Ch. 20

Urinary System

Identify the following structures

o   Kidney

o   Ureter right/left

o   Renal medulla

o   Renal cortex

o   Renal pyramid

o   Renal pelvis

o   Glomerulus

o   Capsular space

o   Fibrous capsule = renal capsule

o   Urethra

o   Right/left renal veins

o   Urinary bladder

Organs of urinary system

• 2 Kidneys

  • Contain nephrons that are responsible for removing waste producrs, stray ions, & excess water from the blood

    • *Nephron producces urine by filtration, absorption, & secretion 

  • Structure of Renal pelvis & the calyces

    • Funnel-shaped sac that connects directly into the ureter, along w/many tubes that converge to form the funnel-shaped sac

  • 2 Ureters

    • The ureters are bilateral, muscular, tublar, structures, each one responsible for taking urine from one kidney to the urinary bladder for storage, prior to excretion

      • Urine flows from renal calyces down ureters into bladder for storage

  • Bladder

    • Stores urine, allowing urination to be infrequent & controlled

    • The normal capacity of the bladder is 400-600 mL

  • Urethra

    • The tube that conveys urine outside the body

Kidneys

• Locate retroeritoneally-outside of/behind the peritoneum

  • The kidneys aren’t located in the peritoneal cavity, but outside of it 

  • Recall that the peritoneum is the double-layed serous membrane of the abdominal cavity

• Function of Kidneys

  • Primary function of the kidneys

    • To regulate the volume & composition & pH of body fluids 

    • In the process, the kidneys remove metabolic wastes & excess chemicals as well as excess water 7 electrolytes from the blood &, along w/the ureters, urinary bladder, & urethra, excrete them to the outside as urine

      • Electrolytes

        • Molecules that release ions in water

        • Role in water balance

        • Intracellular fluids

          • High concentration of potassium K+

    • These wastes include nitrogenous & sulfur-containing products of protein metabolism, such as certain metabolic acids

    • *The kidneys convert vitamin D from supplements or the sun to the active form of vitamin D that is needed by the body

Kidneys Pt. 2

• The formation of urine the nephron involves 3 processes:

  • Glomerular filtration- filtration that begins at renal corpuscle (Glomerulus)

    • The stage of urine formation that involves the production of filtrate w/the same composition as tissue fluid & plasma, while large molecules such as proteins & blood cells remain in the blood

    • High blood pressure would increase the rate of glomerular filtration

  • Tubular Reabsorption

    • Occurs as substances pass from the renal tubules to the peritubular capillaries

      • Returns usefule substances back into the blood

    • Water reabsorption from the proximal tubule is closely linked to the active reabsorption of Sodium (Na) ions

  • Tubular Secretion

    • Secretion occurs when substances pass from the peritubular capillaries (blood) into the renal tubules

    • Secretion of hydrogen ions (H+) is important for regulating the pH of body fluids

Kidneys function in the homeostatic regulation of pH of body fluids

• Kidneys adjust excretion & absorption of hydrogens ions & bicarbonate as needed

Trace the pathway of urine from the nephron to body’s exterior

• The nephron is the functional unit within the kidney. The nephron has several segments, each w/more specialized functions

• Glomerular capscule → proximal convoluted tubule → Nephron loop (ascending/descending Loop of Henle) → Distal convoluted tubule → Collecting duct

• Sequence through tubules:

  • Proximal tubule, descending limb of nephron loop, ascending limb of nephron loop, distal tubule

What parts make up the nephron?

• Renal corpuscle

• Renal tubule

Flow of urine coming out of nephrons in the kidneys (Plural)

• Urine flows from the major calyces, into the renal pelvis, & then the ureters or another version for 1 kidney:

  • Calyx, renal pelvis, ureter, urinary bladder, urethra

Storage & Elimation of Urine

• Ureters

  • The ureter is a tubular organ that extends from each kidney to the urinary bladder

  • Its wall has mucous, muscular, & fibrous layers

  • Peristaltic waves in the ureter force urine to the urinary bladder

  • Obstruction in the ureter stimulates strong peristaltic waves & a reflex that decreases urine production

• Urinary Bladder

  • The urinary bladder is a distensible organ that stores urine & forces it into the urethra

  • The ureters & urethra open at the 3 angles of the trigone in the floor of the urinary bladder

• Urethra

  • The urethra conveys urine from the urinary bladder to the outside

  • In females, it is short & empties between the labia minora

  • In males, it is long & conveys products of reproductive organs as well as urine

How do the following hormones affect the urinary system: ADH, aldosterone, ANP?

