Urinary System: Kidneys and Ureters

• The urinary system consists of the kidneys, ureters, urinary bladder, and urethra.

Kidneys

• Kidneys are paired organs in the lumbar region that are retroperitoneal on either side of the vertebral column.
• Kidneys are high on the posterior wall of the abdominal cavity.
• The kidneys are covered by the parietal peritoneum
• The kidneys are held in place by the renal fascia and covered by the renal capsule.
• The Hilum is an indentation where the renal artery enters.
• The Hilum is where the renal vein and ureter exit the kidneys.

Ureters

• The Ureters extend from the kidneys to the bladder.
• The layers of the ureter wall are the mucosa (mucous membrane) which is the inner layer.
• There is the smooth muscle, which is the middle layer.
• The Fibrous connective tissue is the outer layer.
• Ureters are 25cm long.
• The Ureters extend downward posterior to the parietal peritoneum
• The Ureters run parallel to the vertebral column.
• Peristaltic contractions of the ureters cause urine to enter the bladder even if a person is lying down.

Urinary Bladder

• The Urinary Bladder is a hollow, distensible, muscular organ located within the pelvic cavity and posterior to the symphysis pubis
• The Bladder is inferior to the parietal peritoneum.
• The Urinary Bladder stores urine.
• The Urinary Bladder has three openings, two for the ureters and one for the urethra.
• The Trigone is an area at the base of the bladder.
• The Trigone is outlined by the openings.
• The Detrusor muscle is the middle layer of circular fibers.
• The Detrusor muscle has two layers of longitudinal muscle.
• There is a Mucosa of transitional epithelium.
• Sphincters have an internal sphincter around the opening to the urethra and an external sphincter composed of skeletal muscle

Urethra

• The Urethra extends from the urinary bladder to an external opening and conveys urine from the urinary bladder to outside of the body
• In males, the Urethra averages 20cm when the penis is not erect
• The male Urethra is encircled by the prostate gland and carries urine during urination and sperm during ejaculation.
• In females, The Urethra, is about 4 cm long and prone to bacterial infection.

Urination (Micturition)

• Stretch receptors in the bladder are stimulated when the bladder is full
• Nerve impulses from the spinal cord cause the bladder to contract and the sphincters to relax.

Anatomy of the Kidney

• The three regions of the kidneys are the Renal cortex, Renal medulla, and Renal pelvis.
• Anatomy of a Nephron 腎元, the basic functional unit of the kidneys, is divided into blood supply and tubules
• Nephron blood supply includes the Afferent arteriole, Glomerulus, Efferent arteriole, and Peritubular capillary network.
• Nephron Tubules include the Glomerular capsule (Bowman's capsule), Proximal convoluted tubule (PCT), Loop of Henle, Distal convoluted tubule (DCT), and Collecting duct.

Excretion

• There are three steps of Excretion, Step 1: Glomerular Filtration, Step 2: Tubular Reabsorption and Step 3: Tubular Secretion

Step 1: Glomerular Filtration

• Whole blood enters the afferent arteriole and the glomerulus
• Glomerular blood pressure causes blood to be filtered
• Filtration extracts, Water, Nitrogenous wastes, Nutrients, and Salts (ions).
• Non-filtered components leave the glomerulus by the efferent arteriole
• This includes Formed elements (blood cells & platelets) and Plasma proteins

Step 2: Tubular Reabsorption

• Tubular Reabsorption involves both passive and active processes.
• It Occurs in the PCT
• Molecules and ions are reclaimed from the filtrate (i.e. from tubule back to blood in the peritubular capillary network)
• Molecules reclaimed include, Water, Nutrients (i.e. glucose and amino acids) and Required salts (ions)
• Every substance has a maximum rate of transport

Step 3: Tubular Secretion

• Substances are removed from the blood & actively transported to the distal convoluted tubule (DCT)
• Substances secreted into the DCT include Hydrogen ions, Potassium ions, Creatinine and drugs.

Measurement of Excretory Function of the Kidneys: Concepts

• Glomerular Filtration Rate (GFR) – measured filtration rate reflects the passive removal of some metabolic wastes from the kidney
• Renal plasma flow (RPF) is the supply of plasma (blood) and reflects the rate of renal metabolism and excretion
• In renal patient, the GFR and RPF usually diminishes to 50% of normal.

