The nephrons are real power houses. They perform many of the tasks of the kidneys. The kidneys are involved in a variety of processes, as the following list shows:
1) Excretion of urinary substances – elimination function
The kidneys constantly filter the blood of metabolic degradation products and foreign substances (e.g. medication) and excrete these via the urine. These urinary substances include substances that no longer have any function for the body, but also toxins and toxic degradation products that arise from protein metabolism, for example. If excretion is reduced, these uraemic toxins can accumulate in the blood and then directly damage the kidneys, blood vessels and bone metabolism.
In the renal corpuscles, substances that are small and uncharged are filtered out of the vascular tangle (= glomerulum) with a lot of water. A selection by size and charge therefore takes place here, which prevents important blood cells or proteins from being filtered out, for example. The filtered substances dissolved in water enter the Bowman capsule surrounding the glomerulus and flow on into the tubular system. In the tubular system, larger substances and/or charged substances such as uraemic toxins or medication are then actively excreted into the urine. On the other hand, the tubular system also recovers water (about 99% of the filtered water is recovered) and filtered electrolytes and sugars (glycose) or smaller proteins.
2) Water and electrolyte balance
The kidneys regulate the excretion of water and electrolytes/minerals* and thus balance the amount of water in the body. Electrolytes are filtered out together with water as a means of transport in the glomerulus. Water and electrolytes/minerals are then recovered in the tubular system, which is very finely regulated. Excess remains in the urine (e.g. phosphate) and is excreted, but substances that are important for the body are selectively recovered. The selection and fine regulation does not take place during filtration from the glomerulus, but during recovery in the tubular system, where hormone-controlled transport systems regulate recovery to some extent. There is a transport system for every important substance.
Nephrons are true water power stations. For example, urine production can be reduced if the amount of water drunk is reduced. As over 75% of the body is made up of water and even a 2% reduction in body fluid is dangerous, it becomes clear what a finely tuned task the kidneys have in regulating and maintaining the water and electrolyte balance. As a rule, 99% of the water filtered out is recovered so that the urine becomes more and more concentrated. Cats have an enormously high urine concentration capacity. This ability to concentrate is lost in the course of CKD and is visible in a reduced specific urine weight.
*Electrolytes are minerals or mineral substances such as sodium, potassium, chloride and calcium that can conduct electricity in an aqueous solution. They can be present as bases, acids or salts….
3) Regulation of blood pressure
The kidneys also have an influence on blood pressure via the amount of water and electrolytes. Above all, however, the kidneys are involved in regulating blood pressure by producing the hormone renin.
4) Regulation of the acid-base balance
The kidneys can excrete more acids and bases as required and thus regulate the blood pH value, which is necessary to maintain metabolic activity. Many enzyme activities are linked to the blood pH value.
5) Formation of hormone-active substances
a) Erythropoietin
Erythropoietin, also known as EPO, is a growth factor that supports the formation of red blood cells (= erythrocytes). Erythrocytes contain the blood pigment haemoglobin, to which oxygen is bound in the lungs and transported to the cells. Haemoglobin also absorbs carbon dioxide and transports it to the lungs, where it is exhaled. Red blood cells are mainly produced in the bone marrow of larger bones.
Biotechnologically produced EPO is used for anaemia, for example in chronic kidney disease, kidney failure or after chemotherapy.
The reduction in erythropoietin production associated with late-stage CKD can lead to anaemia. Anaemia is the term used when there is too little red blood pigment (haemoglobin). As this occurs in the red blood cells (erythrocytes), a lack of red blood cells is often associated with a lack of blood pigment.
b) Renin
Renin controls blood pressure. It is a hormone-like enzyme that is produced and stored in the kidneys. When blood pressure drops, it is released from the kidneys and leads to an increase in blood pressure via a hormonal system. Behind the renin-angiotensin-aldosterone system (= RAAS) is a finely tuned mechanism that reacts to changes in blood pressure and the water and electrolyte balance.
If the blood pressure drops, the kidneys release renin, which splits angiotensinogen into angiotensin I in the liver. Angiotensin I is in turn converted into angiotensin II in the liver by the enzyme ACE (= angiotensin converting enzyme). The hormone angiotensin II has a strong vasoconstrictive effect (= vasoconstriction), which leads to an increase in blood pressure and simultaneously promotes the release of aldosterone from the adrenal cortex. Aldosterone causes an increased recovery of water and sodium in the kidneys and an increased excretion of potassium. The effect of aldosterone also leads to an increase in blood pressure.
A possible disruption of renin formation in CKD can be associated with high blood pressure.
c) Calcitriol
Calcitriol is the active form of vitamin D3. It is formed in the kidneys from vitamin D in order to maintain the calcium level in the blood. Calcitriol is significantly involved in mineral metabolism and bone mineralisation. It ensures that more calcium and phosphate are absorbed from the intestine and at the same time reduces calcium excretion via the urine. This increases the calcium level in the blood. At the same time, calcitriol reduces the release of parathyroid hormone.
Calcitriol is activated independently of each other by 3 factors: increased parathyroid hormone levels, reduced calcium levels or reduced phosphate levels.
Reduced calcitriol (vitamin D) production in CKD can lead to secondary hyperparathyroidism (= excessive release of parathyroid hormone from the parathyroid gland) and thus also to a mineral and bone disease associated with CKD (= CKD-MBD). In the late stages of CKD, almost all cats are affected. In the early stages, it already affects half of CKD cats.
