Kidney function I: Filtration, Reabsorption and Secretion

Question 1

Functions of kidney

Answer 1

-Excretion of metabolites or ingested substances
-Control of body fluid composition
-Endocrine

Question 2

Examples of metabolites/substances kidney excretes

Answer 2

-Urea from protein catabolism
-Uric acid from nucleic acid breakdown
-Creatinine from muscle creatine
-Hormone metabolites e.g. growth hormone
-End products of haemoglobin breakdown
-Foreign chemicals: drugs, pesticides

Question 3

Examples of kidney control of body fluid composition

Answer 3

-Volume regulation - linked to sodium concentration
-Osmoregulation - water balance
-pH regulation

Question 4

Endocrine function of kidneys

Answer 4

Hormones that act on kidneys:
-ADH
-Aldosterone
-Natriuretic peptides
-Parathyroid hormone
-Fibroblast growth factor 23

Hormones produced by kidney:
-Renin
-Activated vitamin D3
-Erythropoietin
-Prostaglandins

Question 5

Gross structure of kidney

Answer 5

Question 6

Microanatomy of kidney

Answer 6

-Each kidney contains >1 million nephrons
-Each nephron contains renal corpuscle and tubule

Question 7

Simplified diagram of renal corpuscle

Answer 7

Question 8

Filtration interface in renal corpuscle

Answer 8

Learn stuff in brackets

Question 9

Breakdown of the nephron

Answer 9

Question 10

2 types of nephron

Answer 10

Cortical (85%)
-outer 2/3 of cortex
-short Loop of Henle

Juxtamedullary (15%)
-inner 1/3 of cortex
-long Loop of Henle
-produces concentrated urine

Question 11

Where are renal corpuscles of nephrons found?

Answer 11

-Outer cortex region of kidney

Question 12

What makes up juxtaglomerular apparatus?

Answer 12

-Juxtaglomerular cells -release renin
-Macula densa

Question 13

The nephron blood supply

Answer 13

-Peritubular capillaries found close to tubule of nephron
-Peritubular capillaries in Loop of Henle are the Vasa Recta
-2 sets of arterioles (afferent and efferent)
-2 sets of capillary beds (glomeruli and peritubular)

Question 14

Basic renal processes

Answer 14

-Glomerular filtration
-Tubular secretion
-Tubular reabsorption
-Metabolism (e.g. glutamine)

Question 15

Glomerular filtration

Answer 15

-Movement of fluid and solutes from glomerular capillaries into Bowman’s space
-20% of the plasma that enters the glomerulus is filtered and enters the Bowman’s space

Question 16

Tubular secretion

Answer 16

-The secretion of solutes from the peritubular capillaries into the tubules
-Occurs in proximal tubule and collecting duct

Question 17

Tubular reabsorption

Answer 17

-The movement of materials from the filtrate in the tubules into the peritubular capillaries
-Mainly occurs in proximal tubule

Answer 18

Amount of substance excreted in urine = amount filtered + amount secreted - amount reabsorbed

Question 19

What does filtration of substances depend on?

Answer 19

-Molecular size
-Charge
-Possibly shape
-Most plasma constituents are freely filtered - except proteins

Question 20

Under what molecular mass are substances freely filtered in the glomerulus

Answer 20

7000 Daltons

Question 21

How does molecular charge affect how freely they are filtered?

Answer 21

-Fixed sited of negative charges will:
-repel negatively charged dextrans
-encourage the filtration of positively charged dextrans

Question 22

Composition of ultrafiltrate

Answer 22

-Cells and large proteins not filtered across barrier
-Certain drugs and ions bind to proteins so are not filtered

E.g.
-Acidic drugs can bind to protein, albumin
-Basic drugs can bind to a1-acid glycoprotein
-40% of plasma Ca is bound to proteins, therefore only 60% can be freely filtered

Question 23

What can infection, damage to glomerulus, hypertension result in?

Answer 23

-Protein in urine (proteinuria)
-Hb in urine (hameoglobinuria)
-Red cells in urine (haematuria)

Question 24

GFR definition

Answer 24

-Volume of fluid filtered from the glomeruli per minute (ml/min)
-Higher GFR, means greater excretion of salt and water
-Regulated by neural and hormonal input

Question 25

What does GFR depend on?

Answer 25

1) Net filtration pressure of Starling's forces 2) Permeability characteristics of filtration interface 3) Surface area of filtration interface

Question 26

What factors affect Starling's forces involved in filtration

Answer 26

1) Hydrostatic pressure difference => Plasma will flow from high to low 2)Colloid osmotic pressure difference => Plasma will flow from a low to high

Question 27

Net glomerular filtration pressure calculation

Answer 27

Question 28

How does blood flow affect GFR?

Answer 28

-Constricting afferent arteriole, reduces hydrostatic pressure, reducing GFR -Dilating afferent arteriole, increases hydrostatic pressure, increasing GFR

Question 29

Which is more permeable? Glomerular capillaries or peritubular capillaries?

Answer 29

Glomerular capillaries

Question 30

How does surface area affect GFR?

Answer 30

-Intraglomerular mesangial cells contain smooth muscle actin -Sympathetic stimulation causes smooth muscle to contract -This reduces surface area for filtration

Question 31

Structure of proximal tubule (cross section)

Answer 31

-Capillary walls are endothelial cells -Proximal tubule is epithelial cells

Question 32

Proximal tubule wall structure

Answer 32

-Walls are a single layer of columnar cells

Question 33

What 3 things help proximal reabsorption of organic nutrients (e.g. glucose and amino acids)

Answer 33

-Na-coupled co-transporter -Tubular maximum (Tm) system -Specific transporters

Question 34

How is glucose reabsorbed?

Answer 34

Na+ dependent glucose co-transporter (SGLT) -Na+ moves into cell down concentration gradient and provides energy fro glucose to move against concentration gradient Facilitated diffusion glucose transporter (GLUT) -Glucose moves down concentration into interstitial fluid -Low Na+ concentration in cell maintained by Na+/K+ ATPase pump activity

Question 35

Where and how are amino acids reabsorbed?

Answer 35

-Proximal tubule -Different transporters for different groups of amino acids -Transporters for positively charged (basic) amino acids -At least 8 amino acid transporters -6 are Na+-dependent

Question 36

Where and how are proteins reabsorbed?

Answer 36

-Vast majority of filtered protein reabsorbed in PCT by endocytosis -Important for inactivation of small polypeptide hormones (e.g. insulin and growth hormone)

Question 37

Summary of PCT reabsorption

Answer 37

Na coupled transporters for: -glucose -amino acids -phosphate -sulphate Passive reabsorption -Urea -Chloride -Potassium -Calcium Bicarbonate -Related to H+ secretion -Important in acid-base balance

Question 38

Secretion in proximal tubule

Answer 38

-Two stage process -Involves basolateral and luminal membrane transporters -Transporters broadly selective -Only means of excretion for some protein bound molecules

Question 39

Which organic acids (anions) are secreted in the proximal tubule?

Answer 39

Endogenous molecules: -bile salts -fatty acids -prostaglandins Drugs: -Penicillin -Furosemide Diagnostic agent: -Para-aminohippuric acid (PAH)