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Despite their relatively small size, the kidneys receive approximately 20% of the cardiac output. [ 1 ] Each renal artery branches into segmental arteries, dividing further into interlobar arteries , which penetrate the renal capsule and extend through the renal columns between the renal pyramids.
The renal medulla (Latin: medulla renis 'marrow of the kidney') is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the segmental arteries which then branch to form interlobar arteries.
The renal lobe is a portion of a kidney consisting of a renal pyramid and the renal cortex above it. [1] In humans, on average there are 7 to 18 renal lobes. [medical citation needed] It is visible without a microscope, though it is easier to see in humans than in other animals.
The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis.The collecting duct participates in electrolyte and fluid balance through reabsorption and excretion, processes regulated by the hormones aldosterone and vasopressin (antidiuretic hormone).
Diagram showing the basic physiologic mechanisms of the kidney The kidney's ability to perform many of its functions depends on the three fundamental functions of filtration , reabsorption , and secretion , whose sum is called renal clearance or renal excretion.
The renal capsule surrounds the functional tissue of the kidney, and is itself surrounded by a fatty adipose capsule, fascia, and fat. From the inner part of the kidney to outside the kidney, the positioning of the capsule is: renal medulla; renal cortex; renal capsule; adipose capsule of kidney (or perirenal fat, or perinephric fat) renal fascia
Abnormal kidney function may cause too much or too little urine to be produced. The ability of the kidneys to filter protein is often measured, as urine albumin or urine protein levels, [2] measured either at a single instance or, because of variation throughout the day, as 24-hour urine tests. [citation needed]
[citation needed] A large "staghorn" kidney stone may block all or part of the renal pelvis. The size of the renal pelvis plays a major role in the grading of hydronephrosis. Normally, the anteroposterior diameter of the renal pelvis is less than 4 mm in fetuses up to 32 weeks of gestational age and 7 mm afterwards. [2]