Chapter 18THE URINARY SYSTEM

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4 FUNCTIONS OF THE URINARY SYSTEM1. Regulating blood volume and blood pressure2. Regulating plasma concentrations of ions3. Helping stabilize blood pH4. Conserving valuable nutrients

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THE ORGANIZATION OF THE US Kidneys: produce urine Ureter: transports urine from the kidneys to the urinary bladder

Urinary Bladder: temporarily stores urine prior to elimination

Urethra: conducts urine to exterior

Urine: liquid composed of water, ions, small soluble compounds

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The Kidneys Ren: Latin for kidney Nephron: basic functional unit of the kidney. Has 2 primary parts:1. Renal corpuscle2. 50mm renal tubule of 2

convoluted segments (in the cortex) separated by a U-tube (U-tube goes partially/completely into the medulla).

◦ Medulla – inner layer/core◦ Cortex – rind ◦ Juxta – close/nearby

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The Nephron (continued, I) Renal corpuscle consists of 2 components:

◦ Glomerulus (glomus: ball): capillary network◦ Bowman’s capsule: outer wall of renal corpuscle and encapsulates the glomerulus.

Renal tubule split into 3 sections:1. PCT: Proximal Convoluted Tubule2. DCT: Distal Convoluted Tubule3. Loop of Henle (has 2 limbs joined at a “U”, the lowest point of the nephron):

◦ Descending: active secretion of ions, acids, drugs, toxins; selective reabsorption of Na+

◦ Ascending: impermeable to water and solutes

Filtrate: protein-free solution from filtration of blood that goes through the glomerulus. As the filtrate travels along the renal tubule, it is called the tubular fluid.

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The Renal Tubule 2 convoluted tubules (CT) (twisted/coiled little tubes)

◦ Proximal – close to (the glomerulus)◦ Distal – distant from (the glomerulus)

Functions of the Renal Tubule:1. Reabsorbing all the useful organic molecules, plasma

proteins, ions, and +90% of the water in the filtrate. (Proximal CT)

2. Secreting wastes missed by filtration; the secretion empties into the tubular fluid. (Distal CT)

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The Renal Corpuscle Consists of 2 components:

◦ Glomerulus (glomus: ball): capillary network◦ Bowman’s capsule: outer wall of renal corpuscle and encapsulates the glomerulus

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Collecting system PRIMARY FUNCTION: Reabsorption of H2O, Na+ and HCO3

- ions. 2 ducts:1. Collecting: takes tubular fluid from nephrons; these

ducts merge to form a papillary duct.2. Papillary: delivers urine to minor calyx.

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BASIC PRINCIPLES OF URINE PRODUCTION

Excretion of 3 metabolic wastes: 1. Urea

◦ From: catabolizing amino acids◦ Make: 21g per day.

2. Uric Acid ◦ From: recycling RNA◦ Make: .480g per day.

3. Creatinine◦ From: skeletal muscle tissue’s catabolism of creatine phosphate, used in muscle contraction.

◦ Make: 1.8g per day.

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BASIC PRINCIPLES OF URINE PRODUCTION Kidney performs 3 distinct functions:1. Filtration (renal corpuscle only)2. Reabsorption (mostly in PCT)3. Secretion (mostly in DCT)

Also, the regulation of the amounts of H2O, Na+ and K+ ions happens between Loop of Henle and collecting system.

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BASIC PRINCIPLES OF URINE PRODUCTION Glomerular filtration occurs as fluids move across the wall of the glomerular capillaries in to the capsular space.◦ That movement is the response to blood pressure in those capillaries.

◦ GFR – is the rate of filtrate produced per minute◦ Things that affect filtration (blood) pressure alter GFR and kidney function.

◦ Declining filtration pressures simulate the juxtaglomerular apparatus to release renin.◦ RENIN release increased blood volume and blood pressure

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BASIC PRINCIPLES OF URINE PRODUCTION

Reabsorption and Secretion along the RT: PCT reabsorbs 60-70% of the volume of the filtrate produced in the renal corpuscle: nutrients, ions, H2O

Loop of Henle: ions and water◦ Ascending limb pumps out Na+ and Cl- into the medulla◦ Descending limb pumps out H2O into the medulla

DCT◦ Changes in filtrate only occur via active transport by pumps that respond to the presence of the hormone aldosterone, which responds to low Na+ or high K+ concentrations.

