Urinary System

Part I

In the nephron, approximately 20 percent of the blood gets filtered under pressure through the walls of the glomerular capillaries and Bowman’s capsule. The filtrate is composed of water, ions (sodium, potassium, chloride), glucose and small proteins (less than 30,000 daltons — a dalton is a unit of molecular weight). The rate of filtration is approximately 125 ml/min or 45 gallons (180 liters) each day. Considering that you have 7 to 8 liters of blood in your body, this means that your entire blood volume gets filtered approximately 20 to 25 times each day! Also, the amount of any substance that gets filtered is the product of the concentration of that substance in the blood and the rate of filtration. So the higher the concentration, the greater the amount filtered or the greater the filtration rate, the more substance gets filtered.

This filtration process is much like the making of espresso or cappuccino. In a cappuccino machine, water is forced under pressure through a fine sieve containing ground coffee; the filtrate is the brewed coffee. The arrangement of the glomerular capillaries in series with the peritubular capillaries is important to maintain a constant pressure in the glomerular capillaries, and thus a constant rate of filtration, despite momentary fluctuations in blood pressure. Once the filtrate has entered the Bowman’s capsule, it flows through the lumen of the nephron into the proximal tubule.

Review Questions

Where are urea and uric acid produced?

Catabolism of amino and nucleic acids

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What 3 categories do the kidneys excrete?

Uric acid and urea
products of blood cells and hemoglobin
hormones, drug metabolism, pesticides

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How does the kidney effect homeostasis?

Fluid volume
Electrolyte composition
Osmolarity

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How do the kidneys contribute to the endocrine system?

Renin-angiotensin
Erythropoietin

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What is the nephron and its components?

The functional unit of the kidney
1. The glomerulus (Bowman’s capsule)
2. Renal tubules

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What is the function of the glomerulus and bowman’s capsule?

Plasma filter

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What is the function of the renal tubules?

Transport, resorption, secretion

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How many arterioles does each nephron have?

2 – afferent and efferent

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How many capillaries does each nephron have?

2 – Glomerulus and peritubular

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What are the 2 types of nephrons?

Cortical nephron
Juxtamedullary nephron

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Where is the cortical nephron located?

Only in the cortex
Shorter loops
Peritubular capillaries, no Vasa Recta

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Where is the juxtamedullary nephron located?

In both cortex and medulla
Longer loops
Vasa Recta

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What is the filtration fraction?

20% of Renal Plasma Flow is filtered at the glomerulus

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What is filtered at the glomerulus?

Wwater, electrolytes, small molecules (urea, glucose, inulin)

No proteins/cells

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What is clearance?

What the kidney can eliminate from plasma

Urine concentration/plasma concentration x urine volume

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What is the average urine volume/day?

1.8L

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What 2 substances can be used to measure clearance?

Inulin

Creatinine – some reabsorbed, so gives lower clearance value

Para Aminohypuric Acid

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What is reabsorption?

Reclaiming of some components in the filtered load – glucose, AAs, sodium, etc

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How are some substances reabsorbed?

Transporter substances
Glucose needs Na to cotransport

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What is transport maximum?

There are no more carriers available, so the substance stops being reabsorbed and gets excreted

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What is the filtered load?

quantity of filtered substance exiting the glomerulus

Concentration in plasma x GFR

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What are the Starlings forces?

Determine glomerular filtration
Plasma/capillary Hydrostatic
Plasma oncotic
Bowman’s capsule pressure

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Where is most of the glucose and sodium reabsorbed?

Proximal tubule

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Where are substances secreted?

From peritubular capillaries into the renal tubules – limited transport

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What substances are secreted?

Para-amino hypuric acid

Other organic acids

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If filtration rate is greater than excretion rate

Then there is a net reabsorption of X

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If excretion rate is greater than filtration rate

Then there is a net secretion of X

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If filtration and excretion are the same

There is no net reabsorption or secretion

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What is the primary function of the proximal convoluted tubule?

Reabsorb H2O and some essential solutes

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How much of the filtered sodium and water is reabsorbed?

2/3’s

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What 3 things use Na+ for cotransport?

Glucose
AAs
HCO3

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What is the action of carbonic anhydrase?

Inhibit HCO3- reabsorption, act as diuretic

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What does the thin descending limb of the loop do?

highly permeable to water
It is the Concentrating loop

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What does the thin ascending limb of the loop do?

Not permeable to water
Permeable to NaCl and urea

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What does the thick ascending limb do?

Impermeable to water
Somewhat permeable to urea
Reabsorb NaCl – diluting loop

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How do loop diurets work and what are they?

Furosamide, ethacrynic acid, butemetamide

Block Na, K, Cl pump

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What is the role of the distal convoluted tubule and collecting duct?

Fine tuning

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Where is ADH secreted?

Supraoptic and para ventricular nuclei of the hypothalamus in response to volume and pressure changes in vascular bed

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What is the effect of ADH?

decreased urine volume

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What is the function of JG cells?

Smooth muscles in the arterioles, that contract/dilate them – contains renin

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What is the function of the macula densa?

Detects solute levels

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What triggers renin release?

low hydrostatic pressure in afferent

Stimualtion of sympatheetics

Changes in fluid at macula densa

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Whaat is the function of renin?

Converts angiotensinogen to angiotensin I

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How do we get angiotensin II?

Angiotensin I circulates through lungs with angiotensin convertince enzyme (ACE)

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What is the function of angiotensin II?

Stimulates aldosterone
Vasoconstriction of arterioles

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What is the function of aldosterone?

Increases reabsorption of Na+ Secretion of K+ and H+