A&P 2
URINARY SYSTEM

Northland College

Instructor Terry Wiseth

Kidney Function

Daily the kidneys filter about 200 quarts of blood

Produces 2 quarts of waste product and extra water

Major Functions of Urinary System

1) Clears blood of waste products of metabolism

Uremia

accumulation of toxic levels of wastes in blood

2) Maintain normal:

H2O and electrolyte balance

fluid volumes

blood pressure

body pH

Detailed Functions of Urinary System

1) Removal of organic wastes

2) Regulate ion concentrations

3) Regulation of the acid-base balance

4) Regulation of RBC production

5) Regulation of blood pressure

6) Control glucose and amino acid concentration

7) Elimination of toxic substances

Functions of Urinary System

1) Removal of organic wastes

Urea

Uric acid

Creatinine

Breakdown products of hemoglobin and hormones

2) Regulation of the concentrations of important ions

Excretion of:

Sodium (Na+)

Potassium (K+)

Calcium (Ca++)

Magnesium (Mg++)

Sulfate (SO4-)

Phosphate (PO4-)

3) Regulation of the acid-base balance of the body

control the elimination of:

Hydrogen (H+)

Bicarbonate (HCO3-)

Ammonium (NH4+)

4) Regulation of RBC production

Release erythropoietin

regulates the production of RBC in the bone marrow

5) Regulation of blood pressure

Regulate the fluid volume of the body and thus blood pressure

Produces the enzyme Renin

regulates blood pressure and water retention

6) Limited control of blood glucose and blood amino acid concentration

Excrete excess amounts of:

Glucose

Amino acids

7) Elimination of toxic substances

Pollutants

Food additives

Drugs

Other chemicals foreign to the body

Kidney Failure

Blood constituents cannot be held in normal concentrations

Urinary Anatomy

Kidney

Ureter

Bladder

Urethra

Kidneys

Excretory Organs

Intestine

Skin

Lungs

Kidney

Excretes

N2 wastes

Toxins

H20

Electrolytes

Kidney Location

Paired bean shaped organs below the diaphragm and liver

About the size of a fist

Microscopic Structure

Nephron

unit of function

1.25 million / kidney

Highly vascular

20% of blood pumped / min

Nephron

Organ of function in the kidney

Glomerulus

Tubule

Nephron Anatomy

1) Glomerulus

Arterioles

Bowman’s Capsule

2) Tubule

Proximal

Loop of Henle

Ascending

Descending

Distal

Collecting

Urine Formation

Actions in forming urine

1) Filtration

2) Reabsorption

3) Secretion

Glomerulus

Glomerular Filtration

blood flows through glomerular capillaries

H2O and solutes filter out into Bowman’s Capsule

Glomerular Filtration

Blood flows through glomerular capillaries

H2O and solutes filter out into Bowman’s Capsule

Renal Blood Vessels

Arterial blood enters the kidney through the Renal artery

Renal artery branches into numerous Afferent arterioles which supply each glomerulus

Afferent arterioles feed capillary networks, Glomerular tuft in the glomerular capsule

Blood remaining leaves the capillary network by way of the  Efferent arteriole

Peritubular capillaries surround the renal tubules where reabsorption and secretion occurs

Venous blood leaves the kidney by way of the renal vein

Glomerular Filtration

Pressure gradient causes filtration

some kidney diseases the permeability of glomerulus increases

allows blood proteins to filter out into the capsule

Glomerulus

The glomerular capsule is made up of two layers of cells

The outer layer is made up of regular epithelial cells

The inner layer is made up of cells called Podocytes

Podocytes

Podocytes are tightly attached to the glomerular capillaries and highly branched with extensions called Pedicels

Glomerulus

Gaps in contacts between cells are called Slit pores

These pores prevent medium size protein molecules from leaving the capillary

Endothelial cells that line the glomerular capillaries have large pores called Fenestrae

Do not allow passage of RBC, WBC and platelets

Glomerular capillaries are 400 times more permeable to plasma water and dissolved solutes than other capillaries

Filtrate passes:

through the capillary fenestrae

through the slit pores

What passes through as filtrate?

Electrolytes, nutrients, nitrogen wastes, small hormones, water

What does not pass through as filtrate?

