Chapter 24 Structure and Function of the Kidney

Chapter 24Structure and Function of the Kidney

Functions • Kidneys filter the blood • Remove water-soluble wastes • Help control blood pressure and composition • Help maintain red blood cell levels

Three Components of Urine Formation • Filtration • Reabsorption • Secretion

Filtration Happens in the Glomerulus

Glomerular Filtration • Glomerular capillary cellssit 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? • 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 False 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 Urine Reaches Distal Tubule • Juxtaglomerular cells measure blood flow in the afferent arteriole and urine flow and composition • They can release renin, 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.)

Renin-Angiotensin-Aldosterone Pathway • 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 pathway? • 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? • Proximal loop • Distal loop • Collecting duct • Glomerulus

Answer • Collecting duct 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 • Antidiuretichormone 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 True 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 Henl 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 • Both cause the kidneys to stop reabsorbing NaCl • 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?

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? • Renin • Erythropoietin • Aldosterone • Angiotensin

Answer • Erythropoietin 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?

Kidney Functions (cont.) If the kidneys fail, waste builds up in the blood What is the percent of kidney function when serum creatinine is: 2 mg/dL? 3 mg/dL? 10 mg/dL?

Kidney Functions (cont.) If the kidneys fail, waste builds up in the blood Kidney function = Normal serum creatinine Current serum creatinine

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

Chapter 24 Structure and Function of the Kidney