Urinary System

• The “functional units” of the kidney.
• Take a simple filtrate of the blood and modify it into urine.
• Cleanse the blood and balance the constituents of the circulation.

Ureter(s)

• The kidneys and ureters are completely retroperitoneal.
• As urine passes through the ureter, it does not passively drain into the bladder but rather is propelled by waves of peristalsis.

• The bladder collects urine from both ureters.

Urethra

• Transports urine from the bladder to the outside of the body for disposal.
• Voiding is regulated by an involuntary autonomic nervous system-controlled internal urinary sphincter, consisting of smooth muscle and voluntary skeletal muscle that forms the external urinary sphincter below it.

Micturition Reflex

• The proper term for urination or voiding.
• Results from an interplay of involuntary and voluntary actions by the internal and external urethral sphincters. When bladder volume reaches about 150 mL, an urge to void is sensed but is easily overridden.
• Voluntary control of urination relies on consciously preventing the relaxation of the external urethral sphincter to maintain urinary continence. As the bladder fills, subsequent urges become harder to ignore.
• Ultimately, voluntary constraint fails with resulting incontinence, which will occur as bladder volume approaches 300 to 400 ml.
• Normal micturition is a result of stretch receptors in the bladder wall that transmit nerve impulses to the sacral region of the spinal cord to generate a spinal reflex. The resulting parasympathetic neural outflow causes contraction of the detrusor muscle and relaxation of the involuntary internal urethral sphincter.
• At the same time, the spinal cord inhibits somatic motor neurons, resulting in the relaxation of the skeletal muscle of the external urethral sphincter.
• Voluntary micturition requires an intact spinal cord and functional pudendal nerve arising from the sacral micturition center.
• Since the external urinary sphincter is voluntary skeletal muscle, actions by cholinergic neurons maintain contraction (and thereby continence) during filling of the bladder.
• At the same time, sympathetic nervous activity via the hypogastric nerves suppresses the contraction of the detrusor muscle.
• With further bladder stretch, afferent signals traveling over sacral pelvic nerves activate parasympathetic neurons.
• This activates efferent neurons to release acetylcholine at the neuromuscular junctions, producing detrusor contraction and bladder emptying.

Glomerular Filtration Rate (GFR)

• The volume of filtrate formed by both kidneys per minute is termed the glomerular filtration rate (GFR).
• GFR is influenced by the hydrostatic pressure and colloid osmotic pressure on either side of the capillary membrane of the glomerulus.
• A proper concentration of solutes in the blood is important in maintaining osmotic pressure both in the glomerulus and systemically.

Hormones and Enzymes

• Renin is an enzyme that is produced by the granular cells of the afferent arteriole. It enzymatically converts angiotensinogen (made by the liver, freely circulating) into angiotensin I. Its release is stimulated by prostaglandins to decreased extracellular fluid volume.
• Angiotensin II is a potent vasoconstrictor that plays an immediate role in the regulation of blood pressure.
  • It acts systemically to cause vasoconstriction as well as constriction of both the afferent and efferent arterioles of the glomerulus.
  • In instances of blood loss or dehydration, it reduces both GFR and renal blood flow, thereby limiting fluid loss and preserving blood volume.
  • Its release is usually stimulated by decreases in blood pressure, and so the preservation of adequate blood pressure is its primary role.
• Aldosterone is often called the “salt-retaining hormone,” is released from the adrenal cortex in response to angiotensin II or directly in response to increased plasma potassium.
  • It promotes sodium reabsorption by the nephron, promoting the retention of water.
• Antidiuretic Hormone (ADH) promotes the recovery of water, decreases urine volume, and maintains plasma osmolarity and blood pressure.
• Parathyroid hormone (PTH) is produced by the parathyroid glands in response to decreased circulating calcium levels.