Cells, large proteins

Glomerular Filtration

High blood pressure in the glomerulus forces small molecules from blood into the Bowman’s capsule

Glomerular Filtration

TEM of filtration slits from capillaries in Bowman’s Capsule

Glomerular Filtration

Stress can lead to constriction of afferent and efferent arterioles

Causes filtration rate to lower and renal suppression “kidney shutdown”

The speed of filtration is called the Glomerular Filtration Rate (GFR)

Kidneys able to filter:

125 mL per minute

Forms 1 mL of urine

180 L per day (45 gallons)

5 L blood filtered 56 times per day

Glomerular Filtration Rate

Directly related to systemic blood pressure

↓ BP = ↓ glomerular filtration

↑ BP = ↑ glomerular filtration (slight)

Glomerular Filtration

During exercise blood pressure rises

Incoming blood pressure is regulated by vasoconstriction of the afferent arterioles

Accomplished by increased sympathetic nerve activity

Action of exercise induced vasoconstriction

Preserves blood volume

Diverts blood to muscles and heart

The same sympathetic response and vasoconstriction occurs during cardiovascular shock

Vasoconstriction decreases the blood pressure entering the glomerular capillaries enough to compensate the increased BP from exercising

This limits the formation of overwhelming amounts of urine during exercise

Convoluted Tubule

Fluid from glomerulus is called filtrate

Filtrate flows through a long convoluted tubule

Proximal Convoluted Tubule

The first portion of the tubule is called the Proximal Convoluted Tubule (PCT)

Loop of Henle

The middle portion is called the Loop of Henle (LH)

Distal Convoluted Tubule

The terminal end is called the Distal Convoluted Tubule (DCT)

Collecting Duct

Numerous tubule empty into a common Collecting Duct (CD)

Tubular Reabsorption

Tubular reabsorption involves the movement of solutes from tubular fluid back to the blood

Water follows passively by osmosis

Movement of solutes and water from the tubule back to blood occurs by a number of different mechanisms:

1) simple diffusion (Na+, Cl-)

2) facilitated diffusion (glucose, urea)

3) active co-transport (glucose)

4) active transport pumps (Na+, K+)

5) osmosis (water)

Simple Diffusion

Diffusion is the movement of molecules from areas of higher concentration to areas of lower concentration

Movement of solutes occurs as long as there is a concentration difference and the molecule can pass by the membrane

Active Transport

Glucose, Ions, Proteins are able to be moved against their concentration gradients

These processes require input of large amounts of energy from the cell (ATP)

Glucose, amino acids, ions and other useful substances are actively transported from the tubule into blood

Protein pumps embedded in cell membranes are able to selectively transport materials in and out of the cell

Sodium – Potassium Pump

Osmosis

Osmosis is the diffusion of water from areas of higher concentration to areas of lower concentration

Consider a 6 % NaCl solution

6 % NaCl

94 % H20

Given a 6% solution on one side of a semipermeable membrane and a 7% solution on the other side

H20 will move from the 6% solution

(94% H20) to the 7% solution (93% H20)

Movement of water between the tubular fluid, interstitial fluid and the vascular system can be controlled by altering the concentrations of solutes in each of the compartments

The trick is to move solutes by active and passive transport mechanisms, changing the relative solute concentrations

Water then moves by osmosis with the changing solute concentrations

Tubular Reabsorption

Most tubular reabsorption occurs in the proximal tubule

Fine tuning and regulation of reabsorption occurs in the more distal regions

Tubular Reabsorption

The proximal tubules and Loop of Henle reabsorb about 95 % of the water completely unregulated

The distal and collecting tubule reabsorb less than 10 % of the water but it is here that water reabsorption is effectively regulated

Proximal Tubular Reabsorption

Na+ concentrations are equal in proximal tubule fluid and peritubular capillaries

Proximal tubule cells have numerous microvilli which border the lumen of the proximal tubule

Microvilli increase the total surface area for reabsorption

Na+ leaves proximal tubule by simple diffusion into proximal tubule cells

Glucose follows by a co-transport mechanism against their concentration gradient

Na+ is transported by sodium-potassium pump from proximal tubule cells into the peritubular capillaries

