Urinary system

What does renal mean

Pertaining to the kidneys

Kidneys have many roles, to start, what does it remove from the blood

Waste products and forge in chemicals

What does it regulate

Body water, salts and acid base balance, excreting just enough to maintain homeostasis

They are involved in what glucose-related process

Gluconeogenesis, producing glucose from amino acids which is then released into the blood

What does it release in order to regulate blood pressure

Hormones like renin and EPO (increases number of red blood cells

Lastly, what vitamin does it produce to influence calcium balance

D

Where are the kidneys locate din the body

To the side of the spine at the lowest point of the ribcage

What is the structure of the kidney

Outer layer = renal cortex

Middle layer = renal medulla

Inner layer = pelvis

Attached to = ureter

What is the functional unit of the kidney

The nephron

What are the 2 types of nephron

Cortical nephron and Juxtamedullary

What is the difference between the 2

Juxtamedullary nephron mainly has a loop of Henle which extends deep into the renal medulla from the cortex, the cortical nephron is primarily located in the cortex, with a very small extension into the medulla

What are nephrons roughly made up of

A network of tubing and blood vessels

What are the 2 main components

The renal corpuscle and the tubule

What 2 things is the real corpuscle comprised of

The Bowman’s capsule (extension of tubing) and the Glomerulus (network of blood vessels)

What does the renal corpuscle do

Where hyper-filtration occurs and creates a filtrate from blood which is free from cells and proteins, which then leaves the corpuscle and enters the tubule

What happens to this fluid as it flows through the tubule

Substances are added/removed

Where does the fluid end up

In the collecting duct and exits the kidneys as urine

What are the 2 types of tubule

The proximal and distal tubules

Starting the chain of events, the filtration of blood begins where

The renal corpuscle

There is a blood flow into and out of the renal corpuscle, what are these respective arterioles called

Blood flows in through the afferent arteriole, and out via the efferent arteriole

What percentage of the blood plasma makes its way into the Bowman’s capsule

20%

The filtrate then makes its way where

The tubule system, specifically the proximal convoluted tubule

Where does it go from the proximal convoluted tubule

The proximal straight tubule

This then goes into the loop of Henle, what is the name of the limb it enters

The descending limb

The ascending limb is split into 2 segments, what are they and what order are they reached

The thin segment of the ascending limb, then the thick segment of the ascending limb

The filtrate then leaves the loop of Henle and goes where

The distal convoluted Tubule

From there it enters the collecting duct system, which contains 2 collecting ducts which are

The cortical collecting duct and the medullary collecting duct

Lastly the filtrate leaves through what

The renal pelvis

The Bowman’s capsule is made up of a layer of what cells which surround the glomerular capillaries

Podocytes - a specialist filtering system

How is the structure of a podocyte related to its function

It contains filtration slits which enable the transfer of fluid

has a large number of cell processes/extensions to help with filtration

How do the capillaries in the nephron differ from normal capillaries

They are specialised and contain small pores called fenestrae

Blood from the glomerulus is separated from fluid in the Bowman’s capsule by a filtration barrier consisting of three layers:

1. The single cell endothelium of the capillaries

2. The basement membrane (non-cellular, protein-rich)

3. Single cell lining of the Bowman’s capsule

What is the method used for filtering blood

Dialysis

What structure is the dialysis imitating in the kidney

The podocytes

The renal circulation has 2 sets of capillaries which are:

What are they specialised for

Glomerular capillaries: filtering of the blood

Peritubular capillaries: Supply nephron/tubing with their own blood supply + form veins where blood leaves the kidney

What percentage of all nephrons are Juxtamedullary

15% - cortical nephrons make up the other 75%

What is special about the role of the Juxtamedullary nephrons

They generate an osmotic gradient for water reabsorption

What is the Macula Densa and where is it located

In the distal tubule, it contains sensors that detect changes in blood composition

Where are the juxtaglomerular cells

Situated in the space between the distal tubule and the glomerulus

What are the 3 main functions of these cells

1. Secretion of renin

2. Influence the formation of angiotensin II

3. Control blood pressure via vasoconstriction and sodium/water retention

How do the kidneys increase blood pressure, starting from the secretion of renin enzyme from the kidney to the production of Angiotensin II

The liver produces angiotensinogen

Renin cleaves angiotensinogen to angiotensin I

Angiotensin converting enzyme in the endothelium converts Angiotensin I to Angiotensin II

What 2 ways does angiotensin II impact blood pressure

1. Impacts the CV system causing the vasoconstriction of arterioles

2. Impacts the adrenal cortex which releases aldosterone

What does aldosterone do to the kidney

Increases sodium and water retention in the blood

What 3 stimuli causes the kidney to secrete renin

1. Renal sympathetic nerves

2. Intrarenal baroreceptors

3. Macula densa

So if a decrease in plasma volume was detected how would these three stimuli cause the increase of renin secretion from renal juxtoglomerular cells

1. Increased activity of renal sympathetic nerves

2. A decrease in arterial pressure

3. A decrease in glomerular filtration rate, causing a decreased flow to the macula dense which decreases NaCl delivery to the macula densa

The osmolarity of extracellular fluid is normally in what range

285-300 mOsm

The concentration and location of what determines where the water goes

Nonpenetrating solutes

If cells with an intracellular osmolarity of 300 mOsm are placed into a solution of non penetrating solute with an osmolarity of 300 mOsm what will happen to them

They will nether swell nor shrink

What is this type of solution called

Isotonic - the solution has the same concentration of nonpenetrating solutes as the normal extracellular fluid

What is a hypotonic solution

A solution with a lower concentration of nonpenetrating solutes compared to normal extrcellular fluid

