CHAPTER 24

STRUCTURE AND FUNCTION OF THE KIDNEY

Essentials of Pathophysiology

PRE LECTURE QUIZ TRUE/FALSE

Large molecules, such as protein, easily cross the glomerular wall.

During times of decreased cardiac output, the glomerular filtration rate is also decreased.

The activation of vitamin A, which is important for intestinal reabsorption of calcium, occurs in the kidney.

The kidneys perform excretory and endocrine functions.

In the adult, the kidneys are perfused with 20% to 25% of the cardiac output.

F

T

F

T

T

PRE LECTURE QUIZ The functional unit of the kidney is the

__________. Fluid and particles from the blood are

filtered through the capillary membrane into a fluid-filled space in the __________ capsule.

The loop of __________ plays an important role in controlling the concentration of the urine, and is divided into three segments.

The kidney synthesizes __________, which stimulates bone marrow production of red blood cells.

Serum __________ reflects the GFR (glomerular filtration rate) and can be used as an estimate of renal function.

Bowman’s

Creatinine

erythropoietin

Henle

nephron

FUNCTIONS

Kidneys filter the blood

Remove water-soluble wastes

Help control blood pressure and composition

Help maintain red blood cell levels

Diaphragm

Adrenal gland

Right kidney

Inferior vena cava

T11T12 Renal

arteryRenal vein

Left kidney

Aorta

Ureter

Bladder

NEPHRONS ARE MICROSCOPIC TUBULES THAT FORM THE URINE

Insert fig 23-4

THREE COMPONENTS OF URINE FORMATION

Filtration Reabsorptio

n Secretion

FILTRATION HAPPENS IN THE GLOMERULUS

Efferent arteriole

Afferent arteriole

Proximal tubule

Bowman’s space

GLOMERULAR FILTRATION Glomerular capillary

cells sit along a basement membrane

Bowman’s capsule epithelium cells sit along the same basement membrane

They stand up away from the membrane on pseudopods

Fluid filters across the basement membrane and between the pseudopods

GLOMERULAR FILTRATION RATE

Glomerular filtration rate = 125 mL/min

Discussion: How would it change if you:

Constricted the efferent arterioles? Constricted the afferent arterioles? Decreased the blood pressure?

Which of the following will increase GFR? Epinephrine – Prostaglandins NO – Endothelin

IF 125 ML OF ULTRAFILTRATE IS FORMED EACH MINUTE, HOW MUCH IS FORMED

IN A DAY? 60 min/hr X 24 hr/day = 1440 min/day X 125

ml/min = 180,000 ml/day =180 L/day of ultrafiltrate

Actual urine excretion is 1.5 L a day Proximal convoluted tubule reabsorbs about 60%

of nutrients and ions in the urine Absorbs at an automatic rate The amount of a solute it can reabsorb is the

transport maximum Nutrients not reabsorbed pass out in the urine

Water follows the solutes back into the blood

QUESTION

Tell whether the following statement is true or false.

If GFR increases, urine output (UO) will decrease.

ANSWER

FalseRationale: If GFR increases, it means

that fluid is moving more quickly across the basement membrane/through the pseudopods. This means that more fluid will become filtrate and less fluid will be reabsorbed into the blood. If less fluid is reabsorbed, more fluid is left to be excreted (increasing UO).

WHEN FILTRATE REACHES DISTAL TUBULE

Juxtaglomerular cells measure blood flow in the afferent arteriole and urine flow and composition

They can release renin(Image modified from Bowne, P.S. [2004]. Kidneys tutorial. Used with

author’s permission.)

Juxtaglomerular cells

RENIN STARTS THE RAA PATHWAY

Which turns on the Na+/K+ ATPase in the distal tubule

Na+ and water are reabsorbed

K+ is secreted

(Image modified from Bowne, P.S. [2004]. Kidneys tutorial. Used with author’s permission.)

