S1) Introduction to the Urinary System

Question 1

What is the mass of the kidneys?

Answer 1

Each weighs approx. 150g

Question 2

Describe the location of the kidneys

Answer 2

Retroperitoneal:

• Left: T11 - L2
• Right: T12 - L3

Question 3

Describe the relation of the ureter to the kidney

Answer 3

• Each kidney has single ureter
• Ureter is smooth muscle tube running from the renal pelvis to bladder

Question 4

Identify and describe the 4 functions of the kidney

Answer 4

• Regulation: control the [substances] in ECF
• Excretion: waste products
• Endocrine: renin, erythropoietin, prostaglandins
• Metabolism: active form of Vitamin D, catabolism of insulin, PTH, calcitonin

Question 5

Movement of water is driven by osmotic forces.

Define osmolarity and osmolality

Answer 5

• Osmolality: number of osmoles of solute per kilogram
• Osmolarity: number of osmoles of solute per litre

Question 6

Define osmole

Answer 6

Osmole is a measure a solution’s ability to create osmotic pressure & thus affect movement of water

E.g. one mole of CaCl₂ forms a 3 osmolar solution in 1L water

Question 7

Relate osmolarity to osmosis in cells

Answer 7

Water moves from low osmolarity solutions to high:

• If ECF Osmolarity is high – water moves out of cells
• If ECF Osmolarity is low – water moves into cells

Question 8

Intra- and extra cellular fluids have different electrolyte compositions. Differences are maintained by active transport.

Demonstrate this

Answer 8

• Intracellular: high K⁺, low Na⁺, many large organic anions
• Extracellular: low K⁺, high Na⁺, mainly Cl^- and HCO₃^-

Question 9

Kidneys affect extracellular fluid concentrations directly.

What happens when this system fails?

Answer 9

• Changes in BP
• Changes in tissue fluid and cell function
• Failure to control ECF osmolarity: cells shrink/swell

Question 10

What is the functional unit of the kidney?

Answer 10

Nephron is a filter unit connected to a long tube for reabsorption

Question 11

Describe the composition of a nephron

Answer 11

• A glomerulus: tuft of capillaries between afferent and efferent arteriole
• A tubule: Bowman’s capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting duct

Question 12

Which parts of the nephron are found in the cortex of the kidney?

Answer 12

• Proximal convoluted tubule
• Distal convoluted tubule

Question 13

Which parts of the nephron are found in the medulla of the kidney?

Answer 13

• Loop of Henle: dips into and out of medulla
• Collecting duct: passes through medulla to pelvis

Question 14

What is epithelium?

Answer 14

Epithelia are continuous sheet of cells covering exposed surfaces and lining internal cavities e.g. digestive, reproductive, urinary and respiratory

Question 15

What is the role of epithelia in the kidney?

Answer 15

• Excrete waste products from the body
• Reabsorb needed materials from the filtrate

Question 16

The kidney is one of the most active organs in the body.

Briefly, describe its metabolic demands.

Answer 16

• Needs blood flow of 4 ml/g/min
• 22% of cardiac output at rest

Question 17

Describe the structure and function of the glomerulus

Answer 17

• Structure: highly specialised filter
• Function: water, electrolytes and small molecules are forced through it by constant filtration pressure in capillaries

Question 18

The proximal convoluted tubule is a major site for reabsorption.

Describe the processes occurring

Answer 18

• PCT absorbs 100% of glucose and amino acids
• Filtrate remains isotonic with plasma (water follows)
• Reabsorbed materials leave by peritubular capillaries

Question 19

Describe the cellular mechanisms of reabsorption in the kidney

Answer 19

• Tubules are lined with polarised epithelial cells
• Different membrane properties on luminal and basolateral membranes

Question 20

Outline the role of sodium pumps in the kidney

Answer 20

• Extrude Na⁺ across basolateral membrane
• Na⁺ enters across luminal membrane down concentration gradient
• Energy from sodium movement drives reabsorption of other substances e.g. glucose and H₂O follows osmotically

Question 21

The Loop of Henle is a further site of reabsorption of salts in the kidney.

Describe its general functions

Answer 21

• Create a gradient of increasing osmolarity in the medulla by counter-current multiplication
• Forms concentrated urine if water has to be conserved

Question 22

Which processes occur in the distal convoluted tubule?

Answer 22

• Variable reabsorption of electrolytes and H₂O
• Removes even more sodium and chloride and actively secretes H⁺ (fluid leaving loop of Henle is hypotonic)

Question 23

What happens to water in the distal convoluted tubule?

Answer 23

Water may or may not follow reabsorption of electrolytes:

• Water is reabsorbed = concentrated urine (natriuresis)
• Water is not reabsorbed = dilute urine (diuresis)

Question 24

Explain the role of the collecting duct in types of urine produced

Answer 24

The collecting duct passes through the high osmolarity environment of the medulla (created by the loop of Henle):

• If water can cross the epithelium = concentrated urine
• If water cannot cross = urine remains dilute

Question 25

Variable reabsorption occurs in the distal nephron.

Explain this in terms of sodium recovery

Answer 25

Sodium recovery controlled by RAAS which controls ECF volume

Question 26

Variable reabsorption occurs in the distal nephron.

Explain this in terms of water recovery

Answer 26

Water recovery controlled by ADH which controls:

• Permeability of DCT and collecting duct to water
• ECF osmolarity