Urinary System 3 - Loop of Henle, DCT and Collecting Duct

osmotic gradient in renal medulla

* The interstitial fluid in the renal medullar gets increasingly hyperosmotic
* The limits for the osmolarity of urine are species specific and determined by the osmolarity of the renal medulla

Maximum Concentrating Ability of the Kidneys Varies with Species

The ability to concentrate depends on the relative length of the loop of henle and the number of juxta medullary nephrons.

Some animals e.g. humans also have cortical nephrons which have little value in concentrating urine.

Why have a medullary osmotic gradient?

We need something against which we can move water by osmosis.

loop of henle

* Descending limb lacks ion pumps but is very permeable to water
* The ascending limb is not permeable to water
* Active transport of ions out of the ascending limb leads to osmotic transport of water out of the descending limb

static effect

1. **Static effect**
* Active transport of solute out of ascending limb to make interstitial space hyper osmotic and draws water out of descending limb making filtrate more concentrated

fluid flow

2. **Fluid flow**
* As filtrate is continually produces new tubular fluid enters the descending limb and pushes the fluid at higher osmolarity down the tube

what happens as static effect and fluid flow are repeated

As these 2 steps are repeated over, the osmotic gradient steadily multiples down the tube.

how would a standard capillary network around the loop of henle impact the concentration gradient

Capillaries that freely exchange solutes and water with the medulla pose a risk to the osmotic gradient.

If there was a standard dense capillary network surrounded the loop of henle, there wouldn't be a concentration gradient.

The Capillary Network Surrounding the Loop of Henle is Highly Specialised

* Called vasa recta
* Supply oxygen and nutrients to the medulla
* Run parallel to the limbs of the loop of henle
* blood flows in the opposite direction to the filtrate and the hairpin loop slows blood flow down

vasa recta

2 components of the countercurrent multiplier

The countercurrent multiplier has 2 components:


1. Specialised peritubular capillaries (vasa recta)
2. Loop of Henle

**Another reason for the hyper osmotic medulla?** *(don’t need to learn - just need to be aware of)*

* Urea - the nitrogenous waste product becomes increasingly concentrated in the filtrate.
* Very highly concentrated in the collecting ducts
* There are urea channels in the medulla which allow urea to escape from the collecting duct and be recaptured by the loop of henle
* ADH unregulated these urea channels in the inner medullary collecting ducts allowing more urea to flow passively into the interstitial fluid and back into thin descending loop of henle
* this process allow urea to contribute to the medullary concentration gradient
* urea recycling is responsible for about 50% of the osmotic gradient

regulated reabsorption of water

Water cannot be actively pumped - it moves by osmosis.

Most of water absorbed in PCT - not here.

fluid that leaves the loop of henle is…

Fluid that leaves the loop of henle is hyposmotic.

endocrine control of water balance

Antidiuretic hormone = ADH

ADH

→ nonapeptide hormone released from the posterior pituitary

→ also known as (arginine) vasopressin

ADH - Sensitive Principal Cells in the Collecting Duct

* ADH secreted
* ADH binds to specific membrane receptor
* Receptor activates cAMP 2nd messenger system
* Cell inserts aquaporins into apical membrane
* Water is absorbed by osmosis into the blood

excessive drinking

**Excessive drinking:**

* No ADH
* No aquaporins
* Large volume of dilute urine/diuresis

dehydration

**Dehydration:**

* Increased ADH
* Increased aquaporins
* Small volume concentrated urine

key points

* The loop of henle continues to reabsorb further water and solute
* Due to the unique permeability’s of different segments of the loop these reabsorption activities act to generate an osmotic gradient through the renal medulla
* This osmotic gradient will allow the tubular fluid to be concentrated in the presence of ADH

sodium

* 100% filtered
* Approximately 90% reabsorbed
* Approximately 70% has been reabsorbed in the PCT
* Approximately 20% in loop of henle

potassium

* 100% filtered
* 100% reabsorbed
* Approximately 70% has been reabsorbed in PCT
* Approximately 30% in loop of henle

what is the sodium and potassium concentration of the filtrate further modified by

The sodium and potassium concentration of the filtrate is further modified by principal cells in the DCT and collecting duct.

Principal Cells in the DCT and Collecting Duct

* Sodium potassium ATPase maintains sodium potassium gradient
* Leak channels on apical side of the principal cells allow passive diffusion of ions between principal cells and filtrate

Principal Cells vs Tubular Cells of the PCT

* Both have asymmetrical arrangement of Na+K+ ATPase
* Principal cells deal with lower volume (no microvilli)
* Principal cells are responsive to hormone
* Water does not necessarily follow Na+

Why doesn't water follow Na+?

Cells are impermeable to water without ADH.

principal cells

Secretion of K+ and reabsorption of Na+ is regulated by the hormone aldosterone.

Cells of DCT and collecting duct are impermeable to water in the absence of ADH - reabsorption of water is therefore not linked to reabsorption of sodium.

phosphorous

**Phosphorous:**

* Inorganic phosphate 100% filtered
* Reabsorption in PCT by Na+ co-transport under hormone control
* Not reabsorbed in DCT and CD
* Dietary excess is excreted via the kidneys

calcium

**Calcium:**

* 50% bound to albumin so only free 50% is filtered
* Approximately 70% reabsorbed in PCT
* Selective reabsorption of Ca2+ in DCT and collecting duct under hormone control
* Normally only 1-2% filtered Ca2+ is excreted

regulation of calcium and phosphorous

Parathyroid hormone (PTH) is released in response to decreased \[Ca2+\] in the blood.

In the kidney, parathyroid hormone (PTH):

* Decreases reabsorption of phosphate in the proximal tubule
* Increases reabsorption of Ca2+ in the ascending loop of Henle, distal tubule, and collecting tubule

key points - ions

* Small volume, regulated reabsorption or secretion of Na+ H+ Ca2+ and water occurs in the DCT and collecting duct
* 100% of potassium and bicarbonate have already been reabsorbed by the start of the DCT
* Homeostatic mechanisms can lead to secretion of potassium, and bicarbonate back into the filtrate