Fig 1. Processes involved in urine formation KIDNEY RLO 1 – function Page 1 Glomerulus Kidney function The role of the kidneys is to control the composition and volume of the blood by eliminating water, salts and waste products in the form of urine. There are three physiological processes that enable the blood to be filtered of waste material, and that allow any beneficial substances to be transferred back into the blood. These processes take place in the nephron and are: a Glomerular filtration b Tubule reabsorption c Tubule secretion Bowman’s Capsule Proximal/ distal tubules Fig 1. Processes involved in urine formation

Glomerular filtration A KIDNEY RLO 2 – function Page 2 Glomerular filtration A Glomerulus Afferent arteriole Efferent arteriole The first process by which the kidneys produce urine is called glomerular filtration. Blood enters the glomerulus under high pressure and forces substances through pores in the blood capillary endothelium into the capsule (Fig 2). Approximately 1 litre of blood passes through each kidney every minute. Substances filter into the funnel-shaped Bowman’s capsule which drains into the proximal tubule. During filtration, roughly one fifth of blood plasma is removed, consisting of water, small proteins, salts (Na+, Cl-, K+), glucose, and nitrogenous waste products such as urea. Bowman’s capsule Fig 2. A section through the glomerulus

Fig 3. Filtration between the glomerular capillary KIDNEY RLO 2 – function Page 3 Glomerular filtration B Bowman’s capsule acts like a sieve allowing small substances to be flow into the nephron, whilst large molecules remain within the blood. The high blood pressure of the glomerular circulation forces small molecules across the capillary wall and through pores in the Bowman’s capsule membrane. These substances drain from the Bowman’s capsule into the poximal tubule for the next step of the process. Fig 3. Filtration between the glomerular capillary and Bowman’s capsule

Fig 4. Tubule reabsorption KIDNEY RLO 1 – function Page 4 Tubule reabsorption A After filtration, many substances that are beneficial to the body are re-absorbed. These are mainly salts (Na+, Cl-), amino acids, glucose and water. Around 99% of substances are reabsorbed. Reabsorption occurs in the proximal convoluted tubule and loop of Henle by passive transport processes (processes that don’t require ATP energy). Beneficial substances are reabsorbed back into the blood stream. The remaining waste substances continue flowing around the tubule to the distal convoluted tubule. Proximal convoluted tubule Loop of Henle Fig 4. Tubule reabsorption

Fig 5. Tubule reabsorption KIDNEY RLO 1 – function Page 5 Distal convoluted tubule Tubule reabsorption B At the distal convoluted tubule, fine adjustments to the composition of the urine are made. This depends on the salt and water status of the body. If salt levels in the plasma are low, the hormone aldosterone drives more reabsorption of sodium. Less reabsorbtion occurs when salt levels of the plasma are high. If the body is thirsty, antidiruectic hormone (ADH) helps regulate the reabsorption of water. In thirst, ADH increases the permeability of the distal tubule and collecting duct and increases water resorption. In this situation, urine produced will be low volume. Fig 5. Tubule reabsorption

KIDNEY RLO 1 – function Page 6 Tubule secretion The final nephron process which is involved in urine formation is tubule secretion. Substances in the blood may be secreted back into the renal tubules. Some metabolic end products and exogenous substances such as penicillin and diuretic drugs are secreted from the circulation into the tubules. In addition, metabolic substances such as ammonium and hydrogen ions produced in the tubule cells themselves may also be transported into the lumen. The process of tubular secretion is an important step in controlling blood pH. Fig 6. Tubule secretion

Fig 7. Average daily water intake and output KIDNEY RLO 1 – function Page 7 Urine formation The end product of kidney filtration, resorption and secretion is urine. Urine comprises of 95% water containing nitrogenous wastes in the form of urea and uric acid, salts and other endogenous and exogenous metabolites. The kidneys provide the major route for water excretion in the body (Fig)m with other losses via the skin, lungs, faeces and sweat. They are an important route for the elimination of drugs and drug metabolites from the body. Fig 7. Average daily water intake and output