Juxtaglomerular cells

RENIN-ANGIOTENSIN-ALDOSTERONE SYSTEM

Turns on the Na+/K+ ATPase in the distal tubule

Na+ and water are reabsorbed Raises blood volume Does not change blood osmolarity

K+ is secreted Lowers blood K+

DISCUSSION

What changes in these variables would turn on the RAA system?

Blood volume Blood K+

Urine volume Urine Na+

REABSORBING WATER ALONE TO REDUCE BLOODOSMOLALITY

• This happens in the inner layer of the kidney, the medulla

• The loop of Henle contains ion pumps

OSMOLALITY CONT.

• The NaCl pumped into the medulla makes it salty

• It is hypertonic to the urine in the collecting duct

• Water moves from the collecting duct into the salty medulla and enters the blood

QUESTION

Which renal structure reabsorbs water?a. Proximal loopb. Distal loopc. Collecting ductd. Glomerulus

ANSWER

c. Collecting duct Rationale: Reabsorption of water occurs

in the medulla (the inner layer of the kidney). Na+Cl- leaves the loop of Henle, which makes it hypertonic to the urine in the collecting duct. Water moves from the collecting duct into the blood (reabsorption).

AMOUNT OF WATER ABSORBED

• Depends on how much can move out of the collecting duct

• Antidiuretic hormone makes the duct permeable to water

ANTIDIURETIC HORMONE (ADH)

Makes collecting duct more permeable to water More water can be reabsorbed from the urine

into the blood Blood osmolarity decreases

QUESTION

Tell whether the following statement is true or false.

Increased ADH decreases urine output (UO).

ANSWER

TrueRationale: ADH makes the collecting

duct more permeable to water, so that more water can leave the duct and be reabsorbed into the blood. More reabsorption means that there is less fluid to be excreted (↓ UO).

SCENARIO

A man was given a drug that stopped the ion pumps in the loop of Henle.

Question: What happened to:

Medulla osmolarity? Na+ levels in the distal convoluted

tubule? Amount of water reabsorbed from the

collecting duct? Renin levels? Blood K+?

REMOVING NaCl AND WATER FROM YOUR BLOOD

Atrial natriuretic peptide and B-type natriuretic peptide ANP is made by overstretched atria BNP is made by overworked ventricles

º Both cause the kidneys to stop reabsorbing NaCl

º The NaCl and water are lost in the urine, reducing blood volume and decreasing the stretch and workload of the heart

DISCUSSION

What will happen to urine if:

Aldosterone is given?

ADH levels are high?

BNP levels are elevated?brain natriuretic peptide made by the ventricles

DISCUSSION

A man has severe renal disease. Question: Why does he develop:

Anemia? Weak bones?

QUESTION

What hormone secreted by the kidneys stimulates RBC formation in the bone marrow?

a. Reninb. Erythropoietinc. Aldosteroned. Angiotensin

ANSWER

b. ErythropoietinRationale: Erythropoietin literally means

“producing erythrocytes/RBCs.” Decreased levels of this hormone lead to anemia; increased levels lead to polycythemia.

KIDNEY FUNCTIONS

The kidneys clear wastes out of the blood Renal clearance is a measurement of how

much blood the kidneys clean in a minute If the blood contains 1 mg waste/100 mL

blood If the person produces 1 mL urine per minute

And if the urine contains 1 mg waste/mL

Question How much blood did the kidney clean in one

minute? RC RC

KIDNEY FUNCTIONS (CONT.)

What is the percent of kidney function when serum creatinine is:

2 mg/dL?

3 mg/dL?

10 mg/dL?

• If the kidneys fail, waste builds up in the blood

KIDNEY FUNCTIONS (CONT.)

Kidney function = Normal serum creatinineCurrent serum creatinine

• If the kidneys fail, waste builds up in the blood

DISCUSSION

What would each of these test results indicate?

Severe proteinuria Casts with red blood cells in them Low specific gravity Serum creatinine = 6 mg/dL BUN = 35 mg/dL, serum creatinine = 1.2

mg/dL