Proximal tubule cells maintain low Na+ concentrations due to sodium-potassium pump action

Glucose is transported by facilitated diffusion from the proximal tubule cell into the peritubular capillaries

Diffusion of Na+ into peritubular capillaries does not occur as fast as sodium-potassium pumps pump Na+ out of the proximal tubule cells

Thus Na+ accumulates in the surrounding interstitial fluid

This accumulation creates a potential difference across the wall of the tubule

The electrical gradient favors the passive movement of Cl-  ions into the interstitial fluid

The resulting accumulation of NaCl increases osmotic pressures in the interstitial fluid in comparison with the proximal tubule

The osmotic gradient causes H2O to move by osmosis from the proximal tubule to the interstitial fluid and into the proximal tubular cells

NaCl and H2O then move passively into the peritubular capillaries and returned to the blood

Tubular Reabsorption

99% of the filtrate is returned to the vascular system

A concentration gradient must be maintained between tubular fluid and blood that favors osmotic return of water to the vascular system

Only 65 % of the salt and water in the original glomerular filtrate is reabsorbed across the proximal tubule

Proximal tubule cells are freely permeable to water, so salt and water are removed in proportionate amounts

20 % of the salt and water in the original glomerular filtrate is reabsorbed in the descending loop of Henle

Neither the Proximal tubule or the Loop of Henle is subject to hormonal regulation

90 % of filtered salt and water is reabsorbed in a constant fashion in the initial sections of the tubule

Reabsorption is costly in terms of energy expenditures

Accounting for 6 % of the calories consumed by the body at rest

% remaining is reabsorbed by distal tubules under the influence of hormone action

Glucose Reabsorption

if blood glucose levels exceed threshold amount (150mg/100ml) not all glucose is reabsorbed

Renal Diabetes

sometimes maximum transfer capacity is reduced and excess glucose appears in urine even though blood glucose level is normal

Loop of Henle

In order for water to be reabsorbed by osmosis the surrounding interstitial fluid must be hypertonic

The osmotic pressure of the interstitial fluid in the renal medulla is raised to over four times that of plasma

Descending Loop

Water diffuses out of the tubule by osmosis

The descending loop does not actively transport NaCl but is permeable to water

Water is drawn out by osmosis and enters blood capillaries of the vasa recta

Reabsorption from Loop of Henle

Tubular fluid

NaCl concentration is thus increased and volume is decreased

Ascending Loop

Salts are actively transported out of the tubule, but water cannot follow because the walls of this portion of the tubule are impermeable to water

NaCl is actively transported from the Ascending Loop into the surrounding interstitial fluid

Na+ diffuses from the filtrate into the cells of the Ascending Loop

Accompanied by the active transport of K+ and Cl-

Ascending Loop

Na+ is then transported to the interstitial fluid by active transport

(Na+/K+ pumps)

Cl- follows the Na+ by electrical attraction

The effect of NaCl movement to the interstitial fluid is the same as in the Proximal Tubule

The Ascending Loop however, is not permeable to water

Tubular fluid becomes dilute as it approaches the Distal Tubule

Loop of Henle

Reabsorption from Loop of Henle

NaCl is trapped in interstitial fluid of kidney medulla

Reabsorption from Distal Tubules

Proximal tubules reabsorb 66% of Na+

Distal tubules reabsorb 10% of Na+

Distal tubules reabsorb H2O if antidiuretic hormone (ADH) is present (also called Vasopressin)

ADH (Antidiuretic Hormone) acts on the last part of the Distal Tubule to increase the permeability of the tubule

Permits water reabsorption to concentrate the urine

ADH also acts on the collecting duct to reabsorb water into the blood

Distal Tubule

Na+ and HCO3- ions are resorbed from the urine in exchange for K+ and H+ excretion

regulating the pH and ionic concentration of the blood

Functions which are dependent on the hormone aldosterone secreted by the adrenal gland

Distal Tubule

Aldosterone increases Na+ permeability and water reabsorption

Lack of aldosterone causes Na+ impermeability

Na+ remains in the filtrate and larger amounts of water are excreted as dilute urine

