Renal System
Functions of the Renal System:
Maintaining the volume and composition of body fluids within normal ranges
Clearing nitrogenous wastes from protein metabolism
Maintaining acid base balance and electrolyte levels
Excreting drugs and drug metabolites.
Regulating vitamin D activation
Regulating blood pressure through the renin–angiotensin–aldosterone system
Regulating red blood cell production through the production and secretion of erythropoietin
Kidneys:
The glomerulus has a barrier that only allows small molecules to be filtered into the nephron. The afferent arteriole brings dirty and unbalanced blood from the capillaries to the glomerulus, the efferent arteriole brings clean and balanced blood away from the glomerulus back to the capillaries. The afferent and efferent arterioles work together to regulate fluid into the glomerulus, this increases and decreases the hydrostatic pressure.
Juxtaglomerular cells on the walls of the afferent and efferent arterioles regulate the filtration and composition of the blood. Juxtaglomerular cells also secrete renin which is the most powerful vasoconstrictor, it can constrict the arterioles and raise the hydrostatic pressure which increases the filtration rate.
Macula densa cells in the distal tubules sense sodium concentration in the blood, if sodium levels are low, the macula densa cells signal the afferent arteriole to decrease resistance and for the juxtaglomerular cells to increase the glomerular filtration rate and increase blood flow to the kidneys.
Glomerular filtration keeps blood cells, proteins, and lipids inside of the vessels, the hydrostatic pressure pushes water and small components like electrolytes and plasma into the tubule to be secreted in the urine.
Damage to the filter can cause larger molecules like blood cells, proteins, and lipids to be pulled into the tubule and lost in the urine. About 180 liters per day are filters, only 1% becomes urine.
Tubular secretion occurs when the epithelial cells line the renal tubules and secrete
unwanted substances like extra electrolytes, acids, and drugs to become urine. Tubular reabsorption occurs when cells lining the renal tubule reabsorb water and various
essential substances like vitamins, glucose, electrolytes, and bicarbonate. These from the filtrate that goes back into the vascular systems. About 99% of water filtered at the
glomerulus is reabsorbed.
Sodium helps to maintain osmotic and electrical balances by staying connected with chloride and water. Sodium is reabsorbed in the proximal convoluted tubule. The distal convoluted tubule adjust for sodium levels by the influence of aldosterone which is produced in the adrenal glands and natiuretic acid which comes from the hypothalamus. Aldosterone is release when potassium levels are high and causes the sympathetic nervous system to stimulate angiotensin 3. Aldosterone stimulates a sodium and potassium exchange at the distal tubule, sodium is reabsorbed and potassium is lost in the urine. Natiuretic hormone causes a decrease in sodium reabsorption in the distal tubule. The result is more diluted urine. Naturetic hormone is released in response to fluid overload.
Countercurrent mechanism is the process used by the nephrons to concentrate or dilute the urine in response to body stimuli to maintain fluid and electrolyte balance. This mechanism prevents you from producing liters and liters of dilute urine every day and is the reason why you don’t need to be continually drinking to stay hydrated.
Renin Angiotensin Aldosterone System:
The Renin Angiotensin Aldosterone System reaction happens when kidneys do not receive enough nutrients and oxygen due to dehydration, blood loss, hypertension, hypotension, or heart failure.
When blood flow to the kidney decreases, juxtaglomerular cells release renin, renin enters the blood stream and causes the liver to activate angiotensinogen which makes angiotensin 1. Angiotensin 1 travels to the lungs where angiotensin converting enzyme or ACE turns it into angiotensin 2 which is a powerful vasoconstrictor. When angiotensin 2 flows through the blood it raises blood pressure and increases blood flow to the kidneys. Angiotensin 2 is the converted angiotensin 3 in the adrenal glands which causes the release of aldosterone. Aldosterone causes the distal convoluted tubules to retain sodium and water which increases blood volume and flow to the kidneys. Aldosterone causes the reabsorption of sodium by getting rid of potassium. The osmotic center in the brain senses the high sodium also known as hypernaturemia and causes the release of antidiuretic hormone (ADH) from the pituitary gland. This decreases urination and increases blood volume.
Diuretics:
Thiazide diuretics (mild)
Drug name: Hydrochlorothiazide (HTCZ)
Works by blocking chloride pump in the ascending loop of henle or distal convoluted tubule
The result is only a small amount of water is lost and chlorine and sodium is lost in urine
Always the first drug to be prescribed for hypertension
Loop diuretic (stronger)
Drug name: furosimide
Works by blocking chloride pump in the ascending loop of henle and stopping the reabsorption of sodium and chloride
The result is more water lost in urine
Carbonic anhydrase inhibitor (very mild)
Drug name: Acetazolamide
Works by blocking effects on carbonic anhydrase in proximal convoluted tubules, it reabsorbs sodium, water, and bicarbonate
Works by slowing down the movement of hydrogen ions
The result is more sodium and bicarbonate is lost in the urine
Very mild and used with other diuretics
Commonly used for glaucoma
Potassium sparing diuretics (weaker than loop)
Drug name: Spironolactone
Works by losing sodium, retains potassium
Patients taking are at risk for hyperkalemia
Osmotic diuretic (very powerful)
Drug name: mannitol
Works by pulling large amounts of fluid into urine quickly
Used for increased intercrannial pressure and acute renal failure due to shock, overdose, or trauma