it solely driven by the difference in blood pressure.

Question 19

Name the fluid that is filtered into the Bowman’s capsule.

Answer 19

Glomerular filtrate

Question 20

State and explain two differences in composition of the glomerular filtrate and the blood.

Answer 20

Blood contains plasma proteins and blood cells, while the glomerular filtrate does not. This is because they are too large in size to pass through the walls of the glomerulus and the Bowman’s capsule.

Question 21

List 5 substances which are reabsorbed from the glomerular filtrate back into the blood.

Answer 21

Glucose, amino acids, salts, water, urea

Question 22

State the significance of reabsorption.

Answer 22

It prevents the loss of useful substances from the body.

Question 23

State the location at which most reabsorption occurs.

Answer 23

First coiled tubule

Question 24

State how glucose is reabsorbed into the blood in the first coiled tubule.

Answer 24

All glucose is reabsorbed by diffusion and active transport.

Question 25

State how amino acids are reabsorbed into the blood in the first coiled tubule.

Answer 25

**All** amino acids are reabsorbed by **diffusion and active transport**.

Question 26

State how salts are reabsorbed into the blood in the first coiled tubule.

Answer 26

**Most** salts are reabsorbed by **diffusion and active transport**.

Question 27

State and explain how water is reabsorbed into the blood in the first coiled tubule.

Answer 27

As many solute molecules in the glomerular filtrate are reabsorbed into the blood capillary, the **water potential** of the glomerular filtrate becomes higher than that of blood surrounding this part of the first coiled tubule. Water in the glomerular filtrate moves **down the water potential gradient** into the blood surrounding the first coiled tubule by **osmosis**.

Question 28

State and explain how urea is reabsorbed into the blood in the first coiled tubule.

Answer 28

As a **large proportion** of water is reabsorbed from the glomerular filtrate into the blood surrounding the first coiled tubule, the concentration of urea in the glomerular filtrate increases to a value **higher than that in the blood surrounding the tubule**. Urea in the glomerular filtrate travel **down the concentration gradient** into the blood by **diffusion**.

Question 29

Name 4 substances which urine consists of.

Answer 29

Water, salt, urea, and other metabolic wastes.

Question 30

List 5 adaptive features of the first coiled tubule for reabsorption.

Answer 30

1. The first coiled tubule is long and highly coiled. 2. Its wall is **one-cell thick**. 3. The epithelial cells have **many microvilli** on the surface that is in contact with the filtrate. 4. The epithelial cells contain **many mitochondria**. 5. The first coiled tubule is surrounded by a **dense capillary network**.

Question 31

Explain the significance of the first coiled tubule being long and highly coiled.

Answer 31

This increases the **surface area** and **length of time** allowed for the reabsorption of water and useful solutes.

Question 32

Explain the significance of the first coiled tubule having a wall that is one-cell thick.

Answer 32

This **shortens the distance for reabsorption** of useful solutes and water.

Question 33

Explain the significance of the first coiled tubule having microvilli on the surface of epithelial cells that is in contact with the glomerular filtrate.

Answer 33

The microvilli greatly **increaser the surface area for reabsorption** of useful solutes and water.

Question 34

Explain the significance of the first coiled tubule having epithelial cells with many mitochondria.

Answer 34

**Aerobic respiration** at the mitochondria provides the cell with a lot of **energy for the active transport** of useful substances to reabsorb them from the glomerular filtrate into blood, facilitating reabsorption.

Question 35

Explain the significance of the first coiled tubule being surrounded by a dense capillary network.

Answer 35

The blood in the capillaries near the first coiled tubule carries the reabsorbed substances away rapidly. This **maintains a steep concentration gradient** of substances between blood and the glomerular filtrate so that the substances can be reabsorbed into the from the glomerular filtrate into the blood at a higher rate.

Question 36

Explain the difference in glucose content of the blood in the renal artery compared to that of the renal veins.

Answer 36

The glucose content of blood in renal veins is **less than** that in the renal arteries. This is because although all glucose in the glomerular filtrate is reabsorbed into the blood, some glucose is still uptaken by **cells in the kidneys** to release energy for various activities like active transport.

Question 37

What is the full name of ADH?

Answer 37

Antidiuretic hormone

Question 38

State where ADH is synthesised and stored.

Answer 38

It is synthesised by specialised cells in the **hypothalamus** of the brain. It is then transported to the **pituitary gland** for storage and release into the bloodstream.

Question 39

Explain how ADH affects the kidneys.

Answer 39

ADH **increases the permeability** of the **second coiled tubule and collecting duct** to **water**, so that a greater **proportion** of water is reabsorbed from the filtrate.

Question 40

State how the release of ADH is controlled.

Answer 40

The release of ADH is controlled by the hypothalamus, which contains **osmoreceptors** that detect the water potential of blood.

Question 41

Explain the process of osmoregulation when the water potential of the blood is lower than normal.

