15: The Urinary System††


  1. Be able to identify the components of the urinary system and their function.
  2. Describe the structure of the nephron and the processes involved in during urine formation.
  3. What factors influence filtration pressure and the rate of filtrate formation?
  4. How do the kidneys respond to changing blood pressure?
  5. Describe the general symptoms and signs of disorders of the urinary system.
  6. What are some examples of conditions resulting from abnormal renal function.
  7. Urnation has involuntary and voluntary components


1.   Survival when you run out of gas in the desert!?

A.   Without adequate water, humans cannot perform critical reactions within their cells, transport important molecules, and rid the body of metabolic wastes.

B.    In hot and dry climates the body loses water at an alarming rate: fortunately, homeostatic mechanisms including the kidneys step in to do what they can.

C.    So if you run out of gas on a lone desert highway, you have a few hours, even if you forgot to bring drinking water, to hitch a ride to the nearest gas station.

2.     How the body maintains extracellular fluid. (What goes in must equal what goes out)

A.   The volume and chemical composition of extracellular fluid (interstitial plus blood) must be maintained within a set tolerable range.

B.    How extracellular fluid becomes thick or thin.

1)     Water is gained by two processes:

a)     Absorption of water from liquids and solid foods occurs in the gastrointestinal tract.

b)    Metabolism of nutrients during oxidative respiration yields water as a by-product.

2)     Water is lost by at least four processes:

a)     Every time you urinate.

b)    Evaporation occurs from respiratory surfaces and the skin.

c)     Sweating occurs on the skin surface

d)    Elimination of small amounts of water in feces is a normal occurrence.

3)     Most water loses are not preventable, you can only control the amount you urinate.

C.    Solute (things dissolved in water) Gains and Losses

1)     Solutes areadded to the internal environment by four processes:

a)     Nutrients, mineral ions, drugs, and food additives are absorbed by the gastrointestinal tract and cross the epithelial layer into the interstitial fluid or blood.

b)    Secretion form endocrine glands adds hormones.

c)     Respiration ads oxygen to the blood and metabolizing cells add carbon dioxide.

d)    Metabolic reactions contribute waste products.

2)     Extracellular fluid loses mineral ions and metabolic wastes in four ways:

a)     Respiratory exhalation rids the body of carbon dioxide, some water and heat.

b)    Various minerals ions (salt HCO‑3 and Urea) are lost in sweat.

c)     Urinary excretion rids the body of these wastes.

(1)   Uric acid is formed in reactions that degrade nucleic acids.

(2)   Ammonia is formed when amino groups are removed from amino acids

(3)   Urea is formed by reactions in the liver that unite two ammonia molecules with carbon dioxide.

(4)   Phosphoric acid and sulfuring acid are formed during protein breakdown.

d)    Food residues are eliminated by the digestive tract.

3)     The kidneys filter a variety of substances form the blood.

a)     Most of the filtrate (liquid removed from the blood by the kidneys) is returned to the blood: about 1% ends up as urine, a waste fluid of excess water and solutes.

b)    By controlling how much fluids and salts are removed from the blood the kidneys regulate the volume and solute concentrations of extracellular fluid.

3.     The Urinary System

A.   The Urinary system is composed of(16.3)

1)     Two kidneys that filter wastes, regulate blood pressure and pH, release erythropoietin and transforms vitamin D into its active form.

2)     Two ureters, transport urine from kidneys to the urinary blatttter

3)     One urinary blatter the stores uring

4)     one Urethra , that transports urine from urinary blatdder to outside the body

B.    Kidney structure and function

1)     Each kidney is a bean shaped organ about the size of a clenched fist.

2)     Each kidney is composed of two zones-an outer cortex and inner medulla-wrapped with a renal capsule.(It may be best to be able to draw a picture of this)

a)     neprhons filter and retain water and solutes, leaving a concentrated urine to pass through collecting ducts to the central renal pelvis

b)    Urine flows form each kidney through a ureter to a urinary bladder (for storage) and then out of the body through the urethra.

