Kidney

The kidneys are paired retroperitoneal organs that filter the blood and produce urine, thereby regulating the water and ion concentrations of the blood plasma. Besides excreting nitrogen compounds, toxins, water, and electrolytes, kidneys also act as endocrine organs by secreting the hormones erythropoietin, renin, and prostaglandins. They are structurally complex and extraordinarily well vascularized: even though they make up only ~5 % of the human body's weight, they receive ~25% of the cardiac output, amounting to ~1,700 liters of blood per day. Of this large volume of blood flowing through the kidney, ~190 liters is initially processed into urine via filtration. Since daily urine production averages about 1 liter, much of the kidney's energy expenditure is devoted to selective recovery (reabsorption) of water, ions, and small molecules back into the bloodstream. A subspecialty of medicine (nephrology) is devoted to the diagnosis and treatment of kidney disease.

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Anatomy

Gross anatomy

Kidneys are normally present behind the intestines and extend from the level of the last thoracic vertebra (T12) to the third lumbar vertebra (L3). They are roughly oval, with medial indentations. These hila (singular: hilum) are where vessels, nerves, and the ureters enter and leave the organ. A typical adult kidney measures 11 cm (superior-inferior) by 7 cm (medial-lateral} by 3 cm {anterior-posterior], The kidneys receive blood from the descending aorta via the renal arteries and drain through the renal veins to the inferior vena cava. Urine leaves the kidney and travels to the urinary bladder through the ureter.

Microscopic anatomy

The functional unit of the kidney is the nephron. Each kidney contains about 0.8 to 1.25 x l0⁶ nephrons, which resemble tiny funnels. Each nephron produces urine by sequential processing of blood plasma through Bowman's capsule, a proximal convoluted tubule (PCT), a loop of Henle, a distal convoluted tubule (DCT), and a collecting duct.

Bowman's capsule

Bowman's capsule is a specialized cup-shaped structure where blood plasma leaves the cardiovascular system and enters the urinary system as ultrifiltrate. Blood enters and leaves Bowman's capsule through afferent and efferent arterioles, respectively. Within the capsule, the arteriole divides into a network of fine capillaries termed the glomerulus. A glomerulus and its surrounding Bowman's capsule are termed a renal corpuscle; most of these are near the surface of the kidney (the renal cortex). Many small pores are present in the capillary endothelium of the glomerulus, facilitating the transit of water, ions, and small molecules into Bowman's capsule.

Proximal convoluted tubule

The PCT receives filtered plasma from the Bowman's capsule. Cells of the PCT have thousands of microvilli on their luminal surfaces, greatly increasing their ability to reabsorb water and solutes from the filtrate. More than two-thirds of the filtrate is returned to the blood from the PCT by entering the peritubular capillaries. Peritubular capillaries, as the name suggests, surround the PCT, the loop of Henle, and the DCT. They are supplied by the afferent arterioles and drain into interlobular veins.

Loop of Henle

The loop of Henle receives filtrate from the PCT and conducts it through a descending limb, a sharp U-turn, and an ascending limb. Some nephrons' loops of Henle extend deep into the medulla of the kidney. The structure of the loop of Henle allows for the production of highly concentrated urine through a countercurrent mechanism, where the contents of the ascending limb travel in an opposite direction to the flow of urine in the descending limb. An important class of drugs, the loop diuretics, interferes with the ability of the loop of Henle to concentrate urine and results in increased urine production.

Distal convoluted tubule

The DCT lies in the renal cortex and conducts urine from a nephron's loop of Henle to the collecting system.

Collecting duct

The collecting duct receives urine From the DCTs of several nephrons. Collecting ducts begin in the cortex and converge as the urine flows toward the renal pelvis. Urine exits the kidney via the ureter.

Physiology

Sodium reabsorption

Physiology of sodium reabsorption in the kidney

Location in nephron	Proportion of total sodium reabsorption accounted for	Membrane transport protein Ion pump or ion channel	Diuretics that act at this location
Proximal convulated tubule	40%	Carbonic anhydrase Sodium-hydrogen antiporter (Ion pump)	Carbonic anhydrase inhibitors
Late proximal tubule		Chloride-bicarbonate antiporter (Ion pump)
Loop of Henle: thin descending limb	0%	Not applicable	Osmotic diuretics
Loop of Henle: thick ascending limb ('diluting segment')	25%	Sodium potassium chloride symporter (Ion pump)	Loop diuretics
Distal convulated tubule	10%	Sodium chloride symporter (Ion pump)	Thiazides
Collecting tubule	2-5%	Mineralacorticoids receptors Sodium channel (Ion channel)	Potassium-sparing diuretics

The Endocrine kidney

The kidney acts a part of the endocrine system by synthesizing and secreting hormones such as erythropoietin, renin, and prostaglandins.