Urine Formation

What is the osmolarity of blood vs. urine?

• Blood = 300osm/L

• Urine = 500-1200osm/L

What is the pH of blood vs. urine?

• Blood = pH 7.4 - 7.5

• Urine = pH 4.5 - 8.0

How does the volume of urine compare to the amount of blood filtered?

Kidneys filter a large volume of blood (25% of cardiac output) to make 2L urine / day

Is there glucose or protein in urine?

No glucose or proteins in urine

• present in blood

What are the four process of urine formation?

1. Filtration

2. Reabsorption

3. Secretion

4. Excretion

What happens in filtration?

Pressure forces water/solutes from blood into tubule

What happens in reabsorption?

Body reclaims valuable solutes from filtrate

What happens in secretion?

Toxins and solutes are added to filtrate

What is excretion?

Removal of final urine from the system

Three layers of the filtration membrane

1. Fenestrated capillaries - block cells and allows plasma fluid through

2. Basement membrane - blocks large/negatively charged proteins

3. Podocytes(visceral layer) - block small proteins and create filtration slits

What is the difference between blood and filtrate?

Filtrate has no cells or proteins

• Blood has both

What pressures push blood plasma through the filtration membrane?

• Blood pressure (HPgc) = 55 mmHG

• Resistance from capsule (HPcs) = 15 mmHG

• Osmosis from proteins (OPgc) = 30 mmHG

What generates the 3 pressures that affect filtration?

• Blood pressure (HPgc): from glomerular capillaries

• Resistance (HPcs): from filtrate pushing back inside the capsule

• Osmosis (OPgc): from proteins pulling water back into blood by osmosis

What is the net filtration pressure (NFP)?

NFP = 55 - (15 +30) = 10mm Hg

• must stay positive for filtration to occur

What does the PCT (proximal convoluted tubule) reabsorb?

Most H2O, salts, glucose, amino acids, creatine, and some urea

What does PCT (proximal convoluted tubule) secrete?

toxins and solutes (needs ATP)

Is urea reabsorbed?

Yes to help maintain osmotic gradient

Do all nephrons have a Loop of Henle?

Yes, but Juxtamedullary Nephrons have longer loops

What is the descending loop permeable to?

Only H2O

What is the ascending loop permeable to?

Only salts (NaCl)

What gradient do Juxtamedullary Nephrons create?

High → Low osmolarity from medulla to cortex

Countercurrent multiplication

1. Water removed in descending limb = concentrated filtrate

2. Salt removed in ascending limb = dilute filtrate

Why is countercurrent multiplication important?

Makes filtrate dilute so more H2O can be reabsorbed later

What is countercurrent exchange (Vasa Recta)?

Maintains osmotic gradient by removing extra ions/water

What does the distal convulated tubule (DCT) do?

reabsorbs salts, secretes toxins / solutes

Why must filtrate entering DCT be dilute?

Allows more H2O reabsorption and more secretion

Is the DCT regulated by hormones?

Yes by parathyroid hormone (Ca2+) and aldosterone(Na+)

What happens in the collecting duct?

• Reabsorbs water (via ADH)

• Absorbs urea/Na+/Cl-

• Secretes H+/K+

How does ADH work?

ADH adds aquaporins to increase H2O reabsorption

What are aquaporins?

Water channels that increase osmosis

What does the Juxtaglomerular Complex do?

Controls blood pressure using Renin=Angiotensin-Aldosterone system

How does the Juxtaglomerular Complex work?

1. Macula Densa sense low Na+

2. Granular cells sense low BP → releases Renin

3. Renin activates Angiotensin → triggers Aldosterone

4. Aldosterone increases Na+ reabsorption → raises BP