C.    More than a million neprhons are packed inside each kidney (these are the functional unit of the kidney, they do the work)(16.5)

1)     An afferent arteriole delivers blood to the renal corpuscle of each nephron.

a)     filtration occurs in the (renal corpuscle which consists of the)

(1)   glomerulus-a ball of capillaries nestled inside the Bowman's capsule.

(2)   Here some of the plasma is removed from blood.This fluid then moves into the nephron.

b)    the bowmanís capsule collects the filtrate and directs it through the continuous renal tubules: proximal convoluted tubual>>>loop of Henle>>> distal convoluted tubual>>> collecting duct>>>> which dtrains into the renal pelvic connected to the ureter

2)     Blood flow to the nepheron

a)     Blood enteres the renal corpuscle by the afferent arteriole

b)    The capillaries exit the glomerulus, converge into an efferent arteriole, and then branch again to from the pretubular capillaries around the nephron tubules where they participate in reclaiming water and essential solutes that are carried out of the kidney and back to the general circulation.Picture time!

4.     Urine formation

A.   Nephrons perform three functions Filtration, Reabsorption, and Secretion

1)     Urine is a fluid that rids the body of water and solutes that are in excess of the amounts needed to maintain the extracellular body fluid.

2)     Urine forms through a sequence of three processes:

a)     In filtration, blood pressure forces filtrate out of the glomerular capillaries into Bowman's capsule, then into the proximal tubule.

(1)   Blood cells, proteins and other large solutes cannot pass the capillary wall into the capsule

(2)   Water, glucose, sodium and urea are forced out, into the proximal tubule.

b)    Reabsorption takes place in the tubular parts of the nephron where water and solutes move across the tubular wall out the nephron (by diffusion or active transport) and into the surrounding capillaries.

c)     Secretion moves substances form the capillaries into the nephron walls.

(1)   Capillaries surrounding the neprhons secrete excess amounts of hydrogen ions and potassium ions into the nephron tubules.

(2)   Secretion also rids the body of drugs, uric acid, hemoglobin breakdown products, and other wastes, specifically targeted for expulsion from the body.

3)     Urination is a reflex response which empties the bladder

a)     The internal urethral sphincter (involuntary control) regulates flow from the bladder into the urethra.

b)    The external urethral sphincter (voluntary control) opens to void urine form the body

c)     Kidney stones are deposits of uric acid that collect in the renal pelvis or lodge in the ureter: they can be removed by surgery or lithotripsy

B.    Factors that influence blood filtration

1)     the kidneys can process about 1.5 quarts of blood each minute because of two factors

a)     Blood enters the glomerulus under high pressure in arterioles that have wider diameters than most arterioles

b)    Glomerular capillaries are highly permeable to water and small solutes

2)     The rate at which the kidneys filter a given volume of blood depends on the flow of blood through them and the rate of Reabsorption in the tubules is controlled by neural and hormonal signals.

5.     Reabsorption

A.   The role of Proximal tubule

1)     Mechanisms within the kidney carefully regulate the excretion and retention of substances based on intake and bodily need.

2)     Most of the water is reabsorbed in the proximal tubule.

a)     Sodium ions are pumped out of the tubule (filtrate) and into the interstitial fluid surrounding the peritubular capillaries

b)    Most but not all waste follows passively down the gradient that has been created.

B.    Urine concentration and dilution

1)     In the descending limb of the loop of Henle, water moves out by osmosis, but in the ascending portion sodium is pumped out.

2)     this interaction of the limbs of the loop produces a very high solute concentration in the deeper parts of the kidney medulla and delivers a rather dilute urine to the distal tubule.This is possible because water is lost during travel through the descending limb but salts are removed during travel through the ascending limb.

6.     Hormonal adjustments of Reabsorption

A.   How ADH influences Water Reabsorption

1)     Antidiuretic hormone (ADH) from the posterior pituitary is secreted in response to a decrease in extracellular fluid: ADH causes the distal tubules and collecting ducts to decrease in extracellular fluid: ADH causes he distal tubules and collecting ducts to become permeable to water, which moves back into the blood capillaries.

