The Process of Urine Formation

The Process of Urine Formation

Urine is formed as a result of a three phase process

• Simple filtration, selective and passive
  
• Reabsorption and excretion.
  
• Filtration
  

Filtration takes place through the semi- permeable walls of the glomerular capillaries which are almost impermeable to proteins and large molecules. The filtrate is thus virtually free of protein and has no cellular elements. The glomerular filtrate is formed by squeezing fluid through the glomerular capillary bed. The driving hydrostatic pressure (head of pressure) is controlled by the afferent and efferent arterioles, and provided by arterial pressure. About 20% of renal plasma flow is filtered each minute (125 ml/min). This is the glomerular filtration rate (GFR).

In order to keep the renal blood flow and GFR relatively constant hydrostatic pressure in the glomerulus has to be kept fairly constant. When there is a change in arterial blood pressure, there is constriction or dilatation of the afferent and efferent arterioles, the muscular walled vessels leading to and from each glomerulus. This process is called autoregulation; it is the capacity of tissue to control its blood supply. Autoregulation of GFR is achieved by autoregulation of renal blood flow and a feedback mechanism known as "glomerular tubular balance".

When there is a decrease in GFR, there is a resulting decrease in the fluid flow rate within the tubule. At the loop of Henle, there is greater time for reabsorption of sodium and chloride ions. Therefore there is a decrease in the number of sodium and chloride ions reaching the distal tubule which is detected by the macula densa. This in turn decreases the resistance in the afferent arteriole which results in an increase in renal blood flow. It also increases renin release from the juxtaglomerular apparatus which stimulates angiotensin II production causing constriction of the efferent arteriole. These both act to increase the hydrostatic pressure in the glomerular capillary bed and return GFR to normal.

The juxtaglomerular complex consists of macula densa cells, which are special distal tubular epithelial cells which detect chloride concentration and modified smooth muscle cells, juxtaglomerular cells, in the walls of the afferent and efferent arteriole. These cells produce renin.

Renin is an enzyme which converts the plasma protein angiotensinogen to angiotensin I. Angiotensin converting enzyme (ACE) which is formed in small quantities in the lungs, proximal tubule and other tissues, converts angiotensin I to angiotensin II which causes vasoconstriction and an increase in blood pressure. Angiotensin II also stimulates the adrenal gland to produce aldosterone which causes water and sodium retention which together increase blood volume. This is a negative feedback system. In other words the initial stimulus is a fall in blood volume which leads to a fall in perfusion pressure in the kidneys. When blood volume, renal perfusion and GFR improve the system feed back to switch off or turn down the response to the stimulus.

The function of the renal tubule is to reabsorb selectively about 99% of the glomerular filtrate.

The Proximal Tubule reabsorbs 60% of all solute, which includes 100% of glucose and amino acids, 90% of bicarbonate and 80-90% of inorganic phosphate and water. 90% sodium is reabsorbed through the proximal tubular cell both passively and actively

Re absorption is by either active or passive transport. Active transport requires energy to move solute against an electrochemical or a concentration gradient. It is the main determinant of oxygen consumption by the kidney. Passive transport is where reabsorption occurs down an electrochemical, pressure or concentration gradient.

Most of the solute reabsorption is active, with water being freely permeable and therefore moving by osmosis. When the active reabsorption of solute from the tubule occurs, there is a fall in concentration and hence osmotic activity within the tubule. Water then moves because of osmotic forces to the area outside the tubule where the concentration of solutes is higher the substance or interstitium of the medulla.

The Loop of Henle is the part of the tubule which dips or "loops" from the cortex into the medulla, (descending limb), and then returns to the cortex, (ascending limb). It is this part of the tubule where urine is concentrated if necessary. This is possible because of the high concentration of solute in the substance or interstitium of the medulla. The Vasa Recta is a portion of the peritubular capillary system which enters the medulla where the solute concentration in the interstitium is high.

The final concentration of urine depends upon the amount of antidiuretic hormone (ADH) secreted by the posterior lobe of the pituitary. If ADH is present the distal tubule and the collecting duct become permeable to water. As the collecting duct passes through the medulla with a high solute concentration in the interstitium, the water moves out of the lumen of the duct and concentrated urine is formed. In the absence of ADH the tubule is minimally permeable to water so large quantities of dilute urine are formed. There is a close link between the hypothalamus of the brain and the posterior pituitary. There are cells within the hypothalamus, osmoreceptors, which are sensitive to changes in osmotic pressure of the blood. If there is low water intake, there is a rise in osmotic pressure of the blood, and after excess intake of water, the reverse. Nerve impulses from the hypothalamus stimulate the posterior pituitary to produce ADH when the osmotic pressure of the blood rises. As a result water loss in the kidney is reduced because ADH is secreted, and water reabsorbed in the collecting duct.