Urinary system

Urinary system

Objectives

At the end of this lecture, student will be able to

• Describe the functions of urinary system

• Describe the external and internal gross anatomical features of the kidneys

• Explain the structure of renal corpuscles and renal tubules

• Explain the basic functions performed by nephrons and collecting ducts

• Explain the role of renin angiotensin and aldosterone system in renal physiology

• Describe the mechanisms involved in the maintenance of acid-base balance

• Compare the roles of buffers, exhalation of carbon dioxide, and kidney excretion of H+ in maintaining pH of body fluids

• Define acid base imbalance

• Define renal clearance

• Describe various clearance tests for measuring kidney function

• List the forces that contribute for the flow of urine through the urinary system

• Describe the anatomy and histology of urinary bladder

• Define “micturition”

• Describe micturition reflux

• Describe the disorders associated with urinary system

Content

• Anatomical features of the kidneys

• Structure of renal corpuscles and renal tubules

• Functions of Nephrons and collecting ducts

• Renin Angiotensin Aldosterone System

• Maintenance of acid-base balance

• Acid-base imbalance

• Renal clearance

• Clearance tests

• Anatomy and histology of urinary system

• Micturition

• Disorders of urinary system

Urinary system

• Contributes to homeostasis by altering blood composition, pH, volume, pressure & osmolarity

• Maintenance of blood osmolarity, excreting wastes and foreign substances, secreting hormones

• Nephrology – Scientific study of the anatomy, physiology, and pathology of the kidneys

• Urology – The branch of medicine that deals with the male and female urinary systems

• The urinary system consists of two kidneys, two ureters, one urinary bladder, and one urethra

Functions of Urinary system

• Kidney regulate blood volume, composition & BP

• Synthesize glucose

• Synthesis of hormones: Erythropoietin & calcitriol

• Excrete wastes by forming urine

• Ureters transport urine from the kidneys to the urinary bladder

• Urinary bladder stores urine

• Urethra discharges urine

Anatomy of Kidney

• Reddish, kidney-bean-shaped organs

• Retroperitoneal (posterior to peritoneum of the abdominal cavity)

• Located just above the waist between T12 & L3

• Partially protected by the eleventh and twelfth pairs of ribs

• Right kidney is slightly lower than the left

External anatomy of Kidney

• 10–12 cm long, 5–7 cm wide, and 3 cm, mass 135–150 g

• Renal hilum – indentation in center of the concave border

• Three layers of tissue surround each kidney

– Renal capsule (deep layer)

  Adipose capsule (middle layer)

– Renal fascia (superficial layer)

Renal capsule

• Transparent sheet of dense irregular connective tissue

• Serves as a barrier against trauma & helps maintain the shape

Adipose capsule

• Mass of fatty tissue surrounding the renal capsule

• Protects the kidney from trauma and holds it firmly in place

Renal fascia

• Thin layer of dense irregular connective tissue

• Anchors the kidney to surrounding structure & to abdominal wall

Internal anatomy of Kidney

• Two distinct regions:

Renal cortex a superficial, light red area

Renal medulla deep, darker reddish-brown inner region

• Renal medulla consists of several cone-shaped renal pyramids

• Nephrons are the functional units of the kidneys

• 85% - Cortical Nephrons

• 15% - Juxta medullary nephrons

Frontal section of Kidney

Nephron

• Each nephron consists of two parts:

– Renal corpuscle filters blood plasma

– Renal tubule filtered fluid passes

Components of a renal corpuscle

– Glomerulus (capillary network)

– Glomerular (bowman’s) capsule - double-walled epithelial cup that surrounds the glomerular capillaries

• Components of a renal tubule

– Proximal convoluted tubule

– Loop of Henle (nephron loop)

– Distal convoluted tubule

Histology of Renal Corpuscle

Renal Physiology

• 3 Basic processes

– Glomerular filtration

– Tubular reabsorption

– Tubular secretion

Glomerular filtration

• The daily volume of glomerular filtrate - 150 L in females & 180 L in males

• 99% of the glomerular filtrate returns to the bloodstream via tubular reabsorption

• Only 1–2 liters - excreted as urine

Filtration fraction - Fraction of blood plasma in the afferent arterioles of the kidneys

Glomerular filtrate - fluid that enters the capsular space

Filtration Membrane

Net Filtration Pressure

• Glomerular filtration depends on three main pressures

• One pressure promotes filtration and two pressures oppose

Glomerular blood hydrostatic pressure (GBHP)

