11.3 The Kidney and Excretion Excretion. The Kidney Excretion is the removal of metabolic waste products from the body. These include carbon dioxide and nitrogenous waste. In humans, nitrogenous waste is in the form of urea. Ammonia is converted to urea in the ornithine cycle (liver) and urea is excreted in the form of urine.

Humans have two kidneys located low in the abdomen Humans have two kidneys located low in the abdomen. The renal artery brings blood loaded with urea, the renal vein takes the filtered blood away from the kidney. The ureter takes the urine to the bladder. The Nephron The functional unit of the kidney is the nephron. Ultra filtration takes place at the glomerulus, and the filtrate passes along the proximal convoluted tubule, through the loop of Henle and the distal convoluted tubule. At the end of the tubule, the urea passes into the collecting duct and is carried to the bladder.

As it passes along these tubes, valuable substances are removed and returned to the surrounding capillaries. Ultrafiltration Blood arrives at the glomerulus (extensive capillary network) under high pressure in the afferent vessel. Plasma (the liquid part of the blood and many dissolved particles) pass through the walls of afferent capillaries into Bowman’s capsule. Blood cells and large molecules, such as proteins, are too large to pass through.

The filtrate passes through three barriers: The walls of the glomerulus – contains many pores (fenestrated) which allows the plasma through The basement membrane – a protein membrane surrounding the glomerulus which acts as a filter The inner wall of Bowman’s capsule. The cells of the inner wall of Bowman’s capsule are called podocytes. They have extensions called pedicels which fold around the blood vessels. A network of filtration slits prevent blood cells from progressing into the tubule.

Osmoregulation is the control of water balance in the blood, tissues and cytoplasm of a living organism. Reabsorption Bowman’s capsule is connected to the proximal convoluted tubule. The fluid here contains glucose, amino acids, vitamins, hormones, urea, mineral ions and water. All of the glucose, amino acids, vitamins, hormones and most of the mineral ions are reabsorbed into the peritubular capillaries of the vasa recta. Active transport of the glucose and sodium ions takes place. Chlorine ions diffuse passively and osmosis of water follows.

To facilitate all of this transport, the cells lining the lumen of the proximal convoluted tubule have a row of micro villi to greatly increase surface area. They also contain large numbers of mitochondria to provide the necessary ATP for active transport. Loop of Henle 1. Descending limb. Water leaves the nephron by osmosis. It passes into the blood capillaries and is removed from the area. Some salt diffuses into the filtrate inside the loop. 2. Ascending limb. The ascending limb is impermeable to water. Salt is lost from the filtrate by active transport. It remains near the loop of Henle and helps to maintain a high concentration gradient in the medulla.

Medulla The roles of the loop of Henle and the vasa recta are to maintain a high concentration of salts in the tissues of the medulla. This high concentration of salts is maintained by a counter current multiplier mechanism. The collecting ducts run through the medulla, allowing water to be removed by osmosis. This keeps water loss in the urine to a minimum. Counter current exchange. In the ascending limb, salts are removed by active transport, but water remains. In the descending limb, salts diffuse into the tubule and water diffuses out. This makes the filtrate more concentrated as it reaches the bottom of the loop.

At the hairpin of the loop, these salts diffuse out again, ensuring the high concentration of salt in the medulla. YouTube - Function of the Nephron

Blood pH is regulated in the distal convoluted tubule Blood pH is regulated in the distal convoluted tubule. The concentration of hydrogen and hydroxyl ions in the blood is adjusted so that the pH of the blood stays within 7.35 and 7.45, but the pH of the urine varies between 4.5 to 8.2. The Collecting ducts. The walls of the collecting ducts vary in their permeability to water. When antidiuretic hormone, ADH, is secreted from the posterior pituitary gland, the walls become fully permeable. Water moves from the collecting duct to the medulla due to the high concentration of sodium and chloride ions there. This water is then reabsorbed by the blood. When no ADH is secreted, the walls of the collecting ducts become less permeable to water and large amounts of highly dilute urine are produced. The Composition of Plasma, Glomerular Filtrate and Urine. Blood plasma. The composition of the blood plasma remains fairly constant, due to the action of the kidneys. Blood arriving via the renal artery will be high in oxygen, urea and salt, and sometimes water. After ultrafiltration and reabsorption, the blood in the renal vein will contain the optimum amounts of water and salt, less urea, more CO2, and less oxygen.

2. Glomerular filtrate. The filtrate in Bowman’s capsule is similar in composition to blood plasma, without the large proteins. These are filtered out due to their large size. 3. Urine. Urine contains no Glucose Proteins Amino acids Urine contains less water and salts than glomerular filtrate. It contains more urea than glomerular filtrate. The urine of diabetics Due to the absence of insulin, glucose is not converted to glycogen and stored in the bodies of diabetics. As a result, high levels of glucose are present in the blood after a meal. As such, blood arriving at the kidney contains a much higher level of glucose than normal. The kidney is unable to reabsorb all of the glucose in the proximal convoluted tubule of diabetics. It stays in the filtrate and becomes part of the urine.