The Urinary System: Structure and Function

The Urinary System

• This is by Maj FF Otu

Structure of The Urinary System

• Consists of adrenal glands, kidneys, ureters, bladder, and urethra
• The adrenal gland has a medulla and a cortex
• The kidney is the functional unit
• The nephron is partly located in the medulla and partly in the cortex
• Ureters are the exit for urine into the bladder
• The bladder is a temporary storage for urine before it exits the urethra
• The urinary system has upper and lower parts
• The upper part includes the kidneys and ureters
• The lower part includes the bladder and urethra
• Urinary tract infections (UTI) are common

Kidney

• Pelvis, medulla, and cortex are it's constituent parts
• The right sits lower by 1 cm due to pressure from the liver
• The left is bigger than the right
• Protected by the vertebrae, ribs, and muscles
• It is the size of a fist and weighs 135 to 150g
• Has about 1 million nephrons
• The nephrons are the functional unit
• Each nephron has renal corpuscles and tubules

The Kidney: Structure and Function

• About 1300 ml or 25% of blood from the aorta enters the kidney every minute
• Only about 1 ml of urine is produced in that minute
• Functions include:
• Removal of nitrogenous waste like urea, uric acid, and ammonia
• Maintaining water balance
• Maintaining electrolyte balance i.e. Sodium, Chlorine, Potassium, HCO3
• Regulation of acid-base
• Production of erythropoietin and Vitamin D
• Urine is liquid waste from blood after undergoing filtration
• Urine consists of nitrogenous waste, water, salts, and electrolytes

Composition of Normal Urine

• Constituent components and ammount:
• Water: 95%
• Urea: 9.3 to 23.3 gram/liter
• Chloride: 1.87 to 8.4 gram/liter
• Sodium: 1.17 to 4.39 gram/liter
• Potassium: 0.750 to 2.61 gram/liter
• Creatinine: 0.670 to 2.15 gram/liter
• Sulfur: 0.163 to 1.80 gram/liter
• Urea production happens with protein breakdown in the liver where amino acids are formed and ammonia is obtained as a biproduct
• Ammonia combines with water and carbon dioxide to form urea
• Uric acid is a byproduct of purine such as liver, kidney, red meat, alcohol, and seafood
• blood cells and proteins are not filtered
• Creatinine is a biproduct of protein
• Presence of blood cells or protein means kidney compromise especially with bowman's capsule and glomerulus
• Increased urea means low kidney function

Kidney Indicators and Function Tests

• Kidney function levels are indicated by:
• Creatinine in urine
• Urea in blood
• Nitrogen in blood
• Protein (albumen) in urine
• Indicators are assessed with:
• Urinalysis
• Blood Urea Nitrogen (BUN)
• Serum Creatinine
• GFR considers age, size and gender

Intake and Output Chart

• Intake is the amount of consumed fluids
• Output is the amount of expelled fluids
• Amount of intake should equal the amount of output
• Intake sources:
• Oral fluids: 1500 ml/day
• Water in food: 1000 ml/day
• Water of metabolic oxidation: 500 ml/day
• Output sources:
• Urine: 1700 ml/day
• Faeces: 1270 ml/day
• Insensible water loss from skin: 20 ml/day
• Lungs (water vapor lost during exhalation): 10 ml/day

Structure of the Nephron

• Bowman's capsule collects the filtrate
• Consists of:
• Afferent and efferent arterioles
• Distal and proximal convoluted tubules
• Glomerulus (Capillary Network)
• Urine Flow
• Loop of Henle
• Collecting Duct

Nephron Function

• The osmotic gradient is not washed out
• The part of the medulla that carries blood supply is sluggish
• Only 2% of medulla blood flow goes to the kidneys

The Nephron and Urine Formation

• Pathway: Afferent/efferent arteriole -> Glomerulus -> Bowman’s capsule -> Proximal Convoluted Tubule -> Descending loop of Henle -> Loop of Henle -> Ascending loop -> Distal convoluted tubule -> Collecting duct
• Corpuscles include afferent, efferent, glomerulus, & Bowman's capsule
• Tubules: PCT (LH, DCT & duct)
• Afferent arteriole receives raw blood in large quantities
• Glomerular capillaries filter
• Efferent arteriole releases filtered blood back into circulation
• Bowman's capsule collects filtrate for PCT
• PCT reabsorbs most (65%) of blood: glucose, amino acids, urea, Na+, Cl-, K+, S, PO4 & some drugs) and water; 2/3 of each is reabsorbed
• DCT confirms reabsorption in which reabsorption of solute happens but not water, ADH later released to increase porosity for water & solute reabsorption
• Collecting Ducts does as DCT. Secretion also happens
• Movement of substances happens by active or passive transport
• Water movement is passive