– Blood pressure in glomerular capillaries 55 mmHg

– Promotes filtration by forcing water and solutes in blood plasma

The Opposing Pressure

Capsular hydrostatic pressure (CHP) 15 mmHg

• Pressure exerted against the filtration membrane by fluid already in the capsular space and renal tubule

• Represents a “back pressure”

Blood colloid osmotic pressure (BCOP) 30 mmHg

• Due to the presence of proteins in blood plasma:

– Albumin

– Globulins

– Fibrinogen

Net filtration pressure (NFP)

• Total pressure that promotes filtration

• Determined as follows:

• Net filtration pressure (NFP) = GBHP - CHP - BCOP

                                                       = 55 mmHg - 15 mmHg - 30 mmHg

                                                       = 10 mmHg

Glomerular Filtration Rate

• The amount of filtrate formed in all the renal corpuscles of both kidneys each minute

• In adults, the GFR averages 125 mL/min in males

• 105 mL/min in females

• Regulation of GFR

– Renal autoregulation

– Neural regulation

– Hormonal regulation

Tubular reabsorption and tubular secretion

Tubular reabsorption:

• Selective process that reclaims materials from tubular fluid

• Returns them to the bloodstream

• Reabsorbed substances - water, glucose, amino acids, urea, and ions, such as sodium, chloride, potassium, bicarbonate, and phosphate

Tubular secretion:

• Removed from the blood and discharged into the urine

• Ions (K, H, and NH4), urea, creatinine & certain drugs

Hormonal Regulation of Tubular Reabsorption and Tubular secretion

Renin–Angiotensin–Aldosterone System

• Low blood volume and blood pressure

• Juxta glomerular cells secrete the enzyme renin into the blood

• Sympathetic stimulation releases renin

• Angiotensin converting enzyme (ACE) converts angiotensin I to angiotensin II (active form)

Effect of Angiotensin II on renal physiology

• Decreases the glomerular filtration rate

• Enhances reabsorption of Na+, Cl-, and water in the PCT by stimulating the activity of Na+/H+ antiporters

• Stimulates the adrenal cortex to release aldosterone

• Reabsorb more Na+ and Cl- and secrete more K+ by stimulating principal cells

• Reabsorbtion of more Na+ and Cl- ; excreting less water; increases blood volume

Acid-base balance

• Several mechanisms help maintain the pH of systemic arterial blood between 7.35 and 7.45

• Homeostasis of H+ concentration within a narrow range is essential to survival

Removal of H+ from body fluids and its subsequent elimination depends on

I.     Buffer systems

II.   Exhalation of carbon dioxide

III. Kidney excretion of H+

I. Buffer systems

• Buffers act quickly to temporarily bind H+, remove the highly reactive, excess H+ from solution

• Buffers raise pH of body fluids but do not remove H+ from the body

Principal buffer systems are

a.   Protein Buffer System

b.   Carbonic Acid–Bicarbonate Buffer System

c.   Phosphate Buffer System

a. Protein Buffer System

• Buffer in intracellular fluid and blood plasma

• Protein hemoglobin, buffer within red blood cells

• Albumin is the main protein buffer in blood plasma

• Proteins are composed of amino acids

• Contain at least one carboxyl group (COOH) and at least one amino group (NH2)

• When pH is high - Free carboxyl group at one end of a protein acts like an acid and releases H+

• When pH is low - Free amino group at the other end of a protein can act as a base by combining with H+

Hemoglobin as buffer in red blood cells

• Carbon dioxide (CO2) passes from tissue cells into red blood cells

• Combines with water (H2O) to form carbonic acid (H2CO3)



• Oxyhemoglobin (Hb   O2) is giving up its oxygen to tissue cells

• Reduced Hb (deoxyhemoglobin) picks up most of the H+

b. Carbonic Acid–Bicarbonate Buffer System

• The bicarbonate ion (HCO3), act as a weak base

• Carbonic acid (H2CO3), act as a weak acid

When excess of H+ - HCO3 - can function as a weak base and remove the excess H+

When shortage of H+ - H2CO3 can function as a weak acid and provide H+

c. Phosphate Buffer System

Components - Ions Dihydrogen phosphate (H2PO4-) and Monohydrogen phosphate (HPO42-) when pH is high

• Dihydrogen phosphate ion acts as a weak acid and buffer strong bases such as OH-

When pH is low

• Monohydrogen phosphate ion buffers the H+ released by a strong acid & acts as a weak base

II. Exhalation of carbon dioxide

• Increase in the carbon dioxide (CO2) concentration in body fluids increases H+ concentration and thus lowers the pH

