Nephron Function & Urine Formation

Question 1

What is the main function of the kidneys?

Answer 1

1. Remove nitrogenous wastes (urea, uric acid, creatinine)
2. Regulate the composition and concentration of body fluids to maintain homeostasis.

Question 2

Where are the kidneys located and what do they look like?

Answer 2

1. A pair of reddish-brown organs in abdomen, 2. About 11 cm long, each enclosed by a renal capsule with an outer cortex, inner medulla, and a central renal pelvis.

Question 3

What is the functional unit of the kidney?

Answer 3

The nephron, where urine is formed. Each kidney contains about 1.2 million nephrons.

Question 4

What are the main blood vessels associated with the kidneys?

Answer 4

1. Renal artery: brings blood to the kidney
2. Afferent arteriole: brings blood into the glomerulus
3. Efferent arteriole: carries blood away from the glomerulus
4. Peritubular capillaries: surround the nephron for exchange
5. Renal vein: carries filtered blood away from the kidney

Question 5

What are the main parts of a nephron?

Answer 5

1. Renal corpuscle (glomerulus + glomerular capsule)
2. Renal tubule (proximal convoluted tubule → loop of Henle → distal convoluted tubule → collecting duct)

Question 6

What is the role of the glomerular capsule (Bowman’s capsule)?

Answer 6

It surrounds the glomerulus and collects the filtrate forced out of the blood.

Question 7

What are the three main processes of urine formation?

Answer 7

1. Glomerular filtration
2. Selective Absorption
3. Tubular Secretion

Question 8

Where does glomerular filtration occur?

Answer 8

In the renal corpuscle — between the glomerulus and glomerular capsule.

Question 9

How is filtrate formed in the glomerulus?

Answer 9

High blood pressure forces water and small solutes through the capillary and capsule walls into the capsule, forming filtrate.

Question 10

What causes high pressure in the glomerulus?

Answer 10

Afferent arteriole has a wider diameter than efferent arteriole, creating resistance to blood flow and pressure that forces plasma out.

Question 11

How is filtrate forced out of the blood into the glomerular capsule?

Answer 11

1. Blood enters glomerulus under high pressure.
2. This high pressure forces water and small dissolved substances out of blood through differentially permeable membranes of capillary wall and capsule wall — both made of thin, flat cells.

Question 12

What substances are found in the filtrate?

Answer 12

Water, salts, glucose, amino acids, fatty acids, urea, uric acid, creatinine, hormones, toxins, and ions — but no blood cells or plasma proteins.

Question 13

What percentage of plasma is filtered through the glomerulus, and why isn’t all plasma filtered?

Answer 13

1. About 20% of plasma is filtered into glomerular capsule.
2. Complete filtration cannot occur because blood in capillaries is continually pushed forward by blood behind it, preventing all plasma from being filtered at once.

Question 14

How much filtrate do the kidneys produce, and how much actually leaves the body as urine?

Answer 14

1. Rate of filtrate production: ~125 mL per minute
2. Daily filtrate: ~180 L
3. Excreted as urine: ~1% (~1.5 L/day)
4. Most filtrate is reabsorbed back into the blood.

Question 15

What is selective reabsorption?

Answer 15

The reabsorption of some substances and not others in the renal tubules as some filtered components of plasma are of use to the body and are therefore returned to the blood in the peritubular capillaries

Question 16

Where does selective reabsorption occur?

Answer 16

Mainly in the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.

Question 17

What materials are reabsorbed into the blood?

Answer 17

Water, glucose, amino acids, and ions such as sodium, potassium, calcium, chloride, and bicarbonate - (small amounts of urea also reabsorbed.)

Question 18

How do the nephron’s structure and number maximise reabsorption and secretion?

Answer 18

Tubules have a long loop and two convolutions, giving a large surface area.

Each kidney has over 1 million nephrons, so total surface area available for reabsorption and secretion is extremely large

Question 19

What is facultative reabsorption?

Answer 19

The controlled reabsorption of water by carrier proteins from the filtrate back into blood, depending on body’s water needs

Question 20

How does the body control how much water is reabsorbed in the nephron?

