Kidneys, Location of Kidneys
There are two bean shaped kidneys in the human body. A normal kidney is about 2.5 cm thick, 10 cm long and 5 cm wide. They are reddish-brown in color and each weighs approximately 130 gm in adults. Both kidneys receive blood from the renal artery; in a resting adult approximately 1.2 to 1.4 Litres of blood flows through both kidneys in one minute which means every 4 to 5 minutes all the circulating blood in the body passes through the kidneys. Kidneys help remove excess salt, water and waste products from the bloodstream. Like other endocrine organs, kidneys also secrete important hormones such as rennin and erythropoietin into the circulation. These hormones play significant roles in the production of blood cell and in the regulation of blood pressure.
Location of the kidney
Talking about the location of kidneys, each lies in the grooves or gutters alongside the upper vertebral column on the posterior wall of the abdomen at the level of T 12 to L 3 vertebra. The right kidney is about 0.5 inches lower than the left kidney because of its downward displacement by the large right lobe of the liver. From each kidney a ureter of about 25 cm long descends almost vertically downward before entering the urinary bladder. Ureter is a muscular tube that carries urine from the kidney to store temporarily in the bladder before voiding (micturition). About 1 to 1.5 Liters of urine is formed by the kidneys every day. Various metabolic waste products such as urea, uric acid are also eliminated with the urine from the body.
Constituents of urine
The function of kidneys is to produce urine by filtering large quantities of fluid from the blood plasma. Urine consists of approximately 95% water. Urine also contains urea (half of that which was present in the glomerular filtrate), creatinine (all of that which was present in the original glomerular filtrate), phenols, phosphate, potassium, sulfates, nitrates and large amount of uric acid.
Urea, uric acid and creatinine are all nitrogenous waste products. Nitrogen containing compounds are among the most toxic metabolic waste substances that our body produces as a result of various metabolic processes. The level of nitrogenous waste products in our blood is expressed as BUN (blood urea nitrogen). In healthy adults, normal BUN level ranges from 8 mg/dL to 25 mg/dL. An elevated level of BUN indicates renal insufficiency.
The volume and composition of urine produced and excreted every day by the kidneys depend crucially on various factors such as fluid intake, diet, renal functions, climate, metabolic activities of cells and presence or absence of systemic diseases. Dilute urine appears pale and concentrated urine has a dark yellow appearance. Yellow color of the urine is due to biological pigments porphyrins, urochrome and urobilin. The color of urine may change in many diseases (e.g. Jaundice, urinary tract infections).
Abnormal constituents of urine
Normal freshly passed urine is slightly acidic in nature. Abnormal constituents of urine and their associations with various diseases:
- Blood: kidney stones, acute glomerulonephritis (inflammation in glomerulus), bacterial infections, urinary schistosomiasis, sickle cell anemia.
- Ketones: in starvation, uncontrolled diabetes mellitus.
- Proteins: renal tuberculosis, pyelonephritis, glomerulonephritis, nephritic syndrome, urinary tract infections.
- Glucose: diabetes mellitus, sometimes traces can be found in healthy individuals.
- Galactose: galactosemia (rare metabolic disorder)
- Bilirubin: hepatocellular jaundice, obstructive jaundice
- Urobilinogen: abnormal red blood cell breakdown due to other diseases.
Hormonal control of the kidney
Aldosterone is the most physiologically active mineralocorticoid hormone released by the adrenal (suprarenal) glands. There are two adrenal glands on either side of the vertebral column; each sits on the superior poles of each kidney. This hormone is produced in response to changes in the plasma levels of potassium and angiotensin ll of the renin-angiotensin system. Aldosterone acts on the cortical collecting ducts of the nephrons to increase sodium and water reabsorption by the kidneys. Aldosterone also causes increased potassium and hydrogen ion secretion by the kidneys.
Antidiuretic hormone (ADH)
ADH is also called vasopressin. This hormone originates primarily from the cells of the hypothalamus and then stored in the pituitary gland for further release when necessary. Secretion of this hormone is increased during dehydration when the body needs to conserve more water; likewise secretion of this hormone is reduced after ingesting large volume of water. Vasopressin acts on the cortical and medullary collecting ducts of the nephrons and causes increased water reabsorption by the kidneys as a result of which small amount of concentrated urine is produced. Relative or absolute deficiency of vasopressin results in a disease called diabetes insipidus. Patients with diabetes insipidus suffer from polyuria (excretion of high volumes of urine), dehydration, polydipsia (excessive thirst) and elevated metabolic rate.
Angiotensin ll causes increased sodium and water retention by the kidneys.
Parathyroid hormone (PTH)
This hormone is synthesized and secreted by the parathyroid glands which are located just behind the thyroid gland. There are usually four parathyroid glands in a normal individual but the numbers of these glands may vary from two to six. PTH inhibits renal phosphate reabsorption; therefore it increases phosphate excretion in the urine by the kidneys. PTH also causes increased calcium reabsorption by the kidneys which results in an elevated serum calcium level.