Excretory Products and their Elimination Test 11

As blood enters your kidneys, it is filtered by glomerulus. The ultrafiltrate produced by the glomerulus initially contains small molecules like water, glucose, amino acids, bicarbonate, urea, and creatinine as well as ions such as potassium, sodium, chloride, calcium, and phosphate. All of this filtrate is channeled from the Bowman's space inside of the Bowman's capsule into the long series of ducts in the nephron, which are collectively called the renal tubule. During this journey, many things will be added and subtracted to and from the ultrafiltrate by the cells lining these ducts in order to finally form the urine.  After this the tubular fluid leaves the renal tubules and passes to the collecting duct system, which connects the nephron to the ureter. Urine then enters the urinary bladder ( which collects waste excreted by the kidneys prior to disposal by urination) via the ureters and exits via the urethra.

The glomerular-capsular membrane through which ultrafiltration occurs comprises of three layers
(1) endothelium,
(2) the basement membrane of the wall of glomerular capillaries and
(3) squamous epithelial layer.
A special characteristic of the glomerular-capsular membrane is that it is about 100 to 500 times more permeable than the walls of blood capillaries in other tissues of the body. Such a tremendous permeability of this membrane is because of the presence of literally thousands of small pores, called fenestrae, in the walls of glomerular capillaries (fenestrated capillaries).

Mesangial cells have an irregular shape and are thought to be related to smooth muscle cells. They contain similar proteins, such as myosin and actin, and have the ability to contract. They are situated in the gaps between glomerular capillaries, these cells connect to the basement membrane that helps make the capillary walls and forms part of the filter through which blood passes before entering Bowman's capsule.

The renal pelvis is made of two or three divisions called the major calyses. Each major calyx branches into several cup-shaped minor calyces. The apical portion called the papilla, of each renal pyramid projects into a minor calyx. The tip of the papilla is perforated with the openings of 10-25 papillary ducts, the final collecting ducts of the uriniferous tubules, and is therefore called the area cribrosa. 

The diagram depicts a nephron of mammalian kidney. Each kidney has more than 1 million functional units called nephrons. A nephron consists of a cup-like Bowman's capsule connected to a long, partially coiled renal tubule. Positioned within Bowman's capsule is a cluster of capillaries called a glomerulus. Three main regions of the renal tubule are the proximal convoluted tubule (or simply, proximal tubule) (Label B), which conducts the filtrate from Bowman's capsule to the loop of Henle (Label C), an elongated, hairpin-shaped portion; and the distal convoluted tubule (or distal tubule) (Label D), which conducts the filtrate to a collecting duct (Label F). Thus, filtrate passes through the following structures: Bowman's capsule --> proximal convoluted tubule --> loop of Henle--> distal convoluted tubule--> collecting duct.
Blood is delivered to the kidney by the renal artery. Small branches of the renal artery give rise to afferent arterioles (afferent means to carry toward) (Label A). An afferent arteriole conducts blood into the capillaries that make up each glomerulus. As blood flows through the glomerulus, some of the plasma is forced into Bowman's capsule. In a typical circulatory route, capillaries deliver blood into veins. Circulation in the kidneys is an exception because blood flowing from the glomerular capillaries next passes into an efferent arteriole. Each efferent arteriole conducts blood away from a glomerulus. The efferent arteriole delivers blood to a second capillary network, the peritubular capillaries (Label E) surrounding the renal tubule. Blood is filtered as it flows through the first set of capillaries, those of the glomerulus. The peritubular capillaries receive materials returned to the blood by the renal tubule. Blood from the peritubular capillaries enters small veins that eventually lead to the renal vein. Thus blood circulates through the kidney in the following sequence: Renal artery--> afferent arterioles--> capillaries of glomerulus-->efferent arterioles-->peritubular capillaries -->small veins--> renal vein.

Each kidney has more than 1 million functional units called nephrons. A nephron consists of a cuplike Bowman's capsule connected to a long, partially coiled renal tubule. Positioned within Bowman's capsule is a cluster of capillaries called a glomerulus. Three main regions of the renal tubule are the proximal convoluted tubule (or simply, proximal tubule), which conducts the filtrate from Bowman's capsule; the loop of Henle, an elongated, hairpin-shaped portion, and the distal convoluted tubule (or distal tubule), which conducts the filtrate to a collecting duct. Thus, filtrate passes through the following structures- Bowman's capsule then proximal convoluted tubule then a loop of Henle to distal convoluted tubule to collecting duct. Urine, the watery discharge of the urinary system, is produced by a combination of three processes - filtration, followed by reabsorption, and then tubular secretion.

Filtrate passing from Bowman's capsule to the proximal convoluted tubule has an osmolarity of about 300 mosm/L, the same as blood. As the filtrate flows through the proximal convoluted tubule, located in the kidney cortex, a large amount of water and salt is reabsorbed. Thus, the volume of filtrate decreases substantially at this stage, but the osmolarity remains about the same because the reabsorption of water by osmosis keeps pace with reabsorption of solutes all along the proximal convoluted tubule.

A) Correct option D

The nephron is the structural and functional unit of the kidney. Its body is divided into renal corpuscle, proximal convoluted tubule, Henle's loop and distal convoluted tubule (DCT). 

Metabolic wastes must be excreted so that they do not accumulate and reach concentrations that would disrupt homeostasis. Nitrogenous wastes include ammonia, uric acid, and urea. Amino acids and nucleic acids contain nitrogen. During the metabolism of amino acids, the nitrogen-containing amino group is removed (in a process known as deamination) and converted to ammonia. However, ammonia is highly toxic. Some aquatic animals excrete it into the surrounding water before it can build up to toxic concentrations in their tissues. Mammals excrete urea produced in the liver. Birds, reptiles, and terrestrial animals secrete uric acid. So, the correct answer is option D.