Function of the Nephron

• Descending LH is very permeable to water but not solute
• Ascending LH is very permeable to solutes, but not water
• Vasa recta (around the LH) with high osmolarity allows it to diffuse solute into LH. water returns back into LHS
• More NaCl is absorbed when reaching ascending LH
• Medullary osmolarity draws more water from descending LH and is secreted ascending the loop.
• This is called countercurrent multiplication
• Filtrate in ascending and descending LH moves in opposite directions, called Countercurrent
• Blood flowing down descending limbs of vasa recta exchanges solutes with the medullary space which helps increase osmolarity
• ADH released makes distal and collecting ducts permeable to urea and water
• The entire mechanism needs equilibrium and homeostasis

Urine Formation - Key Mechanisms

• Peritubular capillaries lie near the PCT and DCT but branch into the medulla as vasa recta and reabsorb essentials nutrients from renal rubules to the blood stream while supplying cortical and medullary cells with oxygen and nutrients
• Pressure Mounting happens in glomerulus with the coiling of glomerulus and broad afferent/narrow efferent
• Countercurrent multiplier (Loop of Henle) focus ascends and loops to secrete solute into the medullary space and causes more water to passively move into the descending loop
• Countercurrent exchanger (Vasa Recta) moves into the main blood stream while exchanging solute and water with systemic circulation and medullary space
• Concentrated urine happens with ADH making DCT and collecting ducts permeable to water and urea for reabsorption
• Dilute urine happens with less or no ADH as the DCT and collecting duct become impermeable and prevent water reabsorption

The Summary of Urine Formation in the Nephron

• Broad afferent and narrow efferent arterioles (creates pressure difference in glomerulus)
• Glomerulus (Coiling mount pressure, most porous capillary bed)
• PCT: does 65% or 2/3 or reabsorption
• DLH: Large amount of Water leaves the tube into the medullary space
• Loop: high medullary osmolarity. Tube has more solute (Na, Cl, K) than water)
• ALH: Permeable to solute only. Secretes more solute into medullary space. Water stays
• Counter current multiplier (LH)
• DCT is similar to ALH as it is impermeable to water, but allows remaining solute out of tube into cortex, flowing into medullary space
• Collecting tub also does same above as it is Impermeable to water and urea (opens only when ADH is released)
• ADH released (from the hypothalamus and stored in the Pituitary gland) to make DCT + CT permeable to reabsorb water and urea
• High medulla osmolality happens when Vasa recta takes advantage and tape more solute and water by diffusion and osmosis.
• Vasa recta maintains balance of solute/water with circulation/medullary space

Urine Formation

• Glomerulus filtration has size differences that creates efferent/afferent, coiling glomerulus, delay, ↑ porosity
• Proximal Convoluted Tubule majorly reabsorbs solute (65%)
• Descending Loop of Henle confirms reabsorption and reabsorbs main water
• Loop of Henle increases concentration (increased osmolarity)
• Ascending Loop of Henle is permeable to solute (Na, Cl, k) and releases into the medullary.
• Distal tube also permits the same
• It all maintains solute, medullary fluid

Urine Formation Transporters

• Aldosterone increases sodium by reabsorption in the cortex
• Movement of Solutes:
• Water through Passive transport or by osmosis
• Sodium through Active transport via Na/K pump
• Chlorine follows Sodium (Cl-)
• Potassium through Active transport Na/K pump (K+)
• Glucose through facilitated diffusion via GLUTs-glucose transporter or SGLUTs in GIT & Kidney
• Amino acid facilitate diffusion via potein carriers

Urethral Catheter and Urine Bag

• The catheter has urine and balloon drainage
• The tip is inserted into the urethra
• Balloon is filled with sterile water form balloon port

Lower Urinary Tract

• Indications for catheterization include:
• Retention associated with obstruction of lower urinary tract
• Neurological disorders
• The following will need decompressing:
• Bladder before abdominal surgery
• Ill or unconscious patients
• Irrigating the bladder and installing antimicrobial drugs
• Painful urination
• Incontinence
• Skin Problems
• Postoperative Drainage

Ureters, Bladder and Urethra

• Ureters: hollow tube that extends from renal pelvis passing through bladder
• Functions of the bladder: Elastic and expandable
• Renal stones: causes haematuria, vomiting failure
• Urethra: hollow duct outside

Abnormal Micturition

• Postmicturition dribbling happens after emptied bladder
• Incontinence: Passage of urine at unsuitable sites
• Infections and disorders include:
• Cystitis: Bacteria in the urethra or associated bacteria
• Covert bacteriuria: Bacteria Present Without symptoms
• Pyelonephritis localized and causes scarring
• Acute Kidney failure: Caused by infection, stonage, structural damage
• Nephrotic : glomerular compromise
• Glomerulus : Compromised, and leads to proteinuria and fluid retention