Excretory System

• Excretion mechanisms


  • Defecation
  • Urination
  • Exhalation
  • Sweating
  • Other mechanisms done by the body
• Main components of the urinary system

  • Two kidneys
  • Two ureters
  • Urinary bladder
  • Urethra
• Kidneys


  • Left kidney is more superior as compared to the right
  • Functions include: maintaining pH balance and formation of urine whenever it filters blood
• Ureters


  • Ureters are different from urethra
  • You have the left and the right ureter
  • These function to allow urine to flow/pass through from kidney going into urinary bladder
• Urinary bladder


  • Storage and collection of urine coming from the kidneys
• Urethra


  • Passage way of urine as it exits the body
  • From the bladder, it will pass through the urethera to urinate
• The urinary system plays a vital part in maintaining homeostasis of water and electrolytes within the body
• The two main organs regulating our acid-base balance are lungs and kidneys
• Kidneys


  • They produce urine, which contains metabolic waste products, including nitrogenous compounds urea and uric acid, excess ions and, sometimes excreted drugs
  • They are responsible for formation of urine, maintaining water, electrolyte and acid-base balance
  • They are responsible for excretion of waste products
  • They produce and secrete erythropoietin, the hormone that stimulates formation of red blood cells (erythropoiesis)
  • They produce and secrete renin, an important enzyme in the long-term control of blood pressure
• Urine formation


  1. Filtration

  2. Selective reabsorption

  3. Tubular secretion
• Kidneys


  • They contain 1-2 million functional units called nephrons

  
• Nephron tubular system


  1. Proximal Convoluted Tubule

  2. Medullary Loop (Loop of Henle)

  3. Distal Convoluted Tubule

  4. Collecting Duct
• Kidney structure


  • Capsule - outermost layer
  • Cortex - outer layer of the kidney
  • Inner medulla (renal pyramids due to their shape), triangular on their cross section but look pyrimdal in 3D

  
• Kidney blood vessels


  • Aorta gives branch to the kidney to supply blood to it (renal artery)
  • Renal artery - has left and right counterpart (right is inferior since IVC is more anterior)
  • Vena cava has branches coming from renal vein going through IVC
• Blood filtration in kidneys

  1. Renal artery via the afferent arteriole provides blood to the glomerulus (filtration system of kidneys)

  2. Filtered blood enters efferent arteriole and will return to our circulation via the capillaries, will go to renal vein and to the IVC
• Tubular system


  • Proximal convoluted tubule (PCT)
  • Medullary loop or loop of Henle
  • Distal convoluted tubule (DCT)
  • Collecting duct
• Glomerulus


  • Toughed up capillaries found in kidneys or nephron
  • Responsible for filtration of blood
• Blood cells, plasma proteins, and other large molecules are too large to filter through the glomerulus and therefore remain in the capillaries
• Proximal convoluted tubule


  • Most reabsorption (Around 60% of reabsorbed substances become absorbed in PCT) from the filtrate back into the blood takes place here
  • Its epithelial lining possesses microvilli to increase surface area for absorption
• Medullary loop


  • Only 60-70% of filtrate reaches the medullary loop
  • Much of this, especially water, sodium and chloride, is reabsorbed in the loop, so that only 15-20% of the original filtrate reaches the distal convoluted tubule
• Collecting ducts


  • Main function is to reabsorb as much water as the body needs
• Tubular secretion


  1. Substances not required and foreign materials, e.g. drugs including penicillin and aspirin, may not be entirely filtered out of the blood because of the short time they remain in the glomerulus or because the molecules are too large to pass through the filtration pores or glomerular capillaries

  2. Such substances are cleared by tubular secretion from the peritubular capillaries into the filtrate within the convoluted tubules

  3. Tubular secretion of hydrogen ions (H+) is also important in maintaining normal blood pH
• Ureters


  • Carry urine from the kidneys to the urinary bladder
  • Peristalsis is an intrinsic property of the smooth muscle layer that propels urine along the ureter (occurs in a wave-like motion)
• Urinary bladder


  • It is a reservoir for urine
  • It lies in the pelvic cavity and its size and position vary, depending on the volume of urine it contains
  • The bladder is roughly pear-shaped, but becomes more balloon shaped if it is filled with urine
  • The bladder is distensible, but as it fills, awareness of the need to pass urine is felt
  • The total capacity is rarely more than 600 mL
• Urethra


  • It is a canal extending from the neck of the bladder to the exterior, at the external urethral orifice
  • The urethra of males and females differ in terms of length - it is longer in the male than in the female
• Urinary bladder


  Like a balloon, inflates and appears more circular or spherical when distended with urine
• Bladder


  • Roughly pear-shaped, but becomes more balloon shaped if filled with urine
  • Distensible, but awareness of need to pass urine is felt at around 300 mL to 400 mL in adults
  • Total capacity is rarely more than 600 mL
• Urethra


  Canal extending from the neck of the bladder to the exterior, at the external urethral orifice
• Urethra


  • Longer in males than females
  • Shorter female urethra predisposes them more to UTI
  • Associated with both urinary and reproductive systems in males
• Detrusor muscle


  If contracted/pushed, the liquid/urine inside bladder will be pushed out of bladder and out to the urethra
• Sphincter


  A ring of muscle surrounding and serving to guard or close an opening or tube, acting like gates/guards the flow of urine
• If sphincters are contracted or close, no urine will pass in urethra
• Bladder of females is connected to urethra, and for males, bladder is also connected to urethra
• An important structure that can block urine is the prostate gland, especially if enlarged
• Micturition (urination)


  1. Accumulation of urine in bladder activates stretch receptors

  2. Signals transmitted to spinal cord, initiating spinal reflex

  3. Stimulates involuntary contraction of detrusor muscle and relaxation of internal urethral sphincter, expelling urine
• Micturition reflex (in infants)


  Spinal cord immediately sends signals to detrusor muscles to contract and internal urethral sphincter to relax and open to let urine flow
• Micturition (in adults)


  • Signals of bladder distension sent to spinal cord, then passed to brain for processing before urination
  • External urethral sphincter can be consciously contracted to override reflex micturition
• Facilitating urination


  1. Conscious contraction of external urethral sphincter and pelvic floor muscles can inhibit micturition

  2. Increasing pressure in pelvic cavity by lowering diaphragm and contracting abdominal muscles can assist urination
• Over-distension of bladder is extremely painful, and can lead to involuntary relaxation of external sphincter allowing urine to escape
• Incontinence


  Involuntary loss of urine after bladder control has been established