Kidneys, ureters and suprarenal glands

Location

The kidneys are paired retroperitoneal organs of the urinary system. The kidney location is on the posterior abdominal wall at the level of the T12-L3 vertebrae. 

Gross anatomy

Kidneys are bean-shaped structures with superior and inferior poles (extremities), anterior and posterior surfaces, major and minor convexities. The minor convexity is directed towards the median plane and contains the renal hilum. This represents the gateway for passage of the renal artery, renal vein and ureter. Learning a quick mnemonic 'VAD' can help you remember these structures (renal Vein, renal Artery, Duct a.k.a ureter).

The kidneys consist of two portions: an outer renal cortex and an inner renal medulla. The renal cortex shows segmental extensions called renal columns which project into the medulla and separate its pyramids.

The renal medulla consists of 8-18 renal pyramids. The latter are made up of numerous nephrons. The apex of each pyramid (renal papilla) opens medially to the minor calix, which is the first part of the urinary collecting system within the kidney. Minor calices unite to form two to three major calices, which then combine to form the renal pelvis. The ureter emerges from the renal pelvis and leaves the kidney through the hilum. 

Nephron

The nephron is the functional unit of the kidney. Each nephron consists of a renal corpuscle and a renal tubule. Based on where their corpuscle is located, nephrons are either cortical (corpuscle high in the cortex) or juxtamedullary nephrons (corpuscle low in the cortex close to the medulla).

The renal corpuscle consists of the glomerular (Bowman’s) capsule and a web of capillaries called the glomerulus. The function of the corpuscle is to filter blood, while the renal tubule processes and carries the filtered fluid to the system of calices. Going from the corpuscle to the calices, the renal tubules consist of the proximal convoluted tubule, nephron loop (loop of Henle), distal convoluted tubule and collecting tubule.

Collecting tubules from adjacent nephrons empty into a collecting duct, which in turn empties into the system of calices via the renal papilla. The renal tubules are surrounded by a network of peritubular capillaries into which useful substances are reabsorbed from the filtrate.

Functions

The main function of the kidney is to filter the blood by eliminating metabolic wastes and excess water. It also returns useful substances to the blood, such as minerals and nutrients. Through these actions of filtration and reabsorption the kidneys regulate the levels of minerals, electrolytes and the entire biochemical profile of the body. 

By producing urine, the kidneys also regulate the amount of the body fluid, significantly impacting blood pressure.

But that's a part of the wide range of functions the kidneys are responsible for! The whole list is quite easy to learn if you use a mnemonic 'A WET BED', which stands for:

• Maintaining ACID-base balance
• Maintaining WATER balance
• ELECTROLYTE balance
• TOXIN removal
• BLOOD Pressure control
• Making ERYTHROPOIETIN
• Vitamin D metabolism

The kidneys are involved in a large number of pathological conditions. You can check here an interesting case report which describes a case of a horseshoe kidney. To learn more about the kidney and nephrons, check out these study materials.

Adrenal cortex

The adrenal cortex is the outer part of the adrenal gland. It is subdivided into three layers all of which secrete different types of steroid hormones:

• Zona glomerulosa produces mineralocorticoids such as aldosterone. They regulate electrolyte and water balance within the body.
• Zona fasciculata synthesizes glucocorticoids such as cortisol, which coordinates responses to stressful situations.
• Zona reticularis produces adrenal androgens. They regulate sexual development and function.

Adrenal medulla

The adrenal medulla is the central part of the adrenal gland consisting of the chromaffin cells. They release two types of catecholamines into the bloodstream; epinephrine (adrenaline) and norepinephrine (noradrenaline).

These hormones activate the fight-or-flight response, which is a physiological reaction to stressful or dangerous situations, such as a bear attack or an anatomy exam. They elevate heart activity and blood pressure, dilate the bronchioles and increase blood flow to the muscles.

Kidneys and ureters

Each kidney and ureter is supplied by its respective renal artery that arises from the abdominal aorta, just below the superior mesenteric artery. 

The renal artery enters the hilum of the kidney and divides into a series of smaller vessels. They ultimately end as afferent arterioles, which transport blood into the renal glomerulus for filtration. Filtered blood leaves the glomerulus via the efferent arteriole, which becomes the interlobular vein. Venous blood of the kidney is conveyed by the renal vein which drains into the inferior vena cava.

The ureters receive blood supply from three sources:

• Upper third from the renal arteries
• Middle third from the abdominal aorta, gonadal arteries and common iliac arteries
• Inferior third from the internal iliac arteries

Venous blood from the ureters is drained into the respective renal and gonadal veins.

Adrenal glands

The adrenal glands receive arterial supply from three sources:

• Inferior phrenic artery - several superior suprarenal arteries
• Abdominal aorta - several middle suprarenal arteries
• Renal artery - one inferior suprarenal artery

Venous blood from the left and right adrenal glands drains into their respective suprarenal veins. The right suprarenal vein is a direct tributary of the inferior vena cava, whereas the left suprarenal vein drains into the renal vein first.

Kidneys and ureters

The kidneys receive autonomic innervation from the renal plexus. Sympathetic input is provided by the thoracolumbar splanchnic nerves (T10-L1). Parasympathetic input comes from the vagus nerve (CN X).

The ureters receive autonomic nerve supply from the ureteric plexus. The latter receives fibers from the renal, aortic and hypogastric (superior, inferior) plexuses. 

Adrenal glands 

The adrenal glands are stimulated by both neuronal and hormonal pathways. They predominantly receive sympathetic innervation, from the thoracolumbar splanchnic nerves (T10-L1). The fibers synapse directly onto the chromaffin cells of the adrenal medulla, inducing the release of catecholamines. The adrenal cortex receives hormonal stimulation from the pituitary gland by way of adrenocorticotropic hormone (ACTH) to produce corticotropic hormones.