Renal Function Tests: Renal clearance of Inulin for Measuring GFR

1. Inulin is an exogenous polysaccharide excreted by filtration only.
2. Renal Clearance of inulin is the amount of plasma cleared of the inulin by kidney.
3. Golden standard of GFR measurement, for research purpose but need infusion for a stable plasma concentration.
4. GFR = (Rate of Urine Excretion of Inulin) / (Plasma Concentration of Inulin) ≈ 125 ml/min in man.

Renal Function Tests: GFR estimation with Creatinine

1. Creatinine (Cr) is an endogenous metabolites from muscle, excreted by filtration with some secretion
2. Renal Clearance of Cr is the amount of plasma cleared of the Cr by kidney
3. Measurement reflects GFR but slightly overestimates the actual value, for clinical purpose and not need infusion
4. GFR = (Rate of Urine Excretion of Cr) / (Plasma Concentration of Cr)

Renal Function Tests: GFR measurement using Para-AminoHippuric acid (PAH)

• Renal function test, using Para-AminoHippuric acid (PAH) as indicator is also used to estimate Renal Plasma Flow (RPF), which reflects Renal Blood Flow (RBF)
• The principle of this test is the same as GFR measurement
• RPF = (Rate of Urine Excretion of PAH) / (Plasma Concentration of PAH) =~ 600 ml/min
• RBF = RPF / (1-Haematocrit ) = 600/0.57 = ~1 l/min

Regulatory Functions of the Kidneys

• Excretion is the removal of metabolic wastes from the body.
• The Kidneys are the primary organ of excretion
• Homeostasis includes, Excretion of metabolic wastes, Preservation of water-electrolyte balance, Secretion of hormones, Maintenance of acid-base balance.

Excretion of metabolic wastes

• Urea is a by-product of amino acid metabolism.
• Ammonia combines with CO₂ to produce urea which is less toxic than ammonia
• Creatinine is a metabolic breakdown of creatine phosphate
• Uric acid is an insoluble metabolic breakdown of nucleotides
• Gout occurs if excess uric acid accumulate in the joints
• Urochrome occurs when Liver breaks down hemoglobin bilirubin is broken down into smaller compounds, including urochrome in the intestine (yellow colour in urine)

Re-absorption of electrolytes

• Blood volume is closely associated with the salt balance of the body
• Salts have the ability to cause osmosis
• The more salts there are in the blood, the greater the blood volume
• The kidneys also maintain other ions in the blood - Potassium - Bicarbonate - Calcium
• Blood pressure is maintained through the renin-angiotensin-aldosterone (RAA) mechanism

Regulation Through Hormones

• Release renin when blood supply decreases
• Erythropoietin is released when the oxygen-carrying capacity of the blood is reduced
• Erythropoietin stimulates red blood cell production
• the kidneys help activate vitamin D
• Precursor to calcitriol promotes calcium absorption

Maintenance of acid-base balance

• Kidneys excrete hydrogen ions from the blood into the urine
• Kidneys reabsorb bicarbonate ions and return them to the blood
• Urine usually has a pH of 6 or lower

Reabsorption of water

• Reabsorption of salt leads to the reabsorption of water until the osmolarity equals on both sides of a plasma membrane.
• The most reabsorbed water is in the PCT.
• Excretion of hypertonic urine is dependent on the reabsorption of water from, the loop of Henle and the collecting duct.
• Antidiuretic Hormone (ADH), also known as vasopressin, is produced by the hypothalamus when blood osmolarity increases.
• This causes water to be reabsorbed at the distal convoluted tubule and collecting duct, and decreases the amount of urine produced.

