Chapter 18
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Chapter 18. The URINARY SYSTEM. 4 FUNCTIONS OF THE URINARY SYSTEM. Regulating blood volume and blood pressure Regulating plasma concentrations of ions Helping stabilize blood pH Conserving valuable nutrients. THE ORGANIZATION OF THE US. Kidneys : produce urine

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Chapter 18

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Chapter 18

The URINARY SYSTEM


4 FUNCTIONS OF THE URINARY SYSTEM

  • Regulating blood volume and blood pressure

  • Regulating plasma concentrations of ions

  • Helping stabilize blood pH

  • Conserving valuable nutrients


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


The Kidneys

  • Ren: Latin for kidney

  • Nephron: basic functional unit of the kidney. Has 2 primary parts:

    • Renal corpuscle

    • 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


The Nephron (continued, I)

  • Renal corpuscleconsists of 2 components:

    • Glomerulus (glomus: ball): capillary network

    • Bowman’s capsule: outer wall of renal corpuscle and encapsulates the glomerulus.

  • Renal tubulesplit into 3 sections:

    • PCT: Proximal Convoluted Tubule

    • DCT: Distal Convoluted Tubule

    • 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.


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:

  • Reabsorbing all the useful organic molecules, plasma proteins, ions, and +90% of the water in the filtrate. (Proximal CT)

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


The Renal Corpuscle

  • Consistsof 2 components:

    • Glomerulus (glomus: ball): capillary network

    • Bowman’s capsule: outer wall of renal corpuscle and encapsulates the glomerulus


Collecting system

  • PRIMARY FUNCTION: Reabsorption of H2O, Na+ and HCO3- ions.

  • 2 ducts:

  • Collecting: takes tubular fluid from nephrons; these ducts merge to form a papillary duct.

  • Papillary: delivers urine to minor calyx.


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.


BASIC PRINCIPLES OF URINE PRODUCTION

  • Kidney performs 3 distinct functions:

  • Filtration (renal corpuscle only)

  • Reabsorption (mostly in PCT)

  • Secretion (mostly in DCT)

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


BASIC PRINCIPLES OF URINE PRODUCTION

  • Glomerularfiltrationoccurs 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


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.


  • 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.


    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


    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)


    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.


    ACID-BASE BALANCE

    • 3 major buffer systems of the body:

    • H2CO3/HCO3− : primary affecter of ECF pH

    • PO43− : primary affecter of ICF pH

    • 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.


    ACID-BASE BALANCE

    H2CO3/HCO3− : the most important factor affecting pH of ECF

    CO2+ H2O  H2CO3H2CO3  H+ + HCO3−

    CO2 + H2O↔ H2CO3 ↔ H+ + HCO3−

    CO2 + H2O ↔ H++ HCO3−

    • ↓ pH  ↑CO2while↑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.


    ACID-BASE BALANCE

    PO43− : 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+ + HPO42-


    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+.


    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:

      • Secreting or absorbing H+

      • Controlling acid/base excretion

      • Generating more buffers


    ACID-BASE DISORDERS

    • Respiratory acidosis/alkilosis

    • Metabolic acidosis/alkilosis


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