Basic tubular function Flashcards
what is the kidney?
The kidney is a central regulator of homeostasis
What components of urine do we not have a method of transporting on their own?
Urea and Water - so these move in by passive transport
What percentage of the ultrafiltrate is reabsorbed?
99%
Define osmolarity and how can you calculate it
The measure of osmotic pressure exerted by a solution across a semi-permeable membrane.
This is dependent on the number of particles in a solution and not the nature.
Osmolarity = All the concentrations of the different solutes (measured in mmol/l) added together. Each ion is “counted” separately
What is the range for normal plasma osmolarity? What makes up the majority of this?
285-295 mosmol/L
Mainly consists of sodium (140 mmol/L)
What is the range for normal urine osmolarity?
50-1200 mosmol/L
where does absorption take place?What are the two pathways through the urinary epithelium?
It occurs across the tubular wall.
Have an epithelial surface, has tight junction basolateral membrane and lumen.
Transcellular (through the cell) and Paracellular (through tight junctions)
How much paracellular is dependant on how tight the junctions are.
What is the difference between lipophilic passive transport and hydrophilic passive transport?
Lipophilic passive transport has a linear relationship with solute concentration.Protein independent transport (lipophilic molecules)
Hydrophilic passive transport is saturable because it is dependent on the availability of channel proteins.Protein dependent transport (hydrophilic molecules
Describe the relationship between solute concentration and rate in active movement ?
Directly coupled to ATP hydrolysis has the same curve as Indirectly coupled to ATP hydrolysis
As the solute concentration increases, the rate increases until a certain point where it plateaus.
How does water move?
- Water can move through tight junctions if it isn’t too tight
- It also moves through aquaporins
How is a passive system regulated?
The channel is inside the cell. When you want more of the molecule, the channel moves to the cell surface membrane (more of it does)
What are the two routes for water to pass through the renal tubular wall?
Transcellular and Paracellular
How can hydrophilic passive transport be upregulated or downregulated?
By changing the number of transporters available e.g. ADH increases the amount of Aquaporin 2 on the apical membrane
How does protein reabsorption normally happen?
Receptor mediated endocytosis - the protein binds to a receptor and is endocytosed
The acidity of the endosome allows the complex to dissociate and the receptors are recycled
What happens if the concentration of a solute in the urine exceeds the transport maxima and give an example?
It is excreted in the urine e.g. glucose which is an indication of diabetes mellitus, or intake of large amount of vit B and Vit C
Describe fully what happens inside the lumen of the nephron?
Once inside the lumen of the nephron, small molecules, such as ions, glucose and amino-acids, are reabsorbed from the filtrate
- Specialised proteins called transporters are located on the membranes of the various cells of the nephron.
- These transporters trap the molecules as they flow by them
- Each transporter traps only one or two types of molecule. For example glucose is reabsorbed by a transporter that also traps sodium.
- Water gets reabsorbed passively by osmosis in response to build up of Na in intercellular spaces.
- Some transporters require energy, usually in the form of ATP (active transport), while others do not (passive transport).
- Transporters are located in different parts of the nephron. Most of the Na transporters are located in the proximal tubule, while fewer are spread out through other segments.
What are the most important substances that are secreted?
H+
K+
where does secretion take place?
- Moves substances from peritubular capillaries into tubular lumen
- Like filtration, this constitutes a pathway into the tubule
- Can occur by diffusion or by transcellular mediated transport
- Active secretion from blood side into tubular cell (via basolateral membrane) and from cell into lumen (via luminal membrane)
Describe the differences in sodium reabsorption throughout the nephron.
65% reabsorbed in PCT
25% reabsorbed in ascending loop of Henle
8% reabsorbed in DCT
Variable absorption regulated by aldosterone and vasopressin in collecting duct, this is the only part that is regulated.
Where is most bicarbonate reabsorbed?
90% is reabsorbed in the PCT
Where, along the nephron, do you find cells that don’t have that many mitochondria?
Descending loop of Henle and collecting duct
These areas are mainly involved in the passive transport of water
Describe the features of a cell in the proximal convoluted tubule.
Numerous mitochondria
Brush border to increase surface area
Designed for lots of reabsorption
What is the most important protein of the cells lining the tubules throughout the nephron?
Na+/K+ ATPase - responsible for the sodium gradient that drives the movement of most substances.
Na/K pump keeps intracellular [Na] low and [K] high.
Large conc. and electrical gradients favour Na movement into the cell (occurs in most nephron segments)
Which substances move in or out with Na+ in the early proximal tubule?
H+ moves out (Na+/H+ countertransport)
Glucose in (Na+/glucose cotransport)
Amino acids in (Na+/amino acid cotransport)
Na+ entry down a large electrochemical gradient can bring about the “uphill” entry of glucose and a-a’s and exit of H+
