Kidney and Glomerular Filtration Essay

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Chapter 24
Urinary System

Objectives
• 1. Describe the anatomy of a nephron.
• 2. List several kidney functions that help maintain body homeostasis. • 3. Identify the parts of the nephron responsible for filtration, reabsorption, and secretion, and describe the mechanisms underlying each of these functional processes.
• 4. Explain the role of aldosterone and of atrial natriuretic peptide in sodium and water balance.
• 5.Describe the mechanism that maintains the medullary osmotic gradient. • 6. Explain the formation of dilute versus concentrated urine.
• 7. Describe the normal physical and chemical properties of urine.
• 8. List several abnormal urine components, and name the condition when each is present in detectable amounts.
• 9. Define micturition and describe its neural control.

Structure of nephron
• 1. glomerulus: tufts of capillaries from afferent arteriole • 2. Bowman’s capsule: blind, enlarged, cupshaped structure that entirely surrounds the glomerulus.
• 3. proximal convoluted tubule: cuboidal epithelium with mitochondria and microvilli
• 4. loop of Henle: thin descending limb and thick ascending limb
• 5. distal convoluted tubule: cuboidal cells, few microvilli Structure of nephron
• 6. collecting duct: some cells with abundant microvilli but more without.
• 7. peritubular capillary bed: arise from efferent arteriole and surround renal tubules • 8. afferent arteriole: supplies blood to glomerulus • 9. efferent arteriole: carries blood away from glomerulus

Cortical vs. juxtamedullary nephrons • cortical nephrons found almost entirely in renal cortex w/ pertubular network. Juxtamedullary nephrons found in cortex and medulla. Loop of
Henle has vasa recta instead of peritubular network.

Vascular resistance
• begins high in renal artery to protect glomeruli from large fluctuations in systemic blood pressure. Decreases as blood leaves kidneys.

Juxtaglomeruluar apparatus
• plays a role in blood pressure regulation and filtrate formation .

Juxtaglomeruluar apparatus
• 1. JG cells: secrete renin, cells act as mechanoreceptors that directly sense blood pressure in afferent arteriole. Juxtaglomeruluar apparatus
• 2. Macula densa: (in DCT) near JG cells are chemoreceptors that respond to changes in solute content of filtrate.

Filtration membrane
• between blood and Bowman’s capsule. Porous to allow free passage of water and solutes smaller than plasma proteins. Similar to respiratory membrane in thickness and function.

Urine formation
• 1000-1200 ml of blood passes through glomeruli/minute. 120-125 ml is forced out into tubules. The filtrate in the tubules is everything in plasma except proteins and cells. Urine, on the other hand, is metabolic wastes and unneeded substances. • Of the 180 L of plasma processed everyday, only
1.5 L leaves the body as urine. Three processes are involved in urine formation: a) glomerular filtration b) tubular reabsorption c) secretion.

Glomerular filtration
• passive, nonselective process; uses hydrostatic pressure. By keeping proteins (namely, albumin) in plasma of capillaries, colloid osmotic pressure of glomerular blood is maintained. This ensures that all of the water in the plasma is not lost to the tubules.

Glomerular filtration
 Presence of protein or blood cells in urine usually indicates some problem with filtration membrane.

Glomerular filtration
• NFP: responsible for filtrate formation (net filtration pressure).
NFP is about 10 mmHg.
• HPg: pushes water and solutes across membrane into capsule. OPg: pulls water back into glomerulus.
HPc : pushes filtrate back into glomerulus from capsule.

Factors affecting glomerular filtration rate:
GFR = 120-125 ml/min, 7.5 L/hr, 180 L/day

• Total surface area available
• Filtration membrane permeability
• Net filtration pressure: a 15% drop in filtration pressure stops filtration altogether (due to change in glomerular blood pressure).  BP, 
GFR; dehydration  GFR

Regulation of glomerular
filtration…