Renal System and Kidney Functions

Kidney Anatomy and Function

• The kidneys are two small, bean-shaped organs located at the back of the peritoneum (retroperitoneal) in the superior lumbar region, just below the rib cage on each side of the spine.
• The right kidney is located slightly lower than the left kidney to accommodate the liver.

Kidney Structure

• The kidney has three main layers: the renal capsule, renal cortex, and renal medulla.
• The renal capsule is a tough, fibrous connective tissue layer that covers the outside of each kidney, composed primarily of collagen and elastin.
• The renal cortex is the outermost layer, surrounded by the renal capsule, and houses the nephrons.
• The renal medulla is the innermost layer, containing the renal pyramids where urine formation takes place.

Nephron Structure and Function

• The nephron is the functional unit of the kidney, stretching from the cortex to the medulla.
• Each kidney has approximately 1,000,000 nephrons.
• The nephron has three main parts: the glomerulus, proximal convoluted tubule, and distal convoluted tubule.
• The glomerulus is a tangle of capillaries surrounded by the Bowman's capsule, where blood pressure forces water and solutes out of the capillaries and into the lumen of the Bowman's capsule.
• The proximal convoluted tubule and distal convoluted tubule are responsible for reabsorption and secretion of substances.

Kidney Functions

• The kidneys remove excess fluid and waste products from the body.
• The kidneys maintain acid-base balance by regulating the concentration of hydrogen ions.
• The kidneys regulate blood pressure by controlling the amount of salt and water in the body.
• The kidneys produce hormones, such as erythropoietin and calcitriol, and the enzyme renin.

Blood Flow and Filtration

• Blood flows into the kidney through the renal artery and exits the kidney through the renal vein.
• Urine exits the kidneys via the ureter and drains into the bladder.
• Blood is filtered through the kidneys about 6 times a day.
• The glomerulus filters the waste molecules (urea, salt, glucose, amino acids, and water) from the blood.

Diuretics

• Diuretics are used to stimulate the kidneys to excrete an increased amount of fluid.
• Diuretics work in the nephron to increase urine production.
• There are three types of diuretics: carbonic anhydrase inhibitors (CAIs), loop diuretics, and potassium-sparing diuretics.
• Loop diuretics, such as Lasix or furosemide, are the strongest diuretic medications and are used to treat renal failure.
• Thiazide diuretics cause a drop in potassium, while potassium-sparing diuretics are used to help patients who are losing too much potassium in treatment.### Kidney Physiology: Mechanisms of Urine Formation
• The filtrate obtained from the glomerulus is called the primary urine, which is almost equal to blood in chemical composition, except for the presence of blood cells.
• The primary urine passes into the proximal convoluted tubule (PCT), where useful substances such as glucose, amino acids, and vitamins are reabsorbed.
• Filtration occurs due to non-selective forces of blood pressure, resulting in the movement of water and small molecules out of the glomerular capillaries.

Functions of Nephrons

• Filtration: the movement of water and small molecules out of the glomerular capillaries into the Bowman's capsule.
• Tubular reabsorption: the process of reabsorbing useful substances from the primary urine back into the bloodstream in the PCT.
• Tubular secretion: the active transport of solutes from the bloodstream into the filtrate in the distal convoluted tubule (DCT).

Tubular Secretion

• It promotes sodium reabsorption in the PCT.
• It induces the release of aldosterone from the adrenal cortex, which promotes sodium and water retention in the kidneys.

Renin-Angiotensin-Aldosterone System (RAAS)

• A hormonal system that controls blood pressure.
• Renin is an enzyme secreted by the juxtaglomerular cells in the kidneys, which converts angiotensinogen into angiotensin 1.
• Angiotensin 1 is converted into angiotensin 2 by the angiotensin-converting enzyme (ACE) in the lungs.
• Angiotensin 2 stimulates vasoconstriction and the release of aldosterone, leading to increased blood pressure.

Antidiuretic Hormone (ADH) Mechanism

• A hormone released by the posterior pituitary gland, regulated by the hypothalamus.
• ADH stimulates water reabsorption in the collecting duct, leading to concentrated urine and decreased urine production.
• Increased ADH secretion helps conserve water in the body.

Urinary Tract Infections (UTIs)

• Can occur in any part of the urinary system, including the kidneys, ureters, bladder, and urethra.
• Most often occurs in the bladder or urethra, but can progress to the kidneys if left untreated.

Kidneys and Ureters

• The kidneys filter waste and reabsorb what the body needs.
• The ureters are thin tubes of muscle that carry urine from the kidneys to the bladder.

Bladder and Urethra

• The bladder is a hollow, muscular, balloon-shaped organ that expands as it fills with urine.
• The urethra is the tube that carries urine from the bladder out of the body.
• The female urethra is about 4 cm (1.5 inches) long, while the male urethra is about 20 cm (7-7.9 inches) long.

Brain and Urination

• The brain, especially the maturation center in the pons, plays a crucial role in controlling urination.
• Voluntary control over the external urethral sphincter develops as the central nervous system matures.
• The brain receives sensory signals from the bladder wall and communicates with the cortex to determine the appropriateness of urination.### Urination Process
• When the bladder is full, stretch receptors in the wall of the bladder send nerve impulses to the spinal cord.
• The signal returns to the bladder, stimulating contraction of the detrusor muscle and relaxation of the internal urethral sphincter.
• This is an involuntary reflex, predominant in infants and young children.
• Three simultaneous events must occur for urination to happen:
  • Contraction of the detrusor muscle by the ANS
  • Opening of the internal urethral sphincter by the ANS
  • Opening of the external urethral sphincter by the somatic nervous system

Urine Composition

• Urine is predominantly water (over 95%)
• The remaining 5% consists of:
  • Urea
  • Chloride
  • Sodium
  • Potassium
  • Creatinine
  • Other dissolved ions
  • Inorganic and organic compounds (e.g. proteins, hormones, metabolites)

Abnormal Urine Output

• Dysuria: painful or uncomfortable urination, often due to urinary tract infections
• Hematuria: red blood cells in urine, often due to infection or injury
• Proteinuria: protein content in urine, often due to leaky or damaged glomeruli
• Glycosuria: glucose in urine, often due to excess plasma glucose in diabetes
• Oliguria: abnormally small amount of urine, often due to shock or kidney damage
• Polyuria: abnormally large amount of urine, often caused by diabetes