Sodium Reabsorption

• Greater than 99% of filtered sodium is reabsorbed back into the blood
• 67% is reabsorbed at the PCT and 25% is reabsorbed at the ascending limb of the loop of Henle
• The rest is reabsorbed from the DCT and the collecting duct

Reabsorption of electrolyte

• Sodium is necessary to the formation of a nerve action potential and muscle contraction
• Potassium is necessary of nerve and muscle contraction
• Abnormally low K+ concentration can lead to cardiac arrest
• Bicarbonate ion is important in maintaining pH
• Calcium and Phosphate ions are important to bone formation & cellular metabolism

Control of Filtration Rate

Four mechanisms:

1. Autoregulation, the glomerular filtration rate may increase when body fluids are in excess and decrease when the body must conserve fluid
2. Sympathetic Nervous System (Reflex of the Autonomic Motor Nervous System (ANS)), increased sympathetic impulses decrease GFR by causing afferent arterioles to constrict.
   • Drop in blood pressure excites baroreceptors in the large blood vessels. These baroreceptors alert sympathetic nervous system to cause vasoconstriction (via release of epinephrine and norepinephrine), which increases the peripheral resistance. -Conversely, vasodilation of afferent arterioles increases the GFR to counter increased blood volume or blood pressure
3. Renin is released from Juxtaglomerular apparatus when blood pressure is too low to promote glomerular filtration. - Renin converts angiotensinogen (plasma protein) to angiotensin I. Angiotensin I is converted to Angiotensin II (vasoconstrictor) by angiotensin-converting enzyme (ACE) in the lung and in plasma. - Angiotensin II constricts the efferent arteriole, causing blood to back up into the glomerulus, raising glomerular capillary hydrostatic pressure.
   • Angiotensin II stimulates the release of aldosterone (secreted by adrenal cortex)

• Aldosterone controls & promote reabsorption of sodium and water at the distal convoluted tubule and promotes the excretion of potassium

4. Atrial natriuretic hormone/Atrial Natriuretic Peptide (ANH/ANP) is secreted by the atria of the heart when blood volume is high.

• It inhibits the secretion of renin and aldosterone and increases sodium excretion by increasing the GFR.

Acid-base balance

• pH of body fluids stays at about 7.4
• The food we ingest & our metabolism add basic or acidic substances to the blood

The Kidneys

• The kidneys can rid the body of a wide range of acidic and basic substances
• The kidney's lower acting than the respiratory system and have powerful effect on pH
• The Kidney's reabsorb bicarbonate ions and excrete hydrogen ions as needed to maintain a normal pH
• Ammonia and phosphate ions are buffers for hydrogen ions in the urine
• Ammonia is produced in kidney tubule cells by the deamination of amino acids.
• Ammonia Diffuses easily through tubule cells into the urine and combines with hydrogen ions to form ammonium ions (NH4+), which cannot diffuse back out of the kidney tubules

Urine and urinalysis

• Water accounts for ~95% of urine volume
• The remaining 5% consists of electrolytes and various solutes
• Urine usually does not contain proteins and blood cells
• Urinalysis is an examination of the physical, chemical, and microscopic properties of the urine
• Composition of urine changes if disease has altered body metabolism or if kidney function is abnormal.

Problems with Kidney Function

• Infections
• Urethritis
• Cystitis
• Pyelonephritis
• Glomerular damage may allow large substances to be filtered
• Albumin, white blood cells, and possibly red blood cells, are detected by a urinalysis
• Extensive damage can cause uremia

Problems with Kidney: treatment (Dialysis)

• Hemodialysis utilizes either an artificial kidney machine or Continuous Ambulatory Peritoneal Dialysis (CAPD)
• Dialysate is the dialysis solution used to extract waste products, toxic chemicals, and drugs from patient's blood and to add needed substances to the patient's blood
• Three- to six-hour treatment twice a week and can remove from patient can remove between 50 - 250 grams of urea.
• CAPD involves the peritoneal lining of the abdominal cavity which is the dialysis membrane and permanently implant a plastic tube for connecting the bag of dialysate
• Replacing a Kidney (Renal Transplant) happens when a functioning kidney from a donor is received by a patient with renal failure, but presents the possibility of organ rejection
• The one-year survival rate is 97% if the kidney is from a relative and 90% if it is from a nonrelative and the Patient must take anti-rejection medication for the rest of his or her life

Effects of Ageing

• Total renal function may decrease by as much as 50%
• Kidneys decrease in size and have fewer nephrons.
• Kidney stones are more common
• Infections are more common
• Enlargement of the prostate in males is more comoon
• Incontinence is more common