Answer 41

The decrease in water potential is **detected by osmoreceptors in hypothalamus**. The hypothalamus then **stimulates the pituitary gland** to **release more ADH** into the blood. **More ADH** will cause the wall of the second coiled tubule and collecting duct to become **more permeable to water**. A **larger proportion of water** in the glomerular filtrate is **reabsorbed** into the blood. As a result, a **smaller volume of concentrated urine** is produced. As a smaller proportion of water is lost in urine, the water potential of the blood rises. The fall in water potential of blood also **stimulates the thirst centre in the brain** to produce a sensation of thirst. This urges the person to **drink more water** to bring the water potential of the blood back to normal value.

Question 42

Explain the process of osmoregulation when the water potential of the blood is higher than normal.

Answer 42

The increase in water potential is **detected by osmoreceptors in hypothalamus**. The hypothalamus then **stimulates the pituitary gland** to **release less ADH** into the blood. **Less ADH** will cause the wall of the second coiled tubule and collecting duct to become **less permeable to water**. A **smaller proportion of water** in the glomerular filtrate is reabsorbed into the blood. As a result, a **larger volume of diluted urine** is produced. As a larger proportion of water is lost as urine, the water potential of the blood falls and is restored to normal value.

Question 43

Explain the effect of drinking isotonic drinks on urine output.

Answer 43

Isotonic drinks do not affect the eater potential of blood much. However, they increase blood volume and hence **blood pressure**. This results in a higher rate of formation of the glomerular filtrate, and an **increase in urine output**.

Question 44

List two substances which inhibit the release of ADH from the pituitary gland.

Answer 44

Alcohol and caffeine

Question 45

Explain the effect of drinking caffeinated drinks on urine output.

Answer 45

Caffeine in the drink **inhibits the release of ADH**. Moreover, it increases the heart rate and flow pressure, resulting in a higher rate of formation of glomerular filtrate. Thus it leads to an increase in urine output.

Question 46

What is the definition of excretion?

Answer 46

Excretion is the removal of metabolic wastes from the body.

Question 47

List three metabolic wastes in urine.

Answer 47

Urea, uric acid, ammonia

Question 48

List two treatments for kidney failure.

Answer 48

Dialysis or kidney transplant

Question 49

State the main parts of a dialysis machine.

Answer 49

A dialysis machine consists of **differentially permeable dialysis tubing** bathed in **dialysing fluid**. The dialysing fluid has the same composition as blood plasma, but no metabolic wastes.

Question 50

Describe and explain the working mechanisms of a dialysing machine.

Answer 50

1. The patient's blood is pumped out to the dialysis machine. 2. Metabolic wastes like urea **diffuse** from blood to the dialysing fluid down the concentration gradient. 3. Nutrients like glucose are **retained** in the blood as their concentrations in dialysing fluid are the same as blood. 4. Plasma proteins and blood cells are retained in the blood as they are **too large** to pass through the wall of the dialysis tubing. 5. Cleaned blood returns to the patient's body.

Question 51

State the mechanism of transport for substances in a dialysis machine.

Answer 51

By diffusion only

Question 52

List and explain the significances of two features of the dialysis tubing in dialysis machines.

Answer 52

1. It is **differentially permeable** and only allows small molecules like urea to pass through. This helps **remove urea** from the blood while **retaining** large blood components like plasma proteins and blood cells in the body. 2. It is **long**. This increases the **surface area** for the diffusion of metabolic wastes from the blood into the dialysing fluid.

Question 53

List and explain the significances of 5 features of the dialysing fluid in dialysis machines.

Answer 53

1. It contains **no metabolic wastes**. This allows metabolic wastes in the blood to **diffuse** into the dialysing fluid **down the concentration gradient**. 2. It contains the **same concentration of nutrients** as the blood, to **prevent loss of nutrients** from the blood as it passes through the dialysis tubing. 3. It has the **same water potential** as blood, to **prevent loss of water** from the blood. 4. It is kept at the **same temperature** as blood, to **prevent heat loss** from the blood. 5. It flows in the **opposite direction** as compared to blood. This **maintains the concentration gradient** between the blood and the fluid, facilitating diffusion of metabolic wastes.

Question 54

List two disadvantages of haemodialysis treatment.

Answer 54

1. Only a **small volume of blood** enters the dialysis tubing per unit time and wastes are continually produced in the body, resulting in the need of **long and frequent treatment**, having a big impact on social lives of people. 2. The dialysing machine or the dialysing fluid may be **contaminated**, which leads to a rick if infection, poisoning, or death.

Question 55

Name two forms of dialysis.

Answer 55

Haemodialysis, Peritoneal dialysis

Question 56

State the usual location of transplanted kidneys.

Answer 56

They are usually in the **front lower abdomen** near the pelvis.

Question 57

State a possible risk and treatment for kidney transplants.

Answer 57

There is a risk of **immune rejection** of transplanted tissued by the recipient's body. The patient will need to take **immunosuppressants** for life to stop the body from rejecting the kidney.