Kidney

Kidney

The kidneys, part of the urinary system, are the most important excretory organ in vertebrates. Medical terms related to the kidneys either involve renal or nephro-. Nephrology is the study of the kidneys.

Table of contents

1 Basic anatomy

1.1 Location
1.2 Structural details
1.3 Organization
1.4 Terms

2 Renal functions

2.5 Filtering wastes from the bloodstream
2.6 Secretion of hormones
2.7 Balancing the total body intake and excretion of both sodium and water
2.8 Glomerular filtration
2.9 Tubular reabsorption
2.10 Tubular secretion

3 Diseases and disorders

3.11 Congenital diseases of the kidneys
3.12 Acquired diseases of the kidneys

4 Analysis of renal function

4.13 Estimation
4.14 Tests
4.15 Formulas

5 Dialysis and kidney transplants
6 See also

Basic anatomy

Location

In humans the kidneys are the two organs that are located in the posterior part of the abdomen, on either side of the spine just below the liver and spleen on the right and left sides of the body respectively. Superior to each kidney is an adrenal gland (also called the suprarenal gland).

Kidneys viewed from behind with spine removed

The kidneys are retroperitoneal, which means they lie behind the peritoneum, the lining of the abdominal cavity. They are approximately at the vertebral level T12 to L3, and the right kidney usually lies slightly lower than the left, due to the size of the liver.

The upper parts of the kidneys are protected somewhat by the eleventh and twelfth ribs, and each whole kidney is surrounded by two layers of fat, the perirenal fat and the pararenal fat, which help to cushion it.

Section of a kidney

Structural details

In a normal human adult, each kidney is about 11 cm long and about 5 cm thick, weighing 150 grams. The kidneys are 'bean-shaped' organs, and have a concave side facing inwards (medially). On this medial aspect of each kidney is an opening, called the hilus, which admits the renal artery, the renal vein, nerves, and the ureter.

Organization

The outer portion of the kidney is called the renal cortex, the inner portion is called the renal medulla the middle is the pelvis. The outside is covered by collagen.

The basic functional unit of the kidney is the nephron, of which there are more than a million in each normal adult kidney. Nephrons regulate water and soluble substances (especially ions) in the body by filtering it all out first, reabsorbing what should be kept and excreting the rest. They use countercurrent exchange mechanisms.

A nephron consists of an initial filtering component called the renal corpuscle, and a renal tubule that extends out from the renal corpuscle.

Each renal corpuscle contains a compact bunch of interconnected capillaries called the glomerulus. Each glomerulus is supplied with blood by the afferent arteriole. The glomerulus protrudes into the fluid filled Bowman's capsule. Blood leaves the glomerulus through the efferant arteriole.

The Bowman's capsule contains a fluid filled space called Bowman's space.

Blood in Bowman's space is separated from blood in the glomerulus by three layers:

The single celled capillary endothelium
A proteinaceous layer of basement membrane
The single celled epithelial lining of Bowman's capsule (these cells are called podocytes)

The renal tubule is continuous with Bowman's capsule. The segment that drains Bowman's capsule is the proximal tubule. The next portion of the tubule is the loop of Henle, which leads to the distal convoluted tubule. Fluid flows flow the distal convoluted tubule into the collecting duct system.

The site where the ascending loop of Henle touches the affarent arteriol, is called the juxtaglomerular apparatus.

Terms

renal capsule: The membranous covering of the kidney.

cortex: The outer layer over the internal medulla. It contains blood vessels, glomeruli (which are the kidneys' "filters") and urine tubes and is supported by a fibrous matrix.
hilus: The opening in the middle of the concave medial border for nerves and blood vessels to pass into the renal sinus.
renal column: The structures which support the cortex. They consist of lines of blood vessels and urinary tubes and a fibrous material.
renal sinus: The cavity which houses the renal pyramid.
calyces: The recesses in the internal medulla which hold the pyramids. They are used to subdivide the sections of the kidney. (singular - calyx)
papillae: The small conical projections along the wall of the renal sinus. They have openings through which urine passes into the calyces. (singular - papilla)
pyramids: The conical segments within the internal medulla. They contain the secreting apparatus and tubules and are also called malpighian pyramids.
renal artery: Two renal arteries come from the aorta, each connecting to a kidney. The artery divides into five branches, each of which leads to a ball of capillaries. The arteries supply (unfiltered) blood to the kidneys. The left kidney receives about 60% of the renal bloodflow.
renal vein: The filtered blood returns to circulation through the renal veins which join into the inferior vena cava.
renal pelvis: Basically just a funnel, the renal pelvis accepts the urine and channels it out of the hilus into the ureter.
ureter: A narrow tube 40 cm long and 4 mm in diameter. Passing from the renal pelvis out of the hilus and down to the bladder. The ureter carries urine from the kidneys to the bladder.

