15.5 - Structure & Function of the Mammalian Kidney

Question 1

What is the role of the kidney?

Answer 1

The kidney is involved in excretion, osmoregulation – the balance and control of water potential in the blood -, they filter nitrogenous waste (urea) out of the blood

Question 2

What do the renal artey and vein do?

What structures are the kidney made of?

Answer 2

• Blood supply is from the renal artery (which branches off the aorta
• Blood is drained by the renal vein into the inferior vena cava

The kidney comprises of several structures called nephrons – tubules that make-up the structural function of the kidney –

Question 3

How is urine passed from the kidney out the body?

Answer 3

• The kidney produces a sterile product – urine
• Urine passes out of the kidney through the ureter to the bladder to be excreted
• As the bladder becomes full the urine exits to pass out the body through the urethra

Question 4

What are the 3 main sections of the kidney and their functions?

Answer 4

1. Cortex – dark outer layer; blood filtration occurs here, has a dense capillary network carrying blood from the renal artery to the nephrons
2. Medulla – lighter colour; contains nephron tubules that form the kidney pyramids (carries urine from the nephron to the calyces – cup shaped cavities – urine is collected there before passing to through the ureter) and collecting ducts
3. Pelvis - central part of the kidney where the urine collects before passing to the ureter

Question 5

What is the structure and general function of the nephron?

Answer 5

The function of a nephron is to filter the blood – majority of the filtered material returns to the blood – nitrogenous waste is removed and mineral ion conc. is balanced.

Bowman’s Capsule – cup-shaped structure containing the glomerulus (ball of capillaries) (more blood enters the glomerulus than leaves due to ultrafiltration)

Proximal (near the torso) Convoluted Tubule – coiled region of the tubule after the Bowman’s capsule in the cortex where substances are reabsorbed into the body

Loop of Henle – loop of tubule between the proximal and distal tubule which is in high solute conc. in the tissue fluid in the medulla. The ascending limb goes to the cortex

Distal (away from the torso) Convoluted Tubule – 2^nd coiled region where water balancing occurs. ADH varies the permeability of the walls; ion & pH regulation also occurs

Collecting Duct – urine passes to the collecting duct through the medulla to the pelvis. More water-balancing occurs – these walls are also sensitive to ADH

The nephron capillary network leads back to a venule and the renal vein. That blood has reduced levels of urea, but glucose, amino acids and other substances remain the same. (Mineral ion conc. will remain the same)

Question 6

What occurs during ultrafiltration?

(up to the basement membrane)

Answer 6

Afferent arteriole has a larger lumen than efferent, creates a filtration pressure to cause substances to leave the glomerulus

Endothelial layer in the capillary wall has fenestrations allowing substances to leave. There is a basement membrane stopping certain substances from leaving (erythrocytes, WBC, plasma proteins, platelets). This is the 1^st filter stage.

Question 7

What occurs during ultrafiltration?

(after the basement membrane)

Answer 7

Before the substances enter the Bowman’s Capsule lumen, they must pass the capsule wall with cells called Podocytes. The 2^nd filter.

The podocytes have extensions called pedicels that are like the gaps in the endothelium. E.g. if some proteins or erythrocytes pass through (if the basement membrane is damaged), the pedicels will ensure they don’t pass to the rest of the nephron

Substances that don’t end up in the glomerular filtrate will leave via the efferent arteriole. (E.g. rbc, wbc, plasma proteins, platelets)

Glomerular filtrate: Glucose, water, urea, salt (Na⁺, Cl^-), aas, vitamins, hormones

Question 8

What occurs during selective reabsoroption and what structures are involved?

(up to the PCT)

Answer 8

1. In the proximal convoluted tubule, all: Glucose, aas, hormones, vitamins are returned to the blood by active transport. Some of the Na⁺ molecules leave by AT, decreasing the water potential, making water leave by osmosis. Filtrate becomes more concentrated so Cl^- leaves by diffusion.

Substances removed from the nephron travel down the concentration gradient. (Maintained by the continuous flow of blood in the vasa recta)

By the time the filtrate reaches the Loop of Henle it is isotonic with the tissue fluid

Question 9

What are the adaptations of the PCT?

Answer 9

1. Adaptations of PCT: Covered in microvilli, increasing SA over which substances can be reabsorbed
2. Many mitochondria to provide ATP for AT.

Question 10

What is the role of the Loop of Henle?

Answer 10

Main role is to work with capillaries and collecting ducts to create and maintain a very negative water potential in the medulla by building up the conc. of Na⁺ and Cl^- in medulla so that eventually water is removed from tubules by osmosis into the interstitial region and then into the blood

Question 11

Which parts of the Loop of Henle are permeable to water?

Answer 11

Descending limb is permeable to water relatively impermeable to solutes

Ascending limb is permeable to solutes and impermeable to water

Question 12

What is the function of the Loop of Henle?

Answer 12

1. Cells in ascending limb actively pump Cl- out of filtrate into medulla
2. Na+ follow.
3. Water cannot follow as ascending limb is impermeable to water
4. Fluid surrounding L o H becomes very concentrated in the medulla
5. Descending limb is permeable to water and relatively impermeable to solutes so water is drawn from descending limb so filtrate becomes v. conc.
6. Water removed passes to vasa recta (there is an osmotic gradient due to blood proteins)
7. Filtrate becomes more conc. as it goes down descending limb.
8. As it proceeds up ascending limb Na+ and Cl- pumped out so filtrate becomes less concentrated.

Question 13

What occurs in the DTC?

Answer 13

1. Distal Convoluted Tubule. Osmoregulation occurs in the collecting duct and DCT (they are affected by ADH.) Na⁺ reabsorbed by active transport and Cl^- by diffusion if needed. Water reabsorbed by osmosis if needed, in the presence of ADH. DCT plays a role in pH maintenance.

Question 14

What occurs in the collecting duct?

Answer 14

1. Collecting Duct passes through the tissue fluid of the renal medulla. This is where urine conc. is determined. Water is reabsorbed by osmosis, if more ADH is secreted, more aquaporins (protein channels that transport water). Cells around the collecting duct can increase [Na⁺] to increase water potential gradient to move water out.
   * More water will move from the filtrate of higher water potential to the tissue fluid of lower water potential down concentration gradient by osmosis.*

Anything left is urine – urea, excess water and salts.