Tubular Reabsorption

Collecting Duct

As the urine passes down the duct, water moves by osmosis from the duct into the blood

The collecting tubules aid in concentrating the urine by water removal

Water transport is possible by the varying permeability of the collecting duct

Permeability of the collecting duct is under the influence of ADH

High ADH results in more reabsorption of water (more concentrated urine)

Low ADH results in the loss of water and diluted urine

As the urine passes down the duct, water moves by osmosis from the duct into the blood

Collecting Duct

Tubular Secretion

Filtration is main route by which substances enter the urine pathway

Secretion is a second way for substances to enter the urine

Secretion is the opposite of reabsorption

Substances move out of the capillaries into the interstitial fluid and then into the tubular filtrate

Some antibiotic drugs and plasma protein molecules are eliminated from the blood by secretion

Secretion of K+ and H+ occur across the distal tubule

Allows the kidney to finely regulate blood concentrations of K+ and pH

Kidney Physiology

Regulation of Urine Volume

1) ADH

2) Aldosterone

3) Extracellular fluid volume

4) Urine solute concentration

Regulation of Urine Volume

1) ADH

Produced by the hypothalamus

Released from the posterior pituitary

Secretion of ADH is stimulated by increasing solute concentrations in the blood

ADH increases causes a retention of water and thus decrease in solute concentration

Control of ADH

A) ADH Presence

More water is reabsorbed

Decrease in urine volume

B) ADH Absence

Less water is reabsorbed

Increase urine volume

Diabetes Insipidus

Disease associated with inadequate secretion or action of ADH

Causes collecting ducts to not be very permeable to water

Large amounts of dilute urine is produced

Dehydration that results causes intense thirst

2) Aldosterone

Increases Na+ reabsorption in distal tubule with H2O following

3) Extracellular Fluid Volume

Urine volume relates directly to extracellular fluid volume (ECF)

ECF ↓ urine volume ↓

ECF ↑ urine volume ↑

Rapid ingestion of large amount of fluid and resulting increased ECF leads to increased urinary output

4) Urine Solute Concentration

High solute concentration in urine increases urine volume by osmotic pressure

Untreated diabetes void large amounts of urine because excess glucose in blood “spilling over”

Influence of Kidney on Blood Pressure

Renal Hypertension

Decreased blood flow to kidney

Constriction of arterioles

Increased BP

Diuretics

Varying types of diuretics act on different areas of the nephron

Increases the volume of urine excreted

Effectively lowers blood volume

Treats:

Hypertension

Congestive heart failure

Edema

Examples of diuretics

Furosemide

Mannitol

Spironolactone

Complications may arise from the use of diuretics that cause an excessive loss of K+ in the urine

Hypokalemia (low blood K+) may result

Can lead to neuromuscular disorders and electrocardiographic abnormalities

Ureters

Tubes leading from kidney to bladder

Urine moves by peristaltic movement

Renal Calculi (kidney stones)

Stones develop in kidney, washed out by urine into ureter

Distend ureter walls

Pain

Kidney Stones

Renal calculi are composed of crystals and proteins that grow until they break loose and pass into the urine collection systems

Calcium oxalate, calcium phosphate, uric acid, cystine are normally present in urine in a supersaturated state from which they crystalize

Large stones may obstruct the flow of urine

Can be removed surgically or by a noninvasive procedure called shock wave lithotripsy

Bladder

collapsible, elastic bag

Ureters

2

Urethra

1

Bladder

Bladder epithelium is surrounded by thick muscular layer

The urinary bladder is lined by epithelial tissue which is constantly shed

Rapid proliferation of epithelial cells lining the bladder gives rise to a high incidence of cancer

Bladder Endoscopy

Endoscopy is often used to help in diagnosing bladder cancer

Functions of Bladder

1) Reservoir for urine

2) Expels urine

distended causes sensation and desire to void

Micturition Reflex

Involuntary and voluntary muscular sphincters surround the urethra

Micturition is controlled by reflex centers in the spinal cord

Stretch receptors in urinary bladder activate this center

Rhythmic contractions of bladder and relaxation of involuntary urethral sphincter muscles cause a  sense of urgency which is perceived by the brain