2)     When waste intake isexcessive, ADH secretion is inhibited: less water is reabsorbed and thus more is excreted

3)     Caffeine and alcohol are diuretics, a substance that promotes loss of water.

B.    How aldosterone influences Sodium Reabsorption

1)     When sodium levels fall so does the volume of extracellular fluid: this triggers the juxtaglomerular apparatus to secrete rennin, which calls forth angiotensins I and II which act on the adrenal cortex to release aldosteronewhich promotes sodium Reabsorption.

2)     sodium retention is accompanied by water retention which can lead to increased blood pressure- hypertension, which can affect kidney function

C.    Salt water balance and thirst

1)     When solute concentration in the extracellular fluid rises, the thirst center of the hypothalamus responds by decreasing saliva production.

2)     The dry sensation in the mouth causes a liquid seeking behavior.

7.     Acid Base Balance

A.   Kidneys also regulate the acidity and alkalinity of extracellular fluid.

1)     Overall acid base balance is maintained by controlling hydrogen ions through buffer systems, respiration, and excretion by the kidneys

2)     buffers can neutralize hydrogen ions, and the lungs can eliminate carbon dioxide.

B.    Only the urinary system can eliminate excess hydrogen ions, permanently and restore the bicarbonate buffering ions to the blood.

1)     the HCO-3 that forms in the nephron cells is moved to the capillaries where it neutralizes excess acid.

2)     the H+ that forms in the cells is secreted into the tubular fluid where it combines with bicarbonate ions to form carbon dioxide which is returned to the blood and excreted by the lungs) and water (which is excreted in the urine)

8.     Maintaining the bodyís Core Temperature

A.   many different physiological and behavioral responses help to maintain the bodies required internal core temperature (37íC)

1)     If body temperature rises above 41íC enzymes will be denatured

2)     As body core temperature drops below 35íC, enzyme activity decreases, shivering stops breathing may cease, and consciousness is lost: further decreases usually are fatal.

3)     As endotherms, humans have a body temperature regulated by metabolic activity and mechanisms that control heat conservation and dissipation.

B.    Responses to cold Stress

1)     Mammals respond to cold by constriction the smooth muscles in the blood vessels of the skin (peripheral vasoconstriction) which retards heat loss

2)     In the pilomotor response, the hairs become more erect to create a layer of still hair that reduces convection and radiative heat.

3)     rhythmic tremors (shivering) is a common response to cold but is not effective for verylong and comes with a high metabolic cost.

4)     Nonshivering heat can be produce d by a hormonal stimulation of the special brown adipose tissue.

C.    Responses to heat Stress

1)     Peripheral vasodilatation is the enlargement of the diameters of blood vessels to allow greater volume of blood to reach the skin and dissipate the heat

2)     Evaporative heat loss by sweating is common and obvious cooling mechanism

3)     hypothermia is a rise in core temperature, with devastating effects such as heat exhaustion and heat stroke

4)     during a fever, hypothalamus resets the bodyís thermostat to a new temporary core temperature

a)     At the onset to fever, heat loss decreases and heat production increases: the person feels chilled

b)    when the fever breaks, peripheral vasodilatation and sweating increases asthe body attempts to reduce the core temperature to normal

c)     the control increase in body temperature is mediated by interlukins, interferons, and prostaglandin) during fever seems to enhance the bodies immune response.

Possible assignments: things to think about


1.     follow a drop of urine through the urinary system.

2.     Who does he kidney work?

3.     How is excreting distinguished form elimination?

4.     What function of the kidney is accomplished by a dialysis machine?




Siuresis refers to a high urine flow rate, and a diuretic is any substance that increases the formation and excretion of urine.Caffeine is a mild diuretic because it inhibits sodium reabsorpiton, and as sodium is excreted it takes water with it.Alcohol is also a diuretic, although by a different mechanism.It inhibits ADH release.If you drink excessive amounts of alcohol, ADH will be suppressed and you may feel dehydrated and thirsty the next day.Diuretic drugs, such as furosemide (Olasix) are prescribed to reduce blood volume an blood pressure in certain patients, such as people with congestive heart failure.


Ed 102406 by Nick Kapp