Kidney Function 1 - Filtration, Sectetion & Re-absorption Flashcards
3 functions of the kidney?
- Excretion of metabolites or ingested substances
- Control of body fluid composition (3ways)
- Endocrine role
Gross structure of the Kidney
Below Diaphram
Kidney - at the back of the abdominal cavity outside the peritoneum.
(each has a convex surface which fades the abdominal wall, AND concave surface which face each other)
> Ureter > Bladder > Urethra
Urine (produced by the kidney) cane be passed by the process of peristalsis through the Urethra to the bladder
Microanatomy of the kidney
Basic unit of the kidney is Nephron (each kidney has >1mil nephrons)
Each Nephron consists of i) Renal corpuscle & ii) Tubule
Describe the renal corpuscle structure
Include description of the filtration interface
Consists of Glomerulus + Bowman’s capsule
Glomerulus - compact toughed of interconnected capillary loops, which is supplied with blood via Afferent, and blood will leave via Efferent capillaries
20% of the plasma that enters Glomerulus is filtered into the Bowmans space across filtration interface
Bowman’s Capsule - fluid-filled ballon which has been punched with a fist
3 layers of the filtration interface
•Fenestrated capillary endothelium (pores max size ~15nm)
•Basement membrane (fixed polyanions throughout)
•Tubular Epithelium, aka podocytes - cells have octopus-like shape with branching pedicels (they do not touch). B/w Pedicles are filtration slit proteins (diameter ~8nm) - Nephrin & Podocin!
Describe the Tubule’s structure
Thin-walled, hollow tube
Divided into 8 distinct segments
Proximal convoluted + straight Tubule > Descending + Ascending thin limb of Henle’s Loop + thick Ascending “ > Distal convoluted Tubule > Cortical + Medullary conducting duct
Urine drains from the collecting duct into the central cavity of the kidney - ‘Renal Pelvis’
(Remember: Kidney is divided into 2 distinct tissue regions - Cortex (outer) & Medullary (inner))
What are the 2 Nephron types?
•must be aware of the Juxtaglomerular apparatus
State the 2 capillary bed found in series
Type 1: Cortical Nephron (85%) -outer 2/3 of Cortex.
> It has a short loop of Henle
Type 2: Juxtamedullary Nephron (15%) - inner 1/3 of cortex.
> it has a long looo of Henle
> producing concentrated urine
•Juxtaglomerular Apparatus composed of 3 distinct cell types
1) Macula densa - locates in the wall of the Ascending LLH just as it turns into the Distal Tubule
2) Juxtaglomerular cells - located in the walls of the Afferent arterioles
3) Extraglomerular mesangial cells - located in b/w the other 2
Two sets of capillary beds: Glomeruli and Peritubular (vesa recta) - arising from the Efferent arterioles
What are the 3 basic Renal processes?
- Glomerular filtration - movement of fluid and solutes from the glomerular capillaries into Bowman’s space. 20% is filtered. (Occurs in the Renal Corpuscles - cortex)
Filtration - passive process + relatively non-selective (means body able to eliminate w/o…)
The filtrate in the Bowman’s space can be described as an ‘ultrafiltrate’ - Tubular secretion - the movement of solutes from the peritubular capillaries into the linen of tubules. (Occurs predominantly in the proximal Tubule)
- Tubular reabsoption - the movement of materials from the filtrate in the tubules into the peritubular capillaries. (Can occur throughout the whole of the Tubule)
Three hypothetical substances
X is filtered and secreted, but not reabsorbed. Eg: Para-aminohippuric acid.
Y is filtered and some of it is reabsorbed. Eg: water and most electrolytes
Z is filtered and completely reabsorbed. Eg: glucose + bicarbonate
(NOT PRESENT IN UNRIME)
The 4th basic renal process?
Metabolism
> some substances may be completely eliminated from the body be being metabolised within the walls of the Tubules. Eg: Glutamine
What gets through the Glomerular filtration barrier?
Most plasma constituents are freely filtered except proteins (non-selective process)
This is because Filtration depends on the molecular size, shape and charge
•Molecular size: know graph
molecules that have a molecular mass <7000D are found in the filtrate at the same concs as they are in the plasma (filtrate plasma ratio is 1)
Larger proteins can only be partially filtered (ratio <1)
Even larger proteins (eg: Albumin -69k D) are completely excluded from the filtrate
•Molecular charge: know graph
Cationic molecules have a greater ability to reach the filtrate
Similarly sized uncharged molecules have a lower ability
Similarly sized anionic (starting point lower) very slim ability
Due to the fact that the basement membrane has sites of fixed polyanions, i.e attract +, repel -ve.
The composition of Ultrafiltrate
What can infection, damage to Glomerulus, very high BP result in?
The Ultrafiltrate (found in the Bowman’s space) has a fairly similar composition to plasma. But there are important differences in conc of certain molecules
- A molecule maybe be small but bc it’s bound to proteins, it’s prevented from being freely filtered across the filtration interface
Eg: 40% of Ca in plasma is bound to proteins
Albumin binds acidic drugs
Infection, damage to Glomerulus, very high BP result in
•protein in urine
•Haemoglobin in urine
•Red cells in urine
What is Glomerular Filtration rate (GFR)
GFR is the volume of fluid filtered from the glomeruli per minute (ml/min)
GFR depends on a combination of
- Opposing starling forces - Hydrostatic + Colloid osmotic pressure forces
- SA of filtration interface (JG Apparatus)
- Hydraulic permeability of capillaries
GFR is regulated by both neural and hormonal input
How much filtrate formed per min?
GFR: 125 ml/min of filtrate formed (180 l/day) - related to renal corpuscle being v.leaky.
Urine output typically only 1.5 l/day
Know the structure of the proximal tubule
Walls are a single layer of columnar cells.
It has a luminal membrane (brash border), numerous microvilli (better SA) and mitochondira (important for generating ATP)
Proximal reabsorption of organic nutrients
1) Na+ coupled co-transporter - which is 2)A tubular maximum system. 3)specific transporters for glucose + different types of amino acids.
Glucose: freely-filtered (ratio=1), not normally found in urine bc filtered glucose is normally reabsorbed.
Step 1- SGLT-Na+ dependent glucose co-transporter - (Tm system)
Step 2- GLUT facilitated transporter
Step 3- Na+K+ ATPase pump
Amino acid: reaborsbed at PT
