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^-
• if you have a crash injury, lots of K will be released and damage the heart

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** stimulated by sympathetic system

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

Question 27

percentage of molecules recovered

Answer 27

\<99% filtered sodium and chloride \<99% water (it follows sodium) 100% bicarbonate 100% glucose (so should be none in urine)

Question 28

what is renal blood flow

Answer 28

* how much blood flows into the kidney (1L/min)

Question 29

what is renal plasma flow

Answer 29

* volume of plasma that flows into the kidney (600ml/min) * RPF = RBF x (1-hamatocrit levels) * hematocrits levels → % of RBC that fill blood (normally 40-45%)

Question 30

Glomerular filtration rate

Answer 30

GFR = 125mil/min amount of filtrate produced from blood flow normally 80% of substances come back in circulation

Question 31

GFR/RPF

Answer 31

20%

Question 32

What does the glomerular filtrate contain

Answer 32

* no blood or plasma * no proteins * composed of organic solutes

Question 33

in histology how can you determine the difference between proximal and distal convoluted

Answer 33

* proximal has a cloudy lumen and contents * distal is empty - distal is also bunched up near the Bowmans capsule

Question 34

what is the role of podocytes

Answer 34

* small gaps within the membrane to allow substances to pass through * negative charge to repel negative

Question 35

label the difference layers in the glomerular capillary membrane

Answer 35

Question 36

some reasons why GFR may drop?

Answer 36

* decline in number of nephrons * decline in GRF within nephrons

Question 37

kidneys secrete renin, what does renin do?

Answer 37

* tunica media in the thick part of the afferent arteriole wall secrets renin that changes in response to tubular fluid * acts on angiotensin which results in vasoconstriction * sympathetic response and results in a fall in plasma volume

Question 38

Kidneys secrete erythropoietin, what is the relevance of this?

Answer 38

produces red blood cells

Question 39

describe the reabsorption of glucose in the PCT

Answer 39

1. normal conc → 2.5-5.5 mmol/L 2. most reabsorption occurs In the PCT 3. transported via active transport, uses the energy from Na/K pump 4. travels via Na/Glucose carriers

Question 40

how is glycosuria causes

Answer 40

* load of glucose is at excess and the transporters become saturated * Tm reduced (max reabsorbed)

Question 41

where are amino acids mainly reabsorbed?

Answer 41

* Normal conc is 2.5-7.5 mmol/L * PCT * reabsorbed via secondary active transport