1. Blood pressure control
1.1. Macular dense in the wall of the distal tubule make contact with cells in the endothelium of the arterioles which release a hormone called renin. -The macula densa and the renin-releasing cells are collectively called the juxtaglomerular apparatus or complex
1.1.1. The juxtaglomerular apparatus is responsible for maintaining a constant blood flow through the glomerulus and thus a constant GFR despite fluctuations in arterial pressure.
1.1.1.1. Renin acts via the renin–angiotensin system to produce both local vast- constriction of the efferent arteriole (increasing GFR) and peripheral vasoconstriction to increase arterial blood pressure.
2. PH Balance
2.1. The kidney secrete excess hydrogen ion (acidosis) into the urine and in the process generating further bicarbonate for reabsorption into the plasma. Rise in plasma PH (alkalosis )when there is a reduction in the hydrogen ion concentration of the body fluids.
2.1.1. The pH scale is a logarithmic scale (range 1–14) that measures the concentration of free hydrogen ions in a fluid. The normal pH range of our bodily fluids is between 7.34 and 7.45
2.1.1.1. When the plasma bicarbonate concentration is normal (25mmol/l) then all of the filtered bicarbonate is reabsorbed in the proximal tubule and in the distal tubule. When plasma bicarbonate concentration is higher than normal then this excess bicarbonate is lost in the urine. The hydrogen ion is secreted into the tubule to be excreted and the bicarbonate ion diffuses into the plasma. This process is called bicarbonate ‘trapping.
2.2. The bicarbonate ion concentration is carefully controlled by the kidneys.
3. Electrolyte balance
3.1. Potassium is the main intracellular cation.
3.2. Potassium is important in maintaining the membrane potential of nerve and muscle cells and hence affects their excitability.
3.2.1. Aldosterone is the only hormone involved in the control of potassium content and thus has a very important regulatory role.
3.2.1.1. The effect of aldosterone in the distal tubule of the nephron is to increase the secretion of potassium into the urine. When aldosterone levels fall, the reverse occurs and less potassium is secreted.
3.3. Calcium is important in the extracellular fluid in controlling nerve and muscle conduction.
3.3.1. Calcium ions play an important role in the regulation of nerve and muscle function and in blood clotting.
3.3.1.1. Calcium is reabsorbed in the distal tubule depends on the levels of circulating PTH.
3.4. Phosphate intracellular cation is important in buffer systems to maintain the plasma pH and exists in equilibrium with calcium. The normal plasma range of total inorganic phosphate is 0.87–1.45 mmol/l. Phosphate is reabsorbed in the early proximal tubule.
3.4.1. When the plasma phosphate level is below 1 mmol/l, the plasma phosphate level rises above 1mmol/l, the amount of phosphate excreted in the urine.
3.4.1.1. Phosphate reabsorption is controlled by parathyroid hormone (PTH).
3.5. Magnesium is an important intracellular cation involved in energy storage and production.
3.5.1. Parathyroid hormone increases tubular reabsorption of magnesium.
3.6. Sodium is the main extracellular cation (positively charged ion) and is intimately related to extracellular volume.
3.6.1. Sodium levels is regulated by three hormones ADH, aldosterone and atrial natriuretic peptide.
4. Fluid volume balance
4.1. Aldosterone is a steroid hormone secreted from the adrenal cortex and effect the distal tubule of the nephron. The secretion is regulated by a peptide, angiotensin 11.
4.1.1. The more aldosterone secreted, the more sodium is reabsorbed.
4.2. ADH is secreted from the posterior pituitary gland in response a rise in plasma osmolality. ADH receptors are found in the collecting ducts of kidney tubules.
4.2.1. The fall in plasma osmolality then leads to a slowing-down of ADH secretion by negative feedback.
4.3. ANP is released from cardiac atrial cells in response to an increased atrial stretch.
4.3.1. ANP five known major effects; inhibition of aldosterone secretion by the adrenal cortex, reduction of renin release by the kidney, reduction of ADH release from the posterior pituitary, vasodilation, natriuresis and diuresis.
5. Erythropoietin production
5.1. Erythropoietin is a hormone produce by the kidney. It promotes the proliferation and differentiation of erythrocyte precursors in the bone marrow.
5.1.1. Erythropoietin is necessary for the maintenance of a normal red cell count and prevention of anaemia.
5.1.1.1. Erythropoietin production is stimulated by hypoxia and inhibited when the hypoxia is corrected, thus its production is controlled by the negative-feedback principle.