The kidneys receive more blood flow than any single organ, after the liver. 20-25% of the cardiac output of the heart goes to the kidneys for filtration (the liver receives closer to 28%). This is especially significant considering that the kidneys account for approximately 0.5% of total body weight (while the liver makes up closer to 3% of total body weight). 

The basic functional unit of the kidney is the nephron. The nephron is composed of the glomerulus, which is encased in the Bowman's capsule, the proximal convoluted tubule, the loop of Henle, and distal convoluted tubule.

99% of glucose and amino acids are resorbed by the proximal tubule of the nephron. The loop of Henle and the distal tubule primarily resorb electrolytes and water, while the glomerulus is strictly for filtration and does not resorb solutes. 

The GFR represents the rate at which blood is filtered through the glomeruli of the kidneys. Creatinine is a waste product of muscle metabolism. Healthy glomeruli are capable of filtering creatinine very effectively, wherein creatinine is excreted in the urine. Generally an elevated serum creatinine will be accompanied by a decreased glomerular filtration rate and is strongly predictive of kidney dysfunction. 

No amount of protein in the urine is considered normal. The capillaries of the glomerulus are covered in a thin layer of cells called podocytes. These podocytes form a very tight filtration system that, when healthy, prevent all large molecules such as proteins from passing out of the glomerulus and into the Bowman's capsule, while allowing the passage of water, electrolytes, and smaller molecules such as glucose and amino acids.

Aldosterone is a mineralcorticoid hormone secreted by the adrenal cortex in response to aldosterone II. An rise in plasma angiotensin levels will effect an elevation in blood volume (and hence blood pressure) by increasing the resorption of sodium and water in the kidney and increasing renal excretion of potassium. 

While normal function is possible with a urinary output as little as 500ml/day, normal urinary output is between 1000-2000ml per day, with 1200ml being average with adequate fluid intake.

The majority of nutrients and electrolytes from food are absorbed n the small intestine. The remaining material passes into the large intestine for further transformation into stool. The liver and stomach are important sites of digestion and metabolism but are not the primary site of nutrient and electrolyte absorption.

The large intestine serves multiple functions. Large amounts of water are resorbed from stool via aquaporins in the large intestine mucosa, and the remaining matter is passed via a series of slow waves toward the rectum for excretion. In addition the large intestine houses trillions of both beneficial and pathogenic intestinal bacteria and fungi.

Stool that has passed through the large intestine is stored in the rectal ampulla, a dilated portion of the rectum inferior to the sigmoid colon and superior to the anal canal. From there it can be passed into the anal canal and excreted via the anus. The haustra coli are segments or pouches formed along the length of the small intestine, demarcated by the smooth muscle of the taenia coli.