1 - Renal Physiology, Structure of the Kidney, Glomerular Filtration

The kidney helps control or regulate many substances in the body. These include (8)

• Body water

• Electrolyte levels – Na+, K+, Cl

• H+ and bicarbonate – acid base balance

• Elimination of wastes, drugs, toxins ↑ new glucose

• Gluconeogenesis

• Long term regulation of blood pressure

• Regulates red blood cell count or hemoglobin levels

• Involved with Ca++ homeostasis

How does the kidney contribute to gluconeogenesis?

Produces glucose for circulation, liver is main organ for gluconeogenesis

How does the kidney contribute to regulation of red blood cell count or hemoglobin levels

produces erythropoietin → stimulates erythrocyte production

How does the kidney contribute to Ca++ homeostasis

activates vitamin D

What parts of the nephron are mainly located in the cortex of the kidney? (6)

• Bowman’s capsule

• Distal convoluted tubule

• Glomerulus

• Proximal convoluted tubule

• Renal corpuscle

• Part of collecting duct

What parts of the nephron are located in the medulla of the kidney? (2)

• Loop of Henle

• Part of collecting duct

I’m not sure if this happens, but if a blood clot developed in the afferent arteriole, it would most likely

A. Prevent blood from entering the glomerulus

B. Prevent blood from exiting the glomerulus i

C. Prevent filtrate from entering the proximal tubule

D. Prevent filtrate from exiting the proximal tubule

A. Prevent blood from entering the glomerulus

If a water molecule was to be reabsorbed from the proximal tubule back into the blood, where would it enter the blood?

A. Collecting duct

B. Efferent arteriole

C. Glomerulus

D. Peritubular capillary

E. Vasa recta

C. Glomerulus - filtration

D. Peritubular capillary - To enter proximal tubule

E. Vasa recta - a vessle in the medulla

Lable

The 3 processes involved in urine production

1. Glomerular filtration - Blood to tubule

2. .Tubular secretion

3. .Tubular reabsorption - tubule to blood

These 3 basic renal processes determine

how much of a substance is excreted (or how the kidney does some of its main jobs!)

Substance excretion calculation

If the amount excreted is greater than the filtered load, this means that the substance was also

D. Reabsorbed

E. Secreted

E. Secreted

Either none, some, or all of a substance that enters the kidney can be excreted depending on (3)

1. How much is filtered

2. How much is secreted and reabsorbed

3. How much of each substance is excreted

How much of each substance is excreted?

What is the glomerular filtration barrier supposed to do? (3)

1. Enable high filtration rates of water

2. Allow nonrestricted passage of small and middle-sized molecules

3. Prevent almost all serum albumin, large proteins, and cells from passing through

Lable components of the filtration membrane

What are podocytes

Epithelial cell of the visceral layer of Bowman’s capsule

Parts of the filtration membrane in the capillary lumen (3)

1. Basement membrane

2. Endothelial cells

3. Fenestrations

Parts of the filtration membrane in the bowman’s space (2)

1. Pedicel of podocyte

2. Filtration slit with slit diaphragm (web of nephrin)

What is freely filtered?(5) What is not?(2)

Freely filtered

1. water

2. electrolytes

3. glucose

4. amino acid

5. urea

Not

1. Larger proteins

2. Cells

How filtration occurs

Fluid (i.e. what will be the filtrate) moves from the glomerular capillary to Bowman’s space via bulk flow

How does the filtrate that is produced compare to plasma?

Same concentration

The forces that affect filtration

Starling forces

• Hydrostatic pressure - the pressure

  of the fluid

• Oncotic pressure - the pressure of protein in the fluid

Which forces will promote (P) and which forces will oppose (O) filtration (the movement of fluid from the blood to Bowman’s capsule)?

• Glomerular capillary hydrostatic pressure (PGC)

• Glomerular capillary oncotic pressure (πGC)

• Hydrostatic pressure in Bowman’s space (PBS)

• Colloid osmotic or oncotic pressure in Bowman’s space (πBS)

• Glomerular capillary hydrostatic pressure (PGC) - P

• Glomerular capillary oncotic pressure (πGC) - O

• Hydrostatic pressure in Bowman’s space (PBS) - O

• Colloid osmotic or oncotic pressure in Bowman’s space (πBS) - P

Net filtration pressure or ultrafiltration pressure calculation

Net effect: (PGC – PBS – πGC) = Fluid is filtered

*Normally, the amount of protein in the filtrate is so small, that BS pressure is 0)

What else affects glomerular filtration? (2)(1)

• Hydraulic permeability of the filtration membrane

• Surface area of the filtration membrane

Also known as Kf - filtration coefficent

Glomerular filtration equation

Kf x net filtration pressure

All of these same forces affect filtration in other capillary beds (e.g. in skeletal muscle)

So why is the end result so different in the kidney (that is, about 20% of plasma flowing through the glomerulus is filtered)? (1)(4)

Because, compared to other capillary beds:

1. Kf is much higher - Due to the design of the filtration membrane

2. PGC is much higher - Due to the higher resistance in the efferent arteriole compared to a venule!

3. Low resistance in renal arterial

4. High resistance in efferent arterial

GFR in young healthy kidney

125 mL/ min or 180 L/ day

GFR is regulated. But how?

Change in the net filtration pressure

How can the net filtration pressure, and hence GFR, change?

PGC - Controlled changes in GFR. Adjusted to suit the body’s needs

PBS, πGC, πBS - Uncontrolled changes in GFR, Change mainly when something is wrong

Without any type of compensation, what will happen to PGC and GFR if blood pressure or mean arterial pressure goes down?

PGC will decrease

GFR will decrease

But PGC, and hence GFR, can be adjusted by constricting or dilating the afferent or efferent arteriole

How will dilation of an arteriole affect blood flow?

Increase blood flow

What will happen to PGC and GFR if the afferent arteriole is dilated (efferent arteriole stays the same)?

PGC will increase

GFR will increase

More blood flow in

What will happen if you constrict the afferent arteriole?

Decrease GFR

What will happen to PGC and GFR if the efferent arteriole is dilated (afferent arteriole stays the same)?

PGC will decrease

GFR will decrease

What will happen if you constrict the afferent arteriole and the efferent arteriole to the same degree?

GFR stays the same

Take home message about MAP and filtration

• MAP can fluctuate throughout the day.

• Despite changes in MAP, within a certain range, GFR remains fairly constant by dilating or constricting the afferent or efferent arteriole.

Autoregulation

If a person becomes very dehydrated causing the plasma protein concentration to increase, what will happen to GC and GFR?

πGC will increase

GFR will decrease

πGC = blood colloid osmotic or oncotic pressure

Think water stays with or is attrackted to protein

What will happen if the filtration membrane is damaged and becomes more permeable to protein?

πBS will increase

GFR will increase

πBS = Bowman’s space colloid osmotic or oncotic pressure

What will happen if a person develops a kidney stone that gets stuck in the ureter and blocks the outflow of urine from the kidney?

PBS will increase

GFR will decrease

PBS = Bowman’s space hydrostatic pressure

Determinants of PGC (3)

• Mean arterial pressure

• Resistance at the afferent arteriole

• Resistance at the efferent arteriole