Renal functions

Filtering wastes from the bloodstream

The wastes are concentrated to form urine which is passed on to the bladder via the ureters.

Secretion of hormones

Secretion of erythropoietin, which regulates red blood cell production in the bone marrow.
Secretion of renin
Secretion of the active form of vitamin D - 1,25-dihydroxycholecalciferol (calcitriol) and prostaglandins.

Balancing the total body intake and excretion of both sodium and water

There is a stable balance of sodium and water in the body. The major homeostatic control point for maintaining this stable balance is renal excretion.

Cl- always follows Na+.

Glomerular filtration

Glomerular filtration is caused by Starling forces.

Tubular reabsorption

As the filtrate moves through the renal tubules certain substances are reabsorbed. Reabsorption rates are high for nutrients, water, and ions but lower for waste products.

Reabsorption is caused by Starling forces, diffusion, and active transport.

Sodium reabsorption is an active process dependent upon sodium potassium pumps.

Sodium reabsorption creates an osmotic difference across the tubule which drives water reabsorption through water channels.

Water reabsorption also occurs passively by osmosis.

Substances to which the tubular epithelium is permeable are reabsorbed by diffusion because water reabsorption creates a concentration difference between the tubule and the interstitial fluid.

Some key regulatory hormones for reabsorption:

Aldosterone stimulates active NaCl reabsorption and indirectly stimulates passive water reabsorption
Antidiuretic hormone stimulates passive water reabsorption by inserting more aquaporins in both membranes.

Tubular secretion

Tubular secretion is caused mainly by active transport.

Usually only a few substances are secreted. These substances are present in great excess, or are natural poisens.

Diseases and disorders

Congenital diseases of the kidneys

Congenital hydronephrosis
renal dysplasia
Congenital obstruction of urinary tract
horseshoe kidney
duplicated ureter

Acquired diseases of the kidneys

Kidney stones are a relatively common and particularly painful disorder.

Pyelonephritis is infection of the kidneys and is frequently caused by complication of a urinary tract infection.
Azotemia is a toxic condition characterized by abnormal and dangerously high levels of urea, creatinine, various body waste compounds, and other nitrogen-rich compounds in the blood.
Hydronephrosis is the enlargement of one or both of the kidneys caused by obstruction of the flow of urine.
Pyelonephritis
In nephrotic syndrome, the glomerulus has been damaged so that a large amount of protein in the blood enters the urine. Other frequent features of the nephrotic syndrome include swelling, low serum albumin, and high cholesterol.
kidney tumors
- Wilm's tumor
Renal failure (acute and chronic)
Glomerulonephritis
Diabetic nephropathy
Lupus nephritis

Analysis of renal function

Estimation

A simple means of estimating renal function is to measure serum urea, creatinine and basic electrolytes (sodium and potassium). As the kidney is the most important organ in controlling these values, any derangement in these values would suggest renal impairment.

Tests

A more formal test of renal function would be to measure the glomerular filtration rate; usually a creatinine clearance test is performed.

Para-amino-hippuric acid is a renal analysis tool.

Formulas

Excretion rate = urinary concentration x urine flow rate

Renal clearance rate of A = excretion rate of A divided by plasma concentration of A

Clearance ratio = renal clearance rate / glomerular filtration rate

Dialysis and kidney transplants

Generally, one can live fine with just one kidney. If both kidneys don't function properly, dialysis is performed, where the blood is filtered outside of the body. Kidney transplants are now also quite common. The first successful such transplant was announced on March 4, 1954 by Peter Bent Brigham Hospital in Boston.