Chapter 24 Structure and Function of the Kidney Essentials of Pathophysiology Chapter 24 Structure and Function of the Kidney

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.

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 T11 Renal artery T12 Adrenal gland Renal vein Left kidney Right kidney Aorta Inferior vena cava Ureter Bladder

Nephrons are Microscopic Tubules That Form the Urine Insert fig 23-4

Three Components of Urine Formation Filtration Reabsorption Secretion

Filtration Happens in the Glomerulus Proximal tubule Efferent arteriole Bowman’s space Afferent arteriole

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

Renin Starts the RAA Pathway Which turns on the Na+/K+ ATPase in the distal tubule Na+ and water are reabsorbed K+ is secreted Juxtaglomerular cells (Image modified from Bowne, P.S. [2004]. Kidneys tutorial. Used with author’s permission.)

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+

Blood volume Blood K+ Urine volume Urine Na+ 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 Blood Osmolality 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 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 True Rationale: 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 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

brain natriuretic peptide made by the ventricles 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? Renin Erythropoietin Aldosterone Angiotensin

Answer Erythropoietin Rationale: 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 = 100𝑚𝐿 𝐵𝑙𝑜𝑜𝑑 1𝑚𝑔 𝑊𝑎𝑠𝑡𝑒 + 1𝑚𝑔 𝑊𝑎𝑠𝑡𝑒 1𝑚𝐿 𝑈𝑟𝑖𝑛𝑒 + 1𝑚𝐿 𝑈𝑟𝑖𝑛𝑒 1𝑚𝑖𝑛 RC= 𝟏𝟎𝟎𝒎𝑳 𝑩𝒍𝒐𝒐𝒅 𝒎𝒊𝒏

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