• ADH

  • Water concentration in blood too high - ADH increases - results in increased reabsorption of water back into blood - urine volume decreases, darker in color as it is more concentrated

    • Water concentration in blood too high - ADH decreases - resuls in decreased ADH - decreases water reabsorption & urine volume increases, urine color pale, & very dilute concentration

  • If tubular reabsorption in the nephron didn’t occur then there would be excessive water loss

• Aldosterone

  • Aldosterone stimulates Na+ reabsorption (conserved) & K+ to be excreted

• Renin

  • Functions to increase blood pressure

How is active transport used in the renal tubule? Passive transport?

• Ex. Active Transport

  • Active transport occurs in the kidney w/glucose molecules, bc there is a higher concentration of glucose in the blood than in the nephrons so to get the glucose to go into the blood active transport is required

  • This reabsorption of glucose all takes place in the proximal tubule

• Ex. Passive Transport

  • Passive transport is osmosis, which is a passive transport bc it does'n’t require any energy to happen

    • An ex of this is the interstitial fluid (liquid found between the cells in tissue spaces), bc the intersititial fluid found outside of the kidney is more concentrated than the inside of the nephron tubules

    • So the water moves out of the kidney & into the blood by osmosis, a passive transport

Describe the histology of the glomerular capsule

• Glomerulus: Also known as renal corpuscle

  • Renal corpuscle made up of:

    • Glomerulus

    • Glomerular capsule

  • Afferent arterioles delivers blood to each glomerulus

  • Capillary cluster that branches from afferent arteriole is glomerulus

  • Filters blood in each nephron

    • Renal arterioles deliver blood to each of the million or so glomeruli in each kidney

    • Remember capillaries are single layer structures that allows for substances to move in & out of capillary

What is the function of the glomerulus?

• The glomerulus is a small bundle of capillaries (blood vessels) located at the beginning of each nephron in the kidney

  • These capillaries are important in the filtration of blood

    • The glomerulus allows smaller molecules, such as water & nutrients, to pass through its pores into the surrounding Bowman’s capsule

    • Larger molecules, such as waste products & red blood cells, are to large to pass through the pores & remain in the blood

    • The glomerulus transports water & small solutes from the body to the outside

    • Aside from waste, the filtrate contains important ions, glucose, amino acids, & smaller proteins that the body requires

  • Altering the diameter of arterioles leading into & draining from a glomerulus will alter the filtration pressure wthin that glomerulus

    • ** Vasoconstriction of the afferent arteriole &/or vasodilation of the efferent arteriole

      • Results in a decrease in glomerular filtration rate (GFR)

Renal Tubule: tubule w/multiple names for various sections of nephron each having different function:

• Leading away from the capsular space, filtered fluid enters the renal tubule on the proximal end of the nephron, which is appropriately called the proximal tubule, The renal tubule continues as the hairpin-shaped nephron loop (loop of henle). The proximal tubule dips toward the renal pelvis to become the descending limb of the nephron loop. The tubule then curves back toward its renal corpuscle & forms the ascending limb of the nephron loop. The ascending limb returns toward its renal corpuscle of origin, where it becomes the distal tubule

• *Putting together in short form:

  • Extends from glomerular capsule to collecting duct

  • *Filtrate proceeds from Glomerular Capsule → (renal tubules) Proximal tubule → Nephron loop (loop of henle, composed of a descending & an ascending limb) → Distal tubule

  • Distal tubules of several nephrons (remember there are approximately 1 million nephrons/kidney) empty into a collecting duct

