Excretory Products and their Elimination Test 56

Proximal convoluted tubule is about 14 millimeters in length. The wall of the tubule is made up of cuboidal epithelial cells, which have microvilli. Each cell has about 150 microvilli per square micron. Each microvillus is about 1 micron in length. The microvilli together form a brush border region.

The development of the kidney proceeds through a series of successive phases, each marked by the development of a more advanced kidney: the pronephros, mesonephros, and metanephros. The pronephros is the most immature form of the kidney, while the metanephros is most developed. The metanephros persists as the adult kidney. During approximately day 22 of human gestation, the paired pronephric kidney appear towards the cranial end of the intermediate mesoderm. The number of the nephron in the pronephric kidney in human is about 40 and mesonephric is about 34. 

The urinary bladder is lined with transitional epithelium. It does not produce mucus.

A - renal artery; B - proximal tubule; C - loop of Henle; D - collecting tubule. The ability to excrete urine that is more concentrated than the extracellular fluid (ECF) depends on the creation of a hyperosmotic environment in the medulla. An increase in osmolarity reflects a decrease in water concentration. In humans, the osmotic gradient in the medulla allows the urine produced to be roughly 5 times as concentrated as the ECF. The first three steps in urine formation, filtration, reabsorption, and the establishment of an osmotic gradient, result in a fluid that is slightly hypotonic to blood. Electrolytes and water are absorbed by the distal tubule. The epithelial cells of the proximal tubule reabsorb ions, nutrients, and water and transport them to the blood vessels. Water flows from the epithelial cells into the blood via osmosis. Because osmosis occurs, the osmolarity of the filtrate remains isotonic. Hence the urine is less osmotic in the proximal tubule. 

Erythropoietin is an essential hormone for red cell production. Under hypoxic conditions, the kidney will produce and secrete erythropoietin to increase the production of red blood cells.

Nephrons that make up the kidney are responsible for the elimination of wastes from the body and help to regulate the volume and pressure of blood, and also regulates the blood pH. These functions which are vital are regulated by the endocrine system. In the case of humans, there are 800,000 to 1.5 million nephrons in the kidney out of which 70-80% are cortical nephrons and rest are juxtamedullary. Thus, the correct answer is option C.

Fluids from blood in the glomerulus are collected in the Bowman's capsule (i.e., glomerular filtrate) and further processed along the nephron to form urine. This process is known as ultrafiltration. As the blood plasma flows through the nephrons, its composition changes. Fluids comprising excess water, salt and metabolic waste products are extracted from the blood as it enters the Bowman's capsule in the nephrons. This fluid in the Bowman's capsule is called glomerular filtrate. It is similar to blood plasma except that it has almost no protein.

Each kidney receives its blood supply via single renal artery from dorsal aorta and is drained off via a single renal vein, which opens in the inferior vena cava. As the artery enters into the medulla after traveling through the hilum, it divides into a number of branches. These branches enter into renal cortex through the column of Bertin and subdivide into afferent arterioles, which form glomerular capillaries. The glomerulus consists of a network of about 50 capillaries. Therefore, the correct answer is option B.

When renal blood flow is reduced (hypotension) or there is a decrease in sodium and chloride ion concentration, the macula densa of the distal tubule releases prostaglandins, which cause the juxtaglomerular cells lining the afferent arterioles to release renin, activating the renin-angiotensin-aldosterone system, to increase blood pressure and increase reabsorption of sodium ions into the bloodstream via aldosterone.