Detailed Notes – Chapter-16-Excretory Products And Their Elimination

16.5 REGULATION OF KIDNEY FUNCTION

1. Hypothalamic Regulation via ADH:

   • Osmoreceptors in the body detect changes in blood volume, fluid volume, and ionic concentration.
   • Excessive fluid loss triggers osmoreceptors, stimulating the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis.
   • ADH promotes water reabsorption from the latter parts of the tubule, preventing excessive urine production (diuresis).
   • Increased body fluid volume suppresses ADH release, completing the feedback loop.
   • ADH also constricts blood vessels, increasing blood pressure, and subsequently glomerular blood flow and GFR.

2. Juxta Glomerular Apparatus (JGA):

   • JGA plays a complex regulatory role in response to changes in glomerular blood flow, pressure, and GFR.
   • Reduced glomerular blood flow/pressure/GFR triggers JG cells to release renin.
   • Renin converts angiotensinogen to angiotensin I and then to angiotensin II.
   • Angiotensin II acts as a potent vasoconstrictor, increasing glomerular blood pressure and GFR.
   • Angiotensin II also stimulates the adrenal cortex to release aldosterone, promoting Na+ and water reabsorption from the distal tubule, further increasing blood pressure and GFR.
   • This mechanism is known as the Renin-Angiotensin mechanism.

3. Atrial Natriuretic Factor (ANF):

   • Increased blood flow to the atria of the heart triggers the release of atrial natriuretic factor (ANF).
   • ANF causes vasodilation, decreasing blood pressure.
   • This mechanism acts as a counterbalance to the renin-angiotensin mechanism, regulating blood pressure.

16.6 MICTURITION

1. Bladder Filling:

   • Urine produced by the nephrons accumulates in the urinary bladder.
   • As the bladder fills with urine, its walls stretch, stimulating stretch receptors located on the bladder walls.

2. Neural Signal Transmission:

   • Stretch receptors send signals to the CNS, indicating the degree of bladder distension.
   • The CNS processes these signals and sends motor messages back to the bladder and urethra.

3. Micturition Reflex:

   • The motor messages from the CNS initiate the contraction of the smooth muscles in the bladder wall.
   • Simultaneously, the urethral sphincter, a muscular ring that controls the flow of urine out of the bladder, relaxes.
   • This coordinated action allows urine to be expelled from the bladder through the urethra.
   • The entire process, from signal initiation to urine release, is known as the micturition reflex.

4. Characteristics of Urine:

   • Urine is a light yellow-colored watery fluid with a slightly acidic pH (around 6.0) and a characteristic odor.
   • On average, an adult human excretes 1 to 1.5 liters of urine per day, containing approximately 25-30 grams of urea.
   • Various conditions can affect the characteristics of urine, and its analysis can aid in diagnosing metabolic disorders and kidney malfunctions.
   • For example, the presence of glucose (glycosuria) and ketone bodies (ketonuria) in urine may indicate diabetes mellitus.

16.7 ROLE OF OTHER ORGANS IN EXCRETION

1. Lungs:

   • Lungs primarily remove carbon dioxide (CO2), expelling approximately 200mL per minute.
   • They also eliminate significant quantities of water through exhalation.

2. Liver:

   • The liver, the largest gland in the body, secretes bile containing various substances such as bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs.
   • Many of these substances are ultimately excreted along with digestive wastes.

3. Skin:

   • Sweat glands in the skin produce sweat, a watery fluid containing sodium chloride (NaCl), small amounts of urea, and lactic acid.
   • Sweat helps in regulating body temperature by facilitating evaporative cooling and also aids in the removal of certain metabolic wastes.
   • Sebaceous glands in the skin secrete sebum, which contains substances like sterols, hydrocarbons, and waxes. Sebum provides a protective oily covering for the skin.
   • While the primary function of sweat is thermoregulation, it also contributes to the elimination of some waste products.
   • Additionally, small amounts of nitrogenous wastes can be eliminated through saliva.

16.8 DISORDERS OF THE EXCRETORY SYSTEM

1. Uremia:

   • Uremia occurs due to the accumulation of urea in the blood, often resulting from kidney dysfunction or failure.
   • This condition can lead to various symptoms, including nausea, vomiting, fatigue, weakness, and cognitive impairment.
   • Severe uremia can ultimately lead to kidney failure, necessitating treatments like hemodialysis.

2. Hemodialysis:

   • Hemodialysis is a treatment for uremia and kidney failure, where blood is pumped through a dialysis machine to remove waste products.
   • During hemodialysis, blood flows through a dialyzing unit containing a semipermeable membrane, allowing the passage of waste substances into a dialysis fluid.
   • This process helps to clear toxins and excess fluids from the blood, providing relief to uremic patients.

3. Kidney Transplantation:

   • Kidney transplantation is the definitive treatment for acute renal failure (kidney failure).
   • A functioning kidney is transplanted from a donor, preferably a close relative, to replace the failed kidney.
   • Modern clinical procedures, including tissue matching and immunosuppressive medications, have improved the success rate of kidney transplantation.

4. Renal Calculi (Kidney Stones):

   • Renal calculi are stone-like formations composed of crystallized salts (e.g., oxalates) that form within the kidney.
   • These stones can cause severe pain, often as they pass through the urinary tract, and may lead to complications such as urinary tract infections and blockages.

5. Glomerulonephritis:

   • Glomerulonephritis is the inflammation of the glomeruli, the filtering units of the kidney.
   • This condition can result from various factors, including infections, autoimmune diseases, and certain medications.
   • Glomerulonephritis can lead to impaired kidney function, proteinuria (presence of protein in urine), and fluid retention.