Regulation of Kidney Function & Disorders

Regulation of Kidney Function

The kidneys are effectively monitored and regulated by hormonal feedback systems involving the hypothalamus, JGA, and the heart. Osmoreceptors in the body respond to changes in blood volume, body fluid levels, and ionic concentration. When there's excessive fluid loss, these receptors signal the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis. ADH promotes water reabsorption in later segments of the tubule, preventing excessive urine production (diuresis).

Conversely, an increase in body fluid volume inhibits osmoreceptors and reduces ADH release, completing the feedback loop. Additionally, ADH affects kidney function by constricting blood vessels, which raises blood pressure. Elevated blood pressure can enhance glomerular blood flow and, consequently, the glomerular filtration rate (GFR).

• The JGA plays a multifaceted role in regulation. Decreased glomerular blood flow/pressure/GFR can stimulate JG cells to secrete renin, which converts angiotensinogen in the blood to angiotensin I and then angiotensin II. 
• Angiotensin II, a potent vasoconstrictor, raises glomerular blood pressure and thus GFR. It also prompts the adrenal cortex to release aldosterone, which promotes sodium and water reabsorption in the distal tubule, further increasing blood pressure and GFR. 
• This intricate process is commonly referred to as the Renin-Angiotensin mechanism.
• Increased blood flow to the atria of the heart triggers the release of Atrial Natriuretic Factor (ANF). ANF induces vasodilation, reducing blood pressure. Consequently, the ANF mechanism acts as a counterbalance to the renin-angiotensin mechanism.

Micturition

• Urine, originating from nephrons, is conveyed to the urinary bladder for storage until prompted by a voluntary signal from the central nervous system (CNS).
• This signal is initiated by the stretching of the bladder as it fills with urine, activating stretch receptors on the bladder walls, which transmit signals to the CNS.
• The CNS responds by sending motor messages that trigger the contraction of the bladder's smooth muscles and the simultaneous relaxation of the urethral sphincter.
• These coordinated actions result in the release of urine, a process known as micturition, governed by the micturition reflex.
• On average, an adult eliminates 1 to 1.5 liters of urine daily, characterized by its light yellow color, slightly acidic pH (around 6.0), and distinct odor.
• Approximately 25-30 grams of urea are excreted per day.
• Urine analysis is valuable in diagnosing metabolic disorders and kidney malfunctions; for example, the presence of glucose (Glycosuria) and ketone bodies (Ketonuria) may indicate diabetes mellitus.

Role of Other Organs in Excretion

• Apart from the kidneys, other organs such as the lungs, liver, and skin play roles in eliminating waste from the body.
• The lungs expel significant amounts of CO2 (approximately 200mL per minute) and considerable volumes of water daily.
• The liver, the body's largest gland, secretes bile containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs. These substances are mostly expelled with digestive wastes.
• Skin's sweat and sebaceous glands also contribute to waste elimination. Sweat, primarily produced by sweat glands to cool the body, contains NaCl, small amounts of urea, lactic acid, etc., aiding in waste removal. Sebaceous glands eliminate substances like sterols, hydrocarbons, and waxes through sebum, providing a protective oily layer for the skin.
• Saliva can also eliminate small amounts of nitrogenous wastes.

Disorders of the Excretory System

• Kidney malfunction can lead to urea buildup in the blood, known as uremia, which can result in kidney failure.
• Hemodialysis is a process used to remove urea and other waste products from the blood in patients with kidney failure.
• During hemodialysis, blood is drawn from an artery and pumped into an artificial kidney, where it is filtered.
• An anticoagulant like heparin is added to prevent blood clotting during the process.
• The artificial kidney contains a cellophane tube surrounded by dialysing fluid, which mimics plasma composition but lacks nitrogenous wastes.
• The porous membrane of the tube allows waste molecules to pass out of the blood based on concentration gradients.
• After purification, the blood is returned to the body through a vein, with anti-heparin added to prevent clotting.
• Kidney transplantation is the preferred method for treating acute renal failure, where a functioning kidney is transplanted from a donor, often a close relative, to minimize rejection.
• Modern clinical procedures have improved the success rate of kidney transplantation.
• Other kidney disorders include renal calculi, which are crystallized salts formed within the kidney, and glomerulonephritis, an inflammation of the kidney's glomeruli.