Excretory Products and their Elimination Test -4

The substances that are secreted into the tubular fluid (for removal from the body) include:

• Potassium ions (K+),
• Hydrogenions (H+),
• Ammonium ions (NH4+),
• creatinine,
• urea,
• some hormones,
• and some drugs (e.g. penicillin).

Counter current mechanism operates in mammals but not in all vertebrates. Due to counter current mechanism it produces hypertonic urine, highly concentrated.

During urine formation, ultra filtration takes place in which energy is not required by cells of kidney. Selective reabsorption of glucose , water, salt etc. occurs actively and proximal convoluted tubules (PCT) do not have microvilli.

Net force of filtration pressure is equal to the blood pressure of glomerular capillaries minus sum of the osmotic pressure of plasma and hydrostatic pressure of fluid in renal tubule. So, filtration pressure = 75 – [ 30 + 20 ] mm Hg.

A fall in glomerular blood flow/glomerular blood pressure/GFR can activate the JG cells to release renin which converts angiotensinogen in blood to angiotensin I and further to angiotensin II.

An excessive loss of fluid from the body can activate receptors which stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis.

Cortical nephron is about 25% of total nephrons. Glomerular filtration rate is 180 litre per day. Juxtamedullary nephron makes about 15% of total nephrons but lung do not removes 180 L of carbon dioxide per day.

The renal threshold is the state when glucose is present in urine. The renal threshold of glucose is 180 mg/100 ml of blood. If blood glucose level exceeds 180 mg, it leads to diabetes insipidus.

In case of toxicity and solubility ammonia is the most toxic and uric acid is the least toxic. According to toxicity, we can assemble them as Ammonia > urea > uric acid. More toxic substance will require more amount of water for elimination. As aquatic amphibia live in water they are ammonotelic, frog and humans are land animals are ureotelic and birds are uricotelic as they are aves.

The Henle's loop helps to maintain the concentration of urine. The descending loop of Henle is permeable to water but it is impermeable to electrolytes. The ascending loop is impermeable to water but allows transport of electrolytes.

The proximity between loop of Henle and vasa recta as well as counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium. This mechanism help to maintain a concentration gradient in medullary interstitium so human urine is nearly four times concentrated than initial filtrate formed.

In frogs and humans NH$$_{3}$$ is converted into less toxic urea in liver for excretion, i.e., ureotelism.

Correct option: D

Ascending limb of loop of Henle is impermeable to water. Here water is not reabsorbed, rather sodium, potassium, magnesium and chloride are reabsorbed and therefore the filtrate becomes hypotonic to blood plasma.

The urea cycle is also known as Krebs-Henseleit urea cycle. As ornithine is one of the product of this reaction, it is called as ornithine cycle. Urea is the major end product of nitrogen metabolism in humans and mammals.

Podocytes are highly specialised, terminally differentiated cells that are located in the Bowman's capsule of the kidney. These cells have interdigitated foot processes, that are bridged by a membrane, called as the slit diaphragm and have a role in the selective permeability of the glomerular filtration barrier.

The nephron is the basic structural and functional unit of the kidney. A nephron eliminates wastes from the body, regulates blood volume and blood pressure, controls levels of electrolytes and metabolites, and regulates blood pH. Renal artery supply oxygenated blood to the nephron.