Human Excretion and Kidney Physiology

Ammonia which is the major form of nitrogenous waste generated in the body is converted urea in the liver and excreted through the urine by the kidneys. Thus, the correct option is C.

The accessory excretory organs are:-

(A) Spleen - Primarily acts as blood filter. 

Metabolic wastes are substances left over from a metabolic process which cannot be used by the organisms and must, therefore, be excreted. This includes nitrogen compounds. The nitrogen compounds through which excess nitrogen is eliminated from the organism are called nitrogenous wastes. They are ammonia, urea, uric acid, and creatinine. 

The large intestine is the part of the digestive system. Whereas the excretory system of humans consists of two kidneys, two ureters, one urinary bladder, and one urethra. Urine is found in almost all excretory system parts(little or more).

In humans, kidneys are the chief excretory organ, other than kidneys, lungs, liver, and skin also help in the elimination of excretory wastes. Lungs remove a large amount of carbon dioxide and the significant amount of water every day. The liver secretes bile containing substances like bilirubin, biliverdin, degraded steroid hormones, vitamins. Most of these substances pass along with digestive wastes. Skin contains sweat glands through which NaCl, the small amount of urea, lactic acid etc are secreted. The intestine is not an excretory organ but it is a digestive organ.

The human kidney secretes many hormones: erythropoietin (EPO), calcitriol, vitamin $D _3$, as well as the enzyme renin. The kidneys also produce prostaglandins.

Renin is the enzyme secreted by the juxta glomerulus of the kidney to the bloodstream. It regulates the blood pressure by converting the angiotensinogen protein to angiotensin I and then further into the angiotens II

Renin is the enzyme secreted by the kidney. Juxa glomerular cells present in kidney release renin. It activates the renin-angiotensin system and produces a rise in blood pressure.  It has stretch receptors in the vascular walls which sense changes in renal perfusion pressure.

Erythropoietin is a glycoprotein hormone that controls erythropoiesis, or red blood cell production. It is a cytokine (protein signaling molecule) for erythrocyte precursors in the bone marrow.

A.Cholecystokinin is a hormone which is secreted by cells in the duodenum and stimulates the release of bile into the intestine and the secretion of enzymes by the pancreas.
B.Erythropoietin is a hormone produced primarily by the kidneys. 
C.Secretin,a hormone released into the bloodstream by the duodenum.
So,the correct option is 'Epinephrine'.

• Angiotensinogen is a precursor protein made in the liver for a hormone called angiotensin I. 
• And renin catalyzes a reaction that converts the angiotensinogen protein into angiotensin I, which is a precursor hormone that is converted to an active hormone called angiotensin II by an enzyme known as an angiotensin-converting enzyme in the lungs. Hence, Angiotensinogen is converted to angiotensin by renin.

The juxtaglomerular (JG) cells in the kidney secrete the enzyme renin, which initiates the renin-angiotensin-aldosterone system (RAAS) to regulate blood pressure.

• Erythropoietin is a  peptide hormone produced primarily by juxtaglomerular cells of the kidney.
• Erythropoietin stimulates the bone marrow to produce more red blood cells.
• The resulting rise in red cells increases the oxygen-carrying capacity of the blood.
•  As the prime regulator of red cell production, erythropoietin's major functions are to promote the development of red blood cells. Hence, Juxtaglomerular cells of kidney produce a peptide hormone called Erythropoietin.

• Renin is a hormone produced by the juxtaglomerular cells of the kidney.
• Renin also acts as an enzyme in the conversion of angiotensin-1 to its active form, angiotensin-2 involved in constriction of blood vessels. Hence renin is both a hormone and an enzyme. Hence, renin is both a hormone and enzyme.