The Urinary System

Objectives: Describe location of kidneys in body

Identify the following regions of a kidney: hilum, cortex, medulla, medullary pyramids, calyces, pelvis, and renal columns

Recognize a nephron and describe its anatomy

Describe process of urine formation, identify areas of the nephron that are responsible for filtration, reabsorption and secretion

Describe the function of the kidneys in excretion of nitrogen-containing wastes

Define polyuria, anuria, oliguria, and diuresis

Describe the composition of normal urine

List abnormal urinary components

Helpful Word Parts:

-Continence …… to hold

Neph- …… kidney

Pyel …… renal pelvis

Ren- …… kidney

-ur- …… urine

Location & Structure

Soap

Extend from T12 to L3

Small, dark red organs

Kidney-bean shape

Lie against dorsal body wall

Basic Function

Maintain the purity and constancy

Bear the major responsibility of excreting nitrogenous wastes, toxins and drugs from the body

Regulate the blood’s volume and chemical makeup

Produces enzymes renin and erythropoietin

Simplified Structure: Medial indentation - renal hilum

Ureters, renal blood vessels and nerves

Adrenal gland

A transparent fibrous capsule

The perirenal fat capsule surrounds each kidney

The renal fascia (outermost capsule) anchors the kidney

Homeostatic Imbalance

The fat surrounding kidneys is vital

If the fat diminish kidneys may drop to a lower position called ptosis

Ptosis becomes a problem if the ureters, -which do what again?- become kinked

Once kinked, the urine can no longer pass through, causing back up and pressure on the kidney tissue called hydronephrosis

More On Structure

Renal cortex (cortex – bark)

Renal medulla: has many triangular regions with a striped appearance

Renal or medullary pyramids: triangular regions with striped appearance

Separated by renal columns

Renal pelvis

Structure Continued…

Calyces: extensions of the pelvis,

Collect urine which constantly drains from the pyramids

Drain it into the renal pelvis.

Renal pelvis to the ureter

Bladder for temporary storage

Blood Supply

25% each minute

The arterial supply = renal artery

Divides into segmental arteries

Interlobar arteries

Interlobar arteries give off the arcuate arteries

The arcuate arteries then branch off into cortical radiate arteries

Venous blood draining from the kidney flowes through veins that trace the pathway, but run in an opposite direction: cortical radiate veins to arcuate veins to interlobar viens to the renal vein

Nephrons & Urine Formation

Over a million nephrons

Nephrons are the structural and functional units of kidneys

Two main structures: a glomerulus and a renal tubule

A glomerulus is a knot of capillaries

Has a closed end which is enlarged and cup-shaped to completely surround the glomerulus

Called the glomerular (glom = little ball)

The inner layer of the tubule is made of highly modified octopus-like cells called podocytes.

Podocytes

Long branching processes….foot processes

Intertwine and cling to the glomerulus

“Filtration slits” between extensions

Podocytes form a porous membrane around the glomerulus

As the tubule extends, it coils and twists before forming a hairpin loop

More on Podocytes…

Coils and twists again

Collecting tubule called the collecting duct

These regions of the tubule have specific names:

• Proximal convoluted tubule (PCT)

• Loop of Henle

• Distal convoluted tubule

- The lumen surfaces (exposed to filtrate) of the tubule cells in the proximal convoluted tubules are covered with dense microvilli, which increase surface area

Nephrons:

Cortical nephrons are located almost entire in the cortex

Few times nephrons are called juxtamedullary nephrons which are located close to the cortex-medulla junction

- Their loops of Henle dip deep into the medullary pyramids

- This is what gives the pyramids their striped appearance

- Deliver the final urine product into the calyces and renal pelvis

More On Nephrons

Associated with two capillary beds:

The glomerular. Fed and drained by arterioles.

The afferent arteriole is the feeder vessel

The efferent arteriole receives blood

The glomerular is specifically different from other capillary beds

How?

Both fed and drained by arterioles

Diameter of afferent arteriole > diameter of efferent arteriole

Blood pressure is higher in the glomerular capillaries

High pressure forces fluid and solutes out of the blood

99% of this filtrate is reclaimed by the renal tubule cells and returned to the blood

Capillary Beds

Peritubular capillaries arise from the efferent arteriole that drains the glomerulus

Capillaries are low-pressure, porous vessels that are adapted for absorption

Cling closely to the whole length of the renal tubule

Receive solutes and water from the tubule cells

These substances are reabsorbed from the filtrate

The peritubular capillaries drain into interlobular veins leaving the cortex

Urine Formation (1 of 3) Glomerular filtration

a) Nonselective, passive process - fluid passes from blood into glomerular capsule

b) The fluid is then filtrated

c) Both proteins & blood cells = to large

d) Systemic blood pressure is normal, filtrate will be formed

e) If arterial blood pressure drops too low, glomerular pressure will also

Urine Formation (2 of 3)Tubular Reabsorption

a) Begins as soon as filtrate enters the proximal convoluted tubule

b) Tubule cells act as transporters

c) Absorbed into the capillary blood by active transport processes

d) Some reabsorption is done passively (Water – osmosis)

e) Active transport involves membrane carriers which are very selective

f) Abundance of carriers for substances that will be reused

g)Needed substances will be entirely removed from the filtrate

h)Nitrogenous waste products are poorly reabsorbed

i) Urea which is formed by the liver and an end product of protein breakdown

j) Uric acid is released when nucleic acids are metabolized

k) Creatinine associated with creatinine metabolism in muscle tissue

l) These tend to be found in high concentrations in urine

m) Most reabsorption occurs in the proximal convoluted tubules

Urine Formation (3 of 3)

Tubular secretion

a) The opposite of tubular reabsorption

b) Some substances move from the blood of the peritubular capillaries through the tubule cells

c) This process is important to rid the body of substances not previously in the filtrate

Homeostatic Imbalance

Oliguria – abnormally low urinary output

Between 100 and 400 ml per day

Anuria – extremely low urinary output

Less than 100 ml per day

Usually indicate glomerular blood pressure is too low to cause filtration

Anuria a can also be a result from transfusion reactions and acute inflammation of the kidney

Characteristics of Urine: -Filtrate contains everything that blood plasma does

except proteins

Urine remains, containing nitrogenous wastes and unneeded substances

Urochrome is a pigment

More solutes in urine call for a deeper yellow color

Urine may be strange colors at times

Sterile

pH is slightly acidic

Specific gravity is the term used to compare how much heavier urine is than distilled water

Chronic renal failure is a condition in which the kidney loses its ability to concentrate urine

Kidney inflammation is pyelonephritis

Homework:

Read pages 517-527

“Did you get it?” Questions 1-7 … In class

Self Test 15.1

Citations:

Marieb, E. N. (2009). Essentials of human anatomy & physiology. San Francisco, CA: Pearson Education, Inc.