• H2CO3 can be eliminated by exhaling CO2, it is called a volatile acid

• A decrease in the CO2 concentration of body fluids raises the pH

III. Kidney excretion of H+

• In PCT, Na+/H+ antiporters secrete H+ as they reabsorb Na+

• In PCT, Na+/H+ antiporters secrete H+ as they reabsorb

• In collecting ducts, intercalated cells absorb K+   & HCO3; secrete H+

Acid–Base Imbalances

Normal pH range of systemic arterial blood is between 7.35 – 7.45

Acidosis (or acidemia)

• Condition where blood pH is below 7.35

• Results in depression of the CNS

Alkalosis

• Conditions where blood pH is above 7.45

• Results in over excitability of the CNS

Clearance tests

• Measures GFR; useful in assessing renal function

• Volume of plasma that would be completely cleared of a substance per min

• Clearance (C), expressed as ml/min, can be calculated by using the formula

C = U х V / P

Where, U = Concentration of substance in urine

V = Volume of urine in ml excreted per min

P = Concentration of substance in plasma

• Measurement of clearance of substance already present in the blood is preferred

• The compounds measured are: Creatinine and urea

Creatinine clearance test

• Creatinine is the excretory product derived from creatinine phosphate

• Its excretion is constant and not influenced by body metabolism and dietary factor

• It is the volume of plasma that would be completely cleared of creatinine per min

C =U х V / P

Where, U = Concentration of creatinine in urine

V = Volume of urine in ml excreted per min

P = Concentration of creatinine in plasma

Importance

• Values are close to GFR, hence measurement is sensitive to assess the renal glomerular filtration

• Decrease in clearance value serves as an indicator of decreased GFR due to renal damage

Urea clearance test

• Urea is the end product of protein metabolism

• It is the volume (ml) of plasma that would be completely cleared of urea per min

Cm = U х V / P

Where, Cm = Maximum urea clearence (only when urine output is > 2 ml/ min

U = Concentration of urea in urine, V = Volume of urine in ml excreted per min

P = Concentration of urea in plasma

• When urine output is less than 2 ml/min, it is called standard urea clearence

Cs = U х (V) 1/2

P

• Urea clearence is less than GFR

• After being filtered by glomeruli, it is partially reabsorbed by the renal tubule

• Influenced by protein content of diet

Importance

• Value below 75%, indicator of renal damage

• If clearance is below 50%, blood urea levels increases

Urine transportation, storage and elimination

• Urine from collecting duct transported through minor calyx, major calyx, renal pelvis and later drained into urinary bladder through ureters

• Urine is then discharged from the body through the single urethra

Ureters

• Two in number, Retroperitoneal

• 25-30 cm in length; Diameter varies from 1mm to 10mm

• Curved medially & pass obliquely through posterior aspect of urinary bladder

• Carry urine from renal pelvis to urinary bladder

• Peristaltic contraction, hydrostatic pressure & gravity are the contributing force for urine flow

Layers of ureter

3 layers of tissue that form the wall of ureter

– Mucosa; Musclaris; Aventitia

Mucosa

• Mucous membrane with transitional epithelium

• Able to stretch; accommodate variable amount of urine

• Goblet cells produce mucus, prevents cells from coming in contact with urine

Musclaris

• Intermediate coat

• Has inner longitudinal; outer circular smooth muscle fibres

• In the distal third, ureter is made of inner longitudinal, middle circular, outer longitudinal

• Function – Peristalsis

Adventitia

• Superficial coat

• Aerolar connective tissue containing blood vessels, lymphatic vessels & nerves

• Blends in with the surrounding tissue

• Anchors ureter in place

Urinary Bladder

• Hollow, muscular organ

• Situated in the pelvic cavity, posterior to pubic symphysis

• Peritoneum holds the urinary bladder in place

• Usually pear shaped

• Capacity averages 700 - 800 ml

Anatomy and histology of urinary bladder

• In the floor of urinary bladder is a small triangular area, trigone

• Two posterior corners of trigone contain two urethral opening & one external urethral opening, the internal urethral orifice

Layers of urinary bladder

Walls of urinary bladder, made up of 3 coats

Mucosa

• Mucus membrane with transitional epithelium

• Rugae, permits expansion of urinary bladder

Muscularis/ Detrusor muscle

• Has 3 layers of smooth muscle fibres - Inner longitudinal; Middle circular; Outer longitudinal

• External urethral sphincter is made of skeletal muscle

Adventitia

• Layer of aerolar connective tissue

• Continuous with that of uterus

• Over the superior surface of urinary bladder is serosa, a layer of visceral peritoneum