◦ More aldosterone = Na+ kept in blood; K+ lost to urine.

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BASIC PRINCIPLES OF URINE PRODUCTION

Reabsorption along the collecting duct: In the presence of ADH, the collecting duct reabsorbs H2O back into the medulla, sending away a small volume of urine. But, because the H2O was already absorbed, it is highly concentrated.

In the absence of ADH, the collecting duct simply passes along the regular volume of urine. It is diluted, since H2O was not reabsorbed.

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URINE TRANSPORT, STORAGE, AND ELIMINATION

Filtrate modification and urine production end at the renal pelvis.

The rest is responsible for transporting, storing, and eliminating the urine.

Components:◦ Ureters◦ Urinary bladder◦ Internal urethral sphincter

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FLUID, ELECTROLYTE, AND ACID-BASE BALANCE Intracellular fluid: 60% of total body water Extracellular fluid: 40% of total body water

Electrolyte balance is important because total electrolyte concentrations affect water balance.◦ Problems are usually Na+ related.

◦ Gained from diet, lost through urine and sweat. Reabsorbed via aldosterone in the DCT.

◦ K+ imbalance can also occur and is more dangerous, but is rare.◦ Generally low concentrations in ECF. Losses occur when Na+ declines, ECF K+ rises

(aldosterone)

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ACID-BASE BALANCE Normal pH is 7.35-7.45 ACIDOSIS: pH too low ALKILOSIS: pH too high

Cation: ion with + charge. CA+ION: “see a positive ion”. Anion: ion with – charge. ANegativeION.

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ACID-BASE BALANCE 3 major buffer systems of the body:1. H2CO3/HCO3

− : primary affecter of ECF pH2. PO4

3− : primary affecter of ICF pH3. Protein: in the ICF/ECF, amino acids respond to

changes in H+ concentrations.◦ Blood plasma proteins and hemoglobin in RBCs prevent major blood pH changes.

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ACID-BASE BALANCEH2CO3/HCO3

− : the most important factor affecting pH of ECF

CO2 + H2O H2CO3 H2CO3 H+ + HCO3−

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3

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CO2 + H2O ↔ H+ + HCO3−

◦ ↓ pH ↑CO2 while ↑pH ↓CO2 (inverse relationship) ◦ With the exception of RBCs, all cells produce CO2 24hrs/day.◦ Spontaneous reaction

◦ Carbonic anhydrase speeds up this in RBCs, liver cells, kidney cells, parietal cells of the stomach.

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ACID-BASE BALANCEPO4

3− : primary affecter of ICF pH. Technically also plays a role in ECF, but only supporting, since ICF pH is dominated by H2CO3/HCO3

−. Really, this is the dihydrogen phosphate buffer system.

H2PO4- ↔ H+ + HPO4

2-

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ACID-BASE BALANCE Protein: Organic/metabolic acids

◦ Made primarily from metabolic processes◦ Examples:

◦ Lactic acid (from anaerobic metabolism of pyruvic acid)◦ Ketone bodies (from metabolism of fatty acids)

◦ These are either recycled or excreted rapidly, so significant buildups of these do not occur.

If pH goes up, carboxyl group (– COOH) releases a H+.If pH goes down, an amino group (– NH2) takes a H+ for –NH3

+.

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ACID-BASE BALANCE Respiratory: lungs help by affecting the H2CO3/HCO3

− buffer system. Changing the respiratory rate can change the CO2 pressure in the fluids, affecting the buffer capacity.

Renal: vary rates of H+ secretion and HCO3− absorption depending on the ECF pH.

Why these also?◦ Buffer systems only provide a temporary solution by tying up excess H+.◦ For homeostasis, the H+ needs to be removed from the body.◦ If all the buffer molecules are tied up with that excess H+, then the ECF can no longer deal with

excess H+, leaving pH unmaintained.◦ Therefore, there must be a combo between the 3 buffer systems and these 2 additional mechanisms.

These 2 mechanisms do this by:1. Secreting or absorbing H+2. Controlling acid/base excretion3. Generating more buffers

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ACID-BASE DISORDERS Respiratory acidosis/alkilosis Metabolic acidosis/alkilosis