Dynamics of Glomerular Filtration

Question 1

What does the term “Freely Filtered” mean?

Answer 1

A substance is present in the filtrate at the same concentration as it is in the plasma

Question 2

What does filtrate consist of?

Answer 2

Mainly inorganic ions and low molecular-weight organic solutes

Question 3

What is the “Hose Analogy?”

Answer 3

A hose with a bunch of holes in it is a Capillary
A bucket is the Bowman’s Capsule
Water slowly filling the bucket is Filtrate
Substances that freely move into the bucket are “Freely Filtered” substances

Question 4

What are some examples of Freely Filtered Substances?

Answer 4

1. Ions (ClaNK and Bicarb)
2. Neutral Organics (glucose and urea)
3. Amino Acids
4. Peptides (Insulin and ADH)

Question 5

What is normal Glomerular Filtration Rate (GFR)?

Answer 5

125 mL/min which translates into 180 L/day

(All other capillaries in the body only accomplish 4L/day)

Average Plasma Volume in human is 3L so the kidney filters plasma about 60 times a day

Question 6

What does Glomerular Filtrate consist of?

Answer 6

1. Freely Filtered Substances
2. Limited Plasma Proteins
3. NO RBCs

Question 7

What are the different cell types of the Glomerulus? What is their purpose?

Answer 7

1. Podocytes
2. Endothelial Cells
3. Mesangial Cells

Form Glomerular Filtration Barrier which determines the size of the “Holes” in the capillaries and what can be filtered and make it into Bowman’s Space

Question 8

Where are the Endothelial Cells of the Glomerulus located and what is their function?

Answer 8

1. Located right next to the blood

2. Filter by SIZE (due to fenestrations)

Question 9

What is the function of the Podocyte cells of the Glomerulus?

Answer 9

Form Filtration Slits that filter by SIZE

Foot processes interdigitate with the foot processes of other Podocytes to form the Filtration Slits

Question 10

What are the Mesangial Cells of the Glomerulus?

Answer 10

1. Filter by CHARGE because they lay down the negative extracellular proteins that repel the negative cells and proteins from entering the Nephron
2. Modified Smooth Muscle cells that can contract

Question 11

What is Net Filtration Pressure? What is the formula?

Answer 11

1. Pressures associated with the Glomerulus and Bowman’s Space that are pushing fluid into the Nephron
2. NFP= HPG - (HPBS - piG)

Question 12

What causes Oncotic Pressure? Where does Oncotic Pressure not exist?

Answer 12

1. The proteins and cells in a certain space wanting to retain fluid to surround the proteins and cells
2. Oncotic Pressure does NOT exist in Bowman’s Space because it has no proteins or cells

Question 13

What are normal NFP values

Answer 13

HPG= 45 mmHg
piG= 0 mmHg
HPBS= -10 mmHg
piBS= -25 mmHg
NFP= 10 mmHg

Question 14

What would happen if normal Hydrostatic Pressure of the Glomerulus were to drop 10 mmHg?

Answer 14

Since HPG is the only force that favors filtration, if it were to drop from 45 to 35, NFP would be NONEXISTENT, meaning filtration would not happen

Question 15

What is Glomerular Filtration Rate (GFR)? What is the formula?

Answer 15

The rate of filtration in the Glomerulus

GFR= Kf x NFP
OR
GFR= KF x (HPG - HPBS - piG)

Kf is a constant that stands for the surface area/amount of Glomeruli

Question 16

What are the 4 forces that play a role in GFR?

Answer 16

1. Surface Area (Kf)
2. Hydrostatic Pressure of the Glomerulus (HPG)
3. Hydrostatic Pressure of the Bowman’s Space (HPBS)
4. Oncotic Pressure of the Glomerulus (piG)

Question 17

What is Kf and what are some ways that it can be changed?

Answer 17

1. Kf is Surface Area of the Kidney

2. It can be decreased via:
A. Diabetes
B. Kidney Disease
C. Hypertension
D. Kidney Donation
E. Drugs causing Mesangial cells to constrict

Question 18

Describe Hydrostatic Pressure of Bowman’s Space:

Answer 18

1. Very constant
2. ONLY changed if there were to be an obstruction of flow through the Nephron such as a Kidney Stone
3. Increased HPBS would decrease GFR

Question 19

Describe Oncotic Pressure of Glomerulus:

Answer 19

1. Opposes Filtration
2. Increases with an increase in proteins found in the blood
3. Increased piG decreases GFR

Question 20

Describe Hydrostatic Pressure of Glomerulus:

Answer 20

1. Comes from Blood Pressure
2. Can be regulated via the Afferent Arteriole and Efferent Arteriole
   - Constricting Afferent Arteriole lowers HPG and GFR
   - Constricting Efferent Arteriole raises HPG and GFR
   - Dilating Afferent Arteriole raises HPG and GFR
   - Dilating Efferent Arteriole lowers HPG and GFR

Question 21

How could you significantly raise the Hydrostatic Pressure of the Glomerulus and ultimately the GFR?

Answer 21

By Dilating the Afferent Arteriole and Constricting the Efferent Arteriole

Question 22

What is Filtered Load? What is its equation?

Answer 22

The amount of a substance that gets into a Nephron and is filtered per unit time

FL=GFR x Plasma Concentration

Question 23

What is a normal Filtered Load value for Na?

Answer 23

17.5 mEq/min

This comes from:
GFR x Plasma Concentration
125 mL/min x 140 mEq/L

Question 24

What are the Autoregulatory Mechanisms essential to keeping GFR constant?

Answer 24

1. Myogenic Response (smooth muscle contraction/dilation of blood vessels) SHORT TERM
2. Tubuloglomerular Feedback (Renin-Angiotensin-Aldosterone System) LONG TERM