Micturition Reflex

• Discharge of urine from urinary bladder is micturition or urination or Voiding

Volume of urine in urinary bladder exceeds 200- 400 ml

â

Pressure in bladder increases

â

Stretch receptors transmit nerve impulses to spinal cord

â

Impulses reaches micturition centre in S2, S3

â

Trigger spinal reflex, Micturition reflex

• Micturition reflex discharge urine from urinary bladder via parasympathetic impulses

• Contraction of detrusor muscle occur

• Relaxation of internal urethral sphincter muscle

Urethra

• Small tube leading from internal urethral orifice of urinary bladder to exterior of the body

• Helps to discharge urine from the body

• Anatomy and histology of urethra differ in females and males

In female,

• Urethra lie posterior to pubic symphysis

• External urethral orifice located between clitoris and vagina

• Wall consists of deep mucosa and muscularis

In males,

• Urethra extends from internal urethral orifice to the exterior

• First passes through prostate, then through perineum & finally through penis

• Wall of male urethra consists of deep mucosa and superfacial muscularis subdivide into

a.   Prostatic urethra – passes through prostate

b.   Membranous urethra – passes through perineum

c.   Spongy urethra – longest portion, passes through penis

Diseases of the urinary system

Renal calculi

• Crystals of salts present in urine occasionally precipitate and solidify into insoluble stones

• Contains crystals of calcium oxalate, uric acid & calcium phosphate

Causes - Ingestion of excessive calcium

- Low water intake

- Abnormally alkaline or acidic urine

- Over activity of parathyroid glands

Urinary tract infection

• Describes either an infection of a part of urinary system or presence of large number of microbes in urine

• More common in female due to shorter length of urethra

• Includes urethritis, cystitis, pyelonephritis

Symptoms – Painful or burning urination

Frequent urination

Low back pain

Bed wetting

Glomerular diseases

Glomerulo nephritis

• Inflammation of kidney involving glomeruli

• Due to allergic reaction to the toxins produced by streptococcal bacteria

• Symptoms – Haematuria, proteinuria

Nephrotic syndromes

• Condition characterised by proteinuria, hyperlipidemia

• Causes- DM, SLE, Cancer & AIDS

• Symptoms- Edema around eyes, ankle, feet and abdomen

Renal failure

Decrease or cessation of glomerular filteration

Acute renal failure

 • Kidneys abruptly stops working entirely

• Causes – Low blood volume, decreased cardiac output, damaged renal tubules, kidney stones, NSAIDS

• Symptoms – Oliguria, Anuria, Anaemia

Chronic renal failure

• Progressive and usually irreversible decline in GFR

• Causes – chronic glomerulonephritis, pyelonephritis, Polycystic kidney disorders, Traumatic loss of kidney tissues

Polycystic kidney disease (PKD)

• Inherited disorder, kidneys get riddled with 100 – 1000 of cysts (fluid filled cavities)

• Progressive impairment of renal function leading to end stage renal failure

Summary

• The organs of the urinary system are the kidneys, ureters, urinary bladder, and urethra

• Kidneys filter blood and return most water and many solutes to the bloodstream, the remaining water and solutes constitute urine

• Kidneys are retroperitoneal organs; three layers of kidney – renal capsule, adipose capsule, and renal fascia

• Nephron is the functional unit of the kidneys

• Nephron consists of a renal corpuscle and a renal tubule

• Nephrons perform three basic tasks: glomerular filtration, tubular secretion, and tubular reabsorption

• Renal autoregulation, neural regulation, and hormonal regulation determines the renal function

• Renin angiotensin and aldosterone system helps in maintaining the blood volume by releasing renin, activating angiotensin II, and releasing aldosterone

• Various mechanisms help maintain the pH of systemic arterial blood between 7.35 and 7.45

• Buffers act to temporarily bind H+, removing the highly reactive, excess H+ from solution

• Increase in concentration of CO2 lowers the pH and will be reduced by exhalation of CO2

• Kidney excretes excess of H+ ions to maintain the pH of blood

• Acid- base imbalance causes acidosis and alkalosis

• Various clearance tests helps in measuring the GFR and are useful in assessing renal function

• Ureters are retroperitoneal and consist of a mucosa, muscularis, and adventitia

• They transport urine from the renal pelvis to the urinary bladder, primarily via peristalsis

• Urinary bladder is located in the pelvic cavity; its function is to store urine before micturition

• Urethra is a tube leading from the floor of the urinary bladder to the exterior

• Discharge of urine from urinary bladder is micturition

• Micturition reflex discharges urine from the urinary bladder via parasympathetic impulses

• Diseases associated with urinary system include renal calculi, urinary tract infection, glomerular disease, renal failure and polycystic kidney disease

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