Urinary System: Kidney

Kidney is divided into two regions:

Kidney lobes and lobules:

The number of lobes equals the number of renal pyramids (8-18 lobes). A lobe consists of one renal pyramid, which is associated with cortical tissue at its base and sides. Lobation is most evident in fetal kidneys., and disappears after birth (although may exist until a teenager). Each lobe is composed of lobules, which consists of one medullary ray and half of the surrounding cortical labyrinth on either side.

Arterial blood supply:

Each kidney is supplied by a renal artery. The renal artery branches into interlobar arteries. Which travel between the lobes (pyramids) up to the level of the cortex. Then they turn to follow an arched course between the cortex and medulla, hence they are then called arcuate arteries. Arcuate arteries then branch into intralobular arteries, which ascend in the cortex towards the capsule. They give off afferent arterioles, one to each renal corpuscle. These then give rise to the capillary network that forms the glomerulus, which is then drained by the efferent arterioles. This gives rise to a second network of capillaries, the peritubular capillaries, that surround the convoluted tubules. The efferent arterioles from the juxtamedullary glomeruli descent to the medulla, forming the vasa recta, which is part of the countercurrent multiplier system.

There are 2 million Nephrons per kidney, They consist of a renal corpuscle (aka malpighian corpuscle) and a tubule system.

Following the Bowman’s capsule are:

Types of nephrons include:

Filtration apparatus is located in the renal corpuscles and consists of three components:

Mesangial cells are a group of cells in the renal corpuscle. The mesangium is the mesangial cells & their ECM. They are located in the vascular stalk of glomerulus. Like pericytes, they are enclosed by basal lamina of glomerular capillaries. Some mesangial cells are located outside the renal corpuscle, along the vascular pole. They are called Lacis Cells, and form part of the Juxtaglomerular Apparatus. Mesangial cells functions include:

Juxtaglomerular Apparatus consists of three cell types:

Collaboration between the three cell types to maintain blood pressure:

Cells of the juxtaglomerular apparatus work together to maintain blood pressure through the Renin – Angiotensin – Aldosterone System (RAAS). Low Na+ concentration in DCT stimulates macula densa cells (osmoreceptors) to secrete mediators such as: ATP, adenosine, nitric oxide (NO), and prostaglandins (PGE2). Mediator molecules act in paracrine manner and their signals are distributed to JC (probably via the gap junctions of Lacis cells). JC are stimulated to secrete renin and RAAS is activated. RAAS activation results in aldosterone secretion from the adrenal gland. Aldosterone increases Na absorption from the DCTs and Collecting ducts. Water absorption increases as a result and blood pressure is maintained.

Kidney Tubules:

Countercurrent Multiplier System creates hypertonic urine. It’s found in the medulla and consists of the Loop of Henle, Vasa recta, and Medullary collecting ducts.

Interstitial Cells are minimal in health tissue. They increase in amount from cortex (7%) to medulla (20%).