Module 10

Urine formation

starts with filtration in glomerulus, then reabsorption of nutrients and water from renal tubule back to interstitial space and ultimately blood, secretion of wastes and ions into the renal tubule from blood/interstitial space

Nephron loop

most specific reabsorption/secretion of salts and water in the nephron or renal tubule

Descending loop

permeable to water, additional water can further be reabsorbed in the nephron loop

Ascending loop

permeable to salts

Filtrate

moved in the opposite direction to blood flow in the nephron loop

Salts

can be first reabsorbed to the blood of vasa recta around the ascending loop because the blood around the ascending loop has lower concentration

Highest concentration of salt

at the loop of the nephron loop which is where salt reabsorption starts and water reabsorption from the descending loop stops

Blood

moves toward the descending loop so it is highly concentrated in salts and has less water than the filtrate in the descending nephron loop

Fluids outside of cells

plasma, spaces between cells, etc.

Plasma

blood

Spaces between cells

interstitial space or fluids

Cells are dehydrated

ICF goes down

Fluids rom ECF

compensate for loss from ICF, water will move from higher concentration to lower concentration to hydrate cells

Overall volume of ECF

decreases and this is the signal for kidneys to reabsorb more water in order to equalize ICF and ECF volumes in your body

Reabsorption of water

happens in DCT and PCT and descending loop of Henle; happens with the help of aquaporins in the collecting tubule and this is represented on the figure on the right

Inside of collecting tubule

space inside the tube that is filled with filtrate or pre-urine that eventually becomes used as it passes through the kidneys

Wall of collecting tubule

cells help special channels of aquaporins that help transport water into the cells of the conducting duct wall; exist on the basolateral and apical side of the renal tubule wall

Basolateral

close to the body/blood

Apical

close to the lumen/outside of the body

Water concentration inside cell increases

aquaporins on the basolateral side also help transport the water across the interstitial space into the blood/plasma of the blood

Extra water reabsorbed in collecting tubule

can help increase ECF volume and equalize ECF and ICF

ECF volume is lower

body blood volume was decreased and the blood osmolality is increased

Osmolality increased

higher concentration of ions/particles dissolved in the blood/given volume of fluids due to lower concentration water in that fluid

Decreased volume of blood

less blood returning to the heart (venous return is lower) which is detected by the baroreceptors in the blood

Osmoreceptors

detect higher osmolality in hypothalamus

Thirst

ensures increase in ECF

Angiotensin II

secretion increases due to lower blood volume and pressure; causes the muscular walls of small arteries to constrict, increasing blood pressure

Thirst centers

activated due to detection of high osmolality and dry mouth

Thirst increased

water is taken in which increased water EECF (increases blood volume or venous return) and this decreases blood osmolality overall

Sodium

reabsorbed from the proximal convoluted section of the renal tubule through several co transporters including sodium hydrogen pumps but also many other co-transporters on the apical side of the renal wall of the PCT

Sodium hydrogen pump

active on the basolateral side of the renal wall of the PCT

Apical co transporter

net concentration of sodium increases inside the cells of the renal tubule wall which can then move down the concentration gradient towards the blood

Water

will follow ions/salts to decrease blood osmolality

Similar arrangement

is needed for the balance of hydrogen ions and the balance of hydrogen ions is critical for pH balance

Carbonic anhydrase

helps break down the carbonic acid in the lumen of the PCT and the cell walls of the tubule

Build up of hydrogen ions

in the cell of the wall and less hydrogen in the blood; extra will be absorbed

Hydrogen ions in the plasma

more would be secreted into the cell of the tubule and then to the lumen

Decreasing levels of sodium in the blood

implies higher potassium concentration in the blood

Aldosterone

released from the adrenal cortex; stimulates reabsorption of sodium by the kidneys and sodium/potassium levels normalize

Renin-angiotensin-aldosterone system

balances the blood pressure in the body and is connected to both electrolyte and fluid balance in the kidneys

Cells of the renal tubule wall

in DCT detect low fluid flow or low sodium concentration in fluids and renin is secreted

Renin

helps stimulate aldosterone but it is not the only required enzyme

Liver

releases angiotensinogen which helps in a series of enzyme reactions that convert angiotensin I to angiotensin II which stimulates adrenal cortex of the adrenal gland just superior to the kidney to release more aldosterone

Pulmonary blood

contains more angiotensin converting enzyme which helps make angiotensin II

Angiotensin II

helps vasoconstriction of all vessels - if there is not enough fluid, this is a signal for the body to retire less of it locally but for the respiratory and cardiovascular centres that overall more blood and blood at higher pressure is needed to balance the homeostatic blood volume loop

Aldosterone

stimulates uptake of sodium on the apical membrane of the cells from the lumen of the renal tubule to the cells of the wall

Once more sodium is delivered to the cells

more of it can move down its concentration gradient from the cells of the wall to the interstitial space and blood

Anti-diuretic hormone

hormone that increases water reabsorptions

During digestion, liver

makes blood clotting factors to help eliminate wastes and to help the blood flow effectively

Step 1

Sodium ions are reabsorbed from the filtrate in exchange for H+ by an antiport mechanism in the apical membranes of cells lining the renal tubule.

Step 2

The cells produce bicarbonate ions that can be shunted to peritubular capillaries.

Step 3

When CO2 is available, the reaction is driven to the formation of carbonic acid, which dissociates to form a bicarbonate ion and a hydrogen ion.

Step 4

The bicarbonate ion passes into the peritubular capillaries and returns to the blood. The hydrogen ion is secreted into the filtrate, where it can become part of new water molecules and be reabsorbed as such, or removed in the urine.