When urination is consciously allowed to occur the brain sends impulses to inhibit contraction of the voluntary urethral sphincter muscles and urine is expelled

Ability to voluntarily inhibit micturition develops at age 2 or 3

Corticospinal tract needs to grow and connect the motor cortex of the brain to the spinal motor neurons

Urethra

Passageway for eliminating urine

Urine

H2O

95%

N2 wastes

Electrolytes

Toxins

Pigments

Hormones

Urine

Blood in the urine is abnormal and requires a Doctor’s visit

RBC in the Urine

Oxalate Crystals in the Urine

Kidney Disease

Small declines in renal function do not cause a problem

Health is maintained with only 50% of renal function

Dialysis or transplant is required if renal function drops below 10%

Diabetic Nephropathy

High Blood Pressure

Polycystic Kidney Disease

Drug Usage

Acute Renal Failure

Chronic Renal Failure

Glomerulonephritis

Diabetic Nephropathy

If sugar stays in the blood instead of breaking down, it can act like a poison

High blood sugar levels can cause nephron damage

High Blood Pressure

Can cause damage to the small blood vessels in the kidneys

Damaged vessels cannot filter poisons from the blood

Renal Blood Vessel Cast

Polycystic Kidney Disease

A genetic disorder in which many cysts grow in the kidneys

Cysts slowly replace much of the mass of the kidney leading to kidney failure

Drug Usage

Some over the counter medicines can damage the nephrons

Aspirin, acetaminophen, ibuprofen

Recreational drugs also damage the nephrons

Alcohol, cocaine etc

Acute Renal Failure

Losing a large amount of blood can cause sudden kidney failure

Some drugs or poisons can cause the kidneys to stop working

May be reversed if kidneys are not seriously damaged

Chronic Renal Failure

Gradual loss of kidney function

Sometimes called “silent” kidney disease

Glomerulonephritis

Inflammation of the glomeruli

Suspected to be an autoimmune disease

Causative antibodies may be produced as a result of streptococcus infections (strep throat)

Destruction of glomeruli can lead to leakage of proteins into the urine

Can lead to edema

Lab Tests for Kidney Function

Laboratory tests on urine and blood samples can be performed to test for various kidney malfunctions

Creatine

Urea (Nitrogen)

Protein

Electrolytes (K+, Na+)

Cholesterol

RBC Count (Anemia)

Creatine

Waste product of the blood

Created by the normal breakdown of muscle during activity

Normally filtered from the blood to the urine

Urea

Waste product of protein metabolism is urea a nitrogen containing compound

Normally filtered from the blood to the urine

Dehydration or heart failure can also result in high blood urea

Protein

Normally protein is left in the blood and not filtered out to the urine

Proteinuria means protein in the urine

Foamy urine in the toilet can be indicative of high protein in the blood

Electrolytes

Na+

Insufficient removal of Na+ can raise blood pressure

K+

Failure to remove excess K+ can slow the heart rate

Cholesterol

High blood cholesterol levels can accelerate kidney failure

Research has shown that individuals with high cholesterol levels are more likely to have kidney problems

RBC Count

Anemia is a condition in which the blood has a low RBC count

Erythropoietin (EPO) is produced by the kidney which stimulates bone marrow to produce RBC

Diseased kidneys may not make enough EPO

Dialysis

Dialysis refers to the separation of molecules by diffusing through a semi-permeable membrane

Two types of Dialysis

1) Hemodialysis (Artificial Kidney)

2) Peritoneal dialysis (CAPD)

Hemodialysis

Blood is sent through a machine that filters away waste products

Clean blood is returned to the body

Performed three times per week for 3 to 4 hours

Peritoneal Dialysis

A dialysate fluid is inserted into the abdomen

The fluid captures waste products from the blood

After a few hours the fluid containing the wastes is drained

Continuous ambulatory peritoneal dialysis (CAPD) is performed 4 times a day

Can be performed without going to a Dialysis Center

Kidney Transplant

A donated kidney may come from a donor who has recently died or from a living person

The donor kidney has a higher percentage chance of being successful if the recipient and the donor are a “good match”

A “good match” will have a lower chance of being rejected by the recipient’s immune system

Immunosuppressant drugs are used to suppress the immune system to prevent rejection