    • Collecting duct continues through the medulla & drains through the renal papilla into a minor calyx

Detrusor Muscle

• The bladder serves to store urine until an appropriate time for urination. The wall of the bladder is made of the detrusor muscle, which is smooth muscle, & an overlying mucosal layer which is lined by uroepithelium

  • The detrusor muscle is essentially what most people refer to as “the bladder”

• The detrusor muscle relaxes to allow urine to fill the bladder & stretches to hold more urine

  • When it is time to urinate, the detrusor muscle contracts to push urine out of the bladder

• The detrusor muscle can malfunction in 2 main ways

  • It can be overactive (contract too much, when not needed) or underactive (doesn’t contract when it should)

    • If the detrusor muscle is overactive, it contracts to squeeze the urine out even when the person is not ready to urinate

    • This can cause a sense of urgency or severe desire to urinate.

      • When this urge occurs but the patient doesn’t loose urine, we call it overactive bladder/overactive detrusor muscle

      • If the squeeze of the detrusor muscle is strong enough, it can push urine out even if the patient is ready to urinate

        • When you lose urine bc of an overactive detrusor muscle, it is called urge incontinence

  • If the detrusor muscle doesn’t contract (squeeze) or is overstrecthed, it can be difficult to get urine out even when you are trying to urinate

    • Bc of that, you will retain urine, called urinary retention

External Urethral Sphincter

• The urethral sphincters allow regulation of urine flow

• Made up of skeletal muscle

Micturition-the act of urinating

• The micturition reflex can be suppressed by higher centers of the brain

  • So if you aren’t near a restroom & you have to urinate, the brain will send a signal down the spinal cord to the urethra to inhibit the micturition reflex

  • If you are ready to urinate, the brain sends the signal allowing you to voluntarily contract the external urethral sphincter & release urine into the toilet

  • Summary:

    • **The detrusor muscle contracts, the interal urethral sphincter relaxes, & urine is expelled through the urethra

    • The micturition reflex subsides, the detrusor muscle relaxes, the internal urethral sphincter contracts, & the bladder begins to fill w/urine again

Thirst

• Highest percentage of intake comes from beverages we drink

• Water intake must equal water loss to maintain water balance

• Primary regulator of water intake → Thirst

• Thirst Center - Located in brain

What substances should not be found in urine when testing?

• Glucose- all reabsorbed back into bloodstream unless diabetic- exceeds renal threshold of 170mg/dl & above that level the rest remains in urine

• Protein- kidney damage, infection

• WBC- infection, pyelonephritis

• RBC- infection, trauma to kidney, glomerulonephritis, tumor

• Bilirubin- hepatitis, liver disease, etc

What is normal constituents of urine?

• Main constituents of urine: urea & creatinine

  • Urea- a by product of amino acid catabolism in the liver

What terms are used to describe an increase or decrease in urine output?

• Polyuria- increased output of urine

• Oliguria- decrease output of urine

• Anuria- zero output of urine-bad → renal failure

Hormone produced in kidney?

• Erythropoietin

  • Helps to control the rate of red blood cell production

When plasma glucose concentration exceeds the renal plasma threshold → then glucose appears in urine

• Remember as blood enters into the glomerulus for filtration, all of the glucose is filtered out into the filtrate & must be put back into the bloodstream. This occurs through the sodium-glucose cotransporters. If blood glucose exceeds 180 mg/dl, the renal threshold to reabsorb glucose is exceeded & the excess glucose will remain in the urine

Fasting urine sample

• An abnormal finding in the urine would be glucose

  • FYI: If fasting, a normal person’s blood glucose would be between 70-100 mg/dl & glucose does not exceed the renal threshold & would all be reabsorbed back into the blood- no glucose in urine which is normal

  • FYI: If a person is a diabetic, & their fasting blood glucose exceeds about 180 mg/dl then you would expect to find glucose in the urine

Diurectics

• Increase volume of urine excreted

Aging

• Decrease in the release of renin

  • Impacts the control of blood pressure

Which substances get reabsorbed within the & secreted within the different parts of the Nephron?