The kidneys are bean-shaped, paired organs that are involved in a variety of functions of the human organism. The most commonly known function of the organ is urine production. The kidney mainly serves to regulate the electrolyte and water balance, but at the same time it also performs essential functions in the acid-base balance and the elimination of toxins. The kidney also plays an important role in the regulation of blood plasma volume and thus blood pressure. In addition, hormones such as calcitriol (calcium balance) or erythropoietin (blood cell synthesis) are synthesized in the kidney.
The kidney primarily serves to regulate electrolytes: Various ions such as sodium (Na+), chloride (Cl-), calcium (Ca2+) and magnesium (Mg2+) are either excreted/secreted or retained/resorbed. Thus the kidney ensures that ions that are important for the body are made available in sufficient quantities or that excess ions are eliminated. The kidney is also responsible for the excretion of drugs, toxins and metabolic waste products such as ammonia or uric acid.
With the excretion or absorption of ions (especially sodium), water is also excreted or absorbed. Thus, the volume of the extracellular space and the blood volume can be directly influenced and thus indirectly influence the blood pressure. For this reason, drugs that increase urine production, such as loop diuretics, thiazides or aldosterone receptor antagonists, can be used to treat high blood pressure (hypertension).
By eliminating protons (H+) and hydrogen carbonate (HCO3-), the acid-base balance of the body is regulated. This mechanism plays an important role in the compensation of acid-base imbalances, which can occur, for example, in the context of respiratory acidosis (respiration-related acidification of the blood). Such acidosis can, for example, be triggered by increased respiration in the context of a stress situation.
By influencing the phosphate and calcium levels, the kidney controls the bone mineralisation, quasi the incorporation of calcium and phosphate into the bone. The hormone calcitriol is also produced in the kidney and also plays a central role in bone formation. Besides calcitriol, other hormones such as erythropoietin are also synthesized in the kidney.
Erythropoietin influences the production of red blood cells. Kinins, urodilatin, prostaglandins and renin are also produced in the kidney. Kinins are important for the regulation of buttock width and permeability in inflammatory processes and in sensitising pain receptors.
Urodilatin is used to increase renal blood flow, urine production and the ejection capacity of the heart. The hormone renin enables the conversion of angiotensinogen into angiotensin and is thus involved in the regulation of blood pressure. Prostaglandins have important functions in the development of pain, inflammatory processes, fever and as mediators.
Task of the renal cortex
The renal cortex is located between the renal capsule and the renal medulla. The cortex of the kidney is about 10 mm thick. In the renal cortex are the vascular clusters (glomeruli), which are the first stage of urine production.
The glomeruli consist of an afferent vessel (vas afferens) and a conduction vessel. Substances in the blood (electrolytes, drugs, etc.) can escape from the vessels and enter the capsule space between the membranes of the podocytes (star-shaped cells around the capillary).
The filtered plasma liquid (about 150l/day) is called ultrafiltrate. The ultrafiltrate first flows through the first section of the proximal tubule (pars convoluta) where its composition is modulated. Electrolytes such as sodium, chloride, bicarbonate, potassium and calcium can be removed from the ultrafiltrate via various transporters and channels.
About two thirds of the filtered common salt and over 90% of the bicarbonate are returned to the blood in this section. As the process continues, proteins, peptides and amino acids are reabsorbed. Glucose, galactose and other sugars are also extracted from the filtrate in the first section.
The pars convoluta of the distal tubule is also located in the cortex, where the electrolyte concentrations in the urine are finely adjusted. The renal medulla is located between the renal cortex and the renal pelvis. The renal medulla consists of about ten to twelve tissue pyramids, also known as renal pyramids.
These tissue pyramids have a broad surface pointing outwards, while the tips protrude into the renal calices. The kidney pyramids continue into the renal cortex as medullary rays (Radii medullares). Several collecting tubes run through the renal pyramids.
In the collection tubes the composition of the urine is finely regulated and additional water is reabsorbed. At the top of the renal pyramids are the urinary pores from which the secondary urine drips into the renal calices. In the medullary region there are also the entry and exit points of the blood vessels, which are essential for the transport of electrolytes and substances to and from the kidney.