Answer 20

The permeability of tubule cell membranes can change — become more/less permeable to water depending on whether body needs to conserve water or remove excess.

Question 21

Which hormone primarily controls facultative reabsorption?

Answer 21

Antidiuretic hormone (ADH).

• High ADH → more water is reabsorbed → urine becomes more concentrated
• Low ADH → less water is reabsorbed → urine becomes more dilute

Question 22

Why does the kidney use facultative reabsorption?

Answer 22

Helps maintain water balance in body, preventing dehydration when water is low, or allowing excess water to be excreted when hydration is high.

Question 23

What is tubular secretion?

Answer 23

The addition of materials from the blood in the peritubular capillaries into the filtrate in the nephron tubule.

Question 24

What substances are secreted into the filtrate?

Answer 24

Potassium (K⁺) ions, hydrogen (H⁺) ions, creatinine, ammonium (NH₄⁺), and some drugs such as penicillin.

Question 25

What are the two main purposes of tubular secretion?

Answer 25

1. To maintain blood pH of 7.4-7.5 by removing excess hydrogen and ammonium ions. 2. Maintains normal urine pH of 6.

Question 26

How does secretion help with excretion of drugs and toxins?

Answer 26

Allows substances like penicillin and metabolic wastes to be actively transported into filtrate for removal in urine.

Question 27

How does urine travel from the nephron to outside the body?

Answer 27

1. Collecting ducts → renal pelvis 2. Renal pelvis → ureters (25–30 cm long, one from each kidney) 3. Ureters → urinary bladder (muscular storage organ) 4. Bladder → urethra → exterior of body 💡 Note: Urine is pushed through ureters by waves of muscle contraction (peristalsis).

Question 28

How does the glomerular capsule help in urine formation?

Answer 28

Surrounds glomerulus to collect fluid (filtrate) forced out of the blood capillaries.

Question 29

Why is filtration efficient in the kidney?

Answer 29

Filtrate passes through only two thin cell layers: one from the capillary wall and one from the capsule wall, allowing rapid transfer of water and solutes.

Question 30

How does blood flow contribute to effective filtration?

Answer 30

1. High blood volume constantly flows through each kidney, maintaining concentration gradient. 2. Efferent arteriole is smaller than afferent arteriole, raising glomerular pressure and forcing more plasma into capsule.

Question 31

Why is the kidney so effective at forming urine?

Answer 31

1. Glomerular capsule collects filtrate efficiently 2. Thin walls allow easy transfer of substances 3. High blood flow maintains filtration pressure and gradient 4. Long, convoluted tubules + many nephrons → large surface area for reabsorption and secretion

Question 32

How do filtration, reabsorption, and secretion together maintain the composition of body fluids?

Answer 32

1. Filtration removes large amounts of water and solutes from blood. 2. Reabsorption returns essential materials (water, glucose, ions) to maintain balance. 3. Secretion eliminates extra ions, drugs, and wastes, and regulates blood pH. 4. Together, these processes keep blood’s water, ion, and pH levels stable, ensuring proper cell function.

Question 33

What percentage of the filtrate is reabsorbed?

Answer 33

About 99% of the water and useful solutes — only ~1% becomes urine.

Question 34

How much urine does a healthy adult produce daily?

Answer 34

Approximately 1.5 L per day, varying with diet, hydration, and environment.

Question 35

Why must at least 0.5 L of water be excreted daily?

Answer 35

To dissolve and eliminate toxic wastes such as urea, even if the body is dehydrated.

Question 36

What substances normally make up urine?

Answer 36

Water, urea, ions (sodium, chloride, potassium, phosphate, sulfate), uric acid, and creatinine - contains no significant protein/glucose

Question 37

Summarise how the kidney contributes to homeostasis.

Answer 37

1. Filters waste and excess water from blood 2. Reabsorbs needed substances 3. Secretes ions and toxins to maintain pH and solute balance → Regulates blood composition, osmotic pressure, and acid–base balance, keeping internal conditions stable.