What happens to the cells in a hypotonic solution

They swell as water rushes in to dilate the intracellular solutes

What is a hypertonic solution

A solution with a higher concentration of nonpenetrating solute compared to the normal extracellular fluid

What happens to cells in a hypertonic solution

They shrink as water rushes out to dilute extracellular solutes

What is ADH also known as

Vasopressin

What are the 2 regulatory mechanisms of vasopressin secretion

Osmotic regulation and volume regulation

In osmotic regulation, what happens if excess water is ingested, and how does this lead to more H2O being secreted (mention body fluid osmolarity, osmoreceptors, vasopressin)

1. There is a decrease in body fluid osmolarity (H2O concentration)

2. This causes a decrease in firing by hypothalamic osmoreceptors

3. This decreases vasopressin secretion from the posterior pituitary

4. Decreasing plasma vasopressin

5. Decreasing the tubular permeability to H2O in the collecting duct AND decreased H2O reabsorption

6. Causes more H2O to be secreted

In terms of volume regulation, what happens if there is a decrease in plasma volume and how can this lead to a decrease in H2O secretion

1. A decrease in venous, atrial and arterial pressure detected by baroreceptors

2. This increases vasopressin secretion from the posterior pituitary

3. This increases plasma vasopressin

4. Increases the tubular permeability to H2O in the collecting ducts AND increases H2O reabsorption

5. Therefore decreases H2O excretion

How does glomerular filtrate compare with regular blood plasma

It contains all the same substances in the same concentrations apart from large proteins and cells

After glomerular filtration, two possible processes can happen in the tubules either both or one or the other, what are they

Tubular excretion/tubular reabsorption

What are the forces that favour filtration

P-GC = Glmerular capillary blood pressure

Blood pressure itself forces the plasma out of the capillary and into the tubule

What are the forces that oppose filtration

P-BS = Fluid pressure in Bowman’s capsule

pi-GC = osmotic force due to protein in plasma

What is the glomerular filtration rate controlled by

Under physiological control (achieved via neural and hormona input to the afferent and efferent arterioles)

What 2 things can be done to the afferent/efferent arterioles to decrease Glomerular filtration rate

Constricting the afferent arteriole will decrease GFR

Dilating the efferent arteriole will decrease GFR

And to increase GFR?

Constrict the efferent arteriole (pulls blood)

OR dilate the afferent arteriole (more blood available)

How many litres of water do the kidneys filter per day, how much is excreted and how much is reabsorbed

180, 1.8, 99%

What do these figures show

The filtered loads are very high (there’s only 40L of water in the body)

Reabsorption of useful components is complete (waste products is incomplete - urea)

What type of process is sodium reabsorption and where does it occur

It is an active process which occurs in all tubular lumen apart from the descending limb of the loop of Henle

The reabsorption of what is dependent on sodium reabsorption

Water by osmosis

How is sodium reabsorbed into the peritubular capillaries

Na+ passively enters the tubular epithelial cells from the tubular lumen down to conc gradient

It is then actively transported into the interstitial fluid by a Na+/K+-ATPase pump

Then is transported into the peritubular capillaries

In the proximal tubule, how is sodium usually transported out of the lumen

Cotransport with another molecule like glucose or H+ (moves into the lumen)

How is water coupled with Na+ transport (as long as the apical membrane is permeable to water)

Loss of Na+ in the tubular lumen reduces the local osmolarity, creating a gradient against the interstitial fluid

This causes a net movement of water across the tubular epithelial cells into the intersistial fluid, where anything dissolved in the interstitial fluid move by bulk flow into the peritubular capillaries

Even if the osmolarity gradient is high, what else is needed for water to pass the epithelium

For this to happen, water channel proteins (vesicles containing aquaporins) need to be inserted into the epithelium of the collecting ducts (controlled physiologically by vasopressin)

What is the difference between the aquaporins in the proximal/distal tubules and the collecting ducts

The aquaporins in the collecting ducts are the only ones that can be controlled by physiological changes, the others are constant

What is obligatory water loss

The minimal volume of urine water to dissolve a certain mass of a solute

Why does water diffuse out of the medullary collecting ducts

The interstitial fluid is very hyperosmotic so water diffuses out of the ducts and into the IF

Why is the production of urine called the countercurrent multiplier system

The ascending and descending limb transport the filtrate in opposing directions, and the aim is to create a hyperosmotic interstitial fluid to pull water away from the medullary collecting duct

Multiplier is because fluid is constantly flowing through the loop of Henle so the osmolarity difference is multiplied

Why does the fluid entering the descending loop have the same osmolarity as the plasma

As the proximal tubule reabsorbs Na+ and water in the same proportions

How is Na+ reabsorbed in the upper part (thick) of the ascending limb

Through a Na+ Cl- cotransporter pump

What about the lower part

Simple diffusion

Why can’t water be reabsorbed from the ascending limb

There are no aquaporins, it s impermeable

What effect does this impermeability have on the fluid in the medulla

It is hyperosmotic in comparison as the solute is reabsorbed without water

What does the descending limb not reabsorb, what is the consequence of this

NaCl, water diffuses out into the interstitial fluid of the medulla, to make the osmolarities of the descending limb and interstitial fluid the same (both higher than ascending limb)

What therefore draws the water out of the collecting duct

The hyperosmolarity of the interstitial fluid

What is the medullary circulation

Efferent arterioles of the juxtamedullary nephrons which descend from the cortex into the medulla parallel to the loops of Henle and form a hairpin loop back to the cortex

Why does the medullary circulation not eliminate the countercurrent mechanism

As blood descends into the increasingly hyperosmotic medulla, it loses water and gains solutes (NaCl), and the opposite happens as it ascends

Means disruption is minimal because it just recycles water and solutes