Location of the kidney
(This is important to keep in mind, in case you get a question involving trauma. You must know the relations and spinal level of each organ)
The kidneys are located high in the abdominal cavity, one on each side of the spine.
- The left kidney is approximately at the vertebral level T12 to L3, and the right is slightly lower than the left one, because of the position of the liver.
- the right kidney is situated just below the diaphragm and posterior to the liver.
- The left kidney is situated below the diaphragm and posterior to the spleen.
- On top of each kidney is an adrenal gland.
- The upper parts of the kidneys are partially protected by the 11th and 12th ribs.
- Each kidney, with its adrenal gland is surrounded by two layers of fat:
- The perinephric fat present between renal fascia and renal capsule- It is covered by a fibrous capsule composed of dense, irregular connective tissue that helps to hold their shape and protect them.
- The perinephric fat: A collection of adipose superior to the renal fascia.
There are three distinct regions of the kidney: the cortex, the medulla, and the renal pelvis.
Major and minor calyces collect urine and empty it into the renal pelvis.
The functional unit of the kidney: The Nephron
The nephron is the functional unit of the kidney. Below is a brief description of the structure of the nephron, but each part of the Nephron will be examined in more detail in subsequent sections.
The nephron is the functional unit of the kidney. It consists of the renal corpuscles as well as the tubules (tubuli renales) and forms an ingenious filter system. In each kidney, there are approximately 1—1.5 million renal corpuscles. A corpuscle is composed of the following components:
The glomerulus is a tuft of capillaries, woven together by anastomoses. They are supplied with fresh blood by the incoming vas afferents. The blood leaves the glomerulus by the vas efferents.
The Bowman’s capsule surrounds the glomerulus and consists of two layers. The parietal layer, formed by a single-layer epithelium lying on a basal lamina, is located on the outside. The visceral layer, formed by so-called podocytes, is located on the inside. These are specialized cells with many processes lining the capillaries of the glomerulus. Both layers are connected at the vessel pole.
The mesangium consists of mesangium cells connected with gap junctions. They occupy the space in between the capillaries in the nephron. They are responsible for the formation of Extracellular Matrix (ECM) and of components of the glomerular basement membrane. They also have a phagocytic function and can contract when necessary to stabilize capillary walls.
The blood-urine-barrier: filter in the nephron
This important filter consists of 3 layers. Each layer refines the structure of the filter.
- The capillary endothelium: has small slits (100nm wide) and a strong anionic glycocalyx (glycoproteins & glycolipids).
- The glomerular basement membrane (GBM) : is formed by the capillaries’ and podocytes’ basal lamina.
- The podocytes form processes which form gaps (40nm).
- These pores are covered by nephrin.
- Podocytes also have a strong anionic glycocalyx facing the urinary side.
Molecules up to a size of 4 nm are filtered through these structures and sorted out based on their electrical charge. This way especially, proteins are prevented from entering into the urine. The ratio of the pressure of capillaries to the capsule chamber (55 mm Hg: 15 mm Hg) also plays a role in infiltration.
In general, the renal corpuscles are only located in the renal cortex. Nephrons may, however, be called cortical, mesocortical, juxtamedullary, depending on the position of the renal corpuscle. The blood that has to be filtered passes through the renal corpuscle and then the blood-urine barrier selects the filtrate that enters the Bowman space. From here the ultrafiltrate begins its process of water reabsorption and solute modification.
Schematic structure of the renal corpuscle:
A: Renal corpuscle; B: Proximal tubule; C: Distal convoluted tube; D: Juxtaglomerular apparatus 1: Basement membrane; 2: Bowman’s capsule, parietal layer; 3: Bowman’s capsule, visceral layer; 3a: Podocyte pedicels; 3b: Podocyte; 4: Bowman’s space (urinary space); 5a: Mesangium – intraglomerular cells; 5b: Mesangium – extraglomerular cells; 6: Juxtaglomerular cells; 7: Macula densa; 8: Myocytes (cells of smooth muscle); 9: Afferent arteriole; 10: Glomerulus capillaries; 11: Efferent arteriole
The renal tubules (tubuli) of the nephron
The proximal tubule begins with the convoluted pars contorta and changes into the distal straight pars recta. In this part, 80 % of the primary urine’s water, glucose, amino acids (Na+symport), electrolytes and urea are reabsorbed. This task is carried out by the cubic epithelium studded with a dense brush border.
The cells are connected and tightly sealed by tight junctions and zonulae adherens. The cells appear dark under the microscope. They are rich in mitochondria, vesicles for endocytosis, lysosomes, and peroxisomes. In this part of the renal tubules, calcidiol is converted to calcitriol under the influence of the hormone PTH.
The intermediary tubule, however, shows a flat epithelium that hardly features any vesicles or other of the aforementioned structures.
In the following distal tubule, especially NaCl is absorbed, but only a little water. It is a flatter cubic epithelium with many Na/K-ATPases and many tight junctions. The distal tubule is also divided into the pars recta with the intermediary macula densa (which is next to the extraglomerular mesangium) and the pars contorta.
The loop of Henle sits between the proximal and distal tubules and refers to the part of the renal tubule that comprises the intermediate tubule and the straight parts of the proximal and of the distal tubule.
The distal tubule merges into the junctional tubule and finally into the collecting tubule
The collecting tubule consists of:
- Principal cells: They are cubic, the cytoplasm appears pale. Water can be reabsorbed from the urine under the influence of the hormone ADH. It is transported from cell to cell by aquaporins. The hormone aldosterone also works here in order to absorb sodium.
- Intercalated cells – type A and B: These cells are used for the regulation of protons and potassium. Therefore, they have many H+-ATPases and H+/K+-ATPases. They are darker than the principal cells. Subsequently, several collecting tubules unite and form the ductus papillaris which then runs to the kidney pelvis.
The juxtaglomerular apparatus is a distinctive structure in the kidney and acts as a local controller for the NaCl-concentration and as a regulator for the blood pressure in the body. It consists of many different cells.
The juxtaglomerular cells lie between the endothelium and the media of the vas afferents. They react upon activation of the sympathetic system (e.g. hypovolemia) and upon drops in pressure which leads to the secretion of renin (renin ? angiotensinogen ? angiotensin I ? angiotensin II (by ACE) ? leads to an increase in aldosterone secretion in the adrenal glands and vasoconstriction ? higher blood pressure).
The palisade shaped cells of the macula densa belong to the cells of the distal tubule. They are located next to the vas afferens and continuously measure the concentration of intraluminal sodium. If detecting a rash of sodium, the vas afferens contracts.
The extraglomerular mesangium cells (Goormaghtigh cells) are located between the juxtaglomerular cells and the macula densa cells.
The Interstitium
It consists of connective tissue and free cells as well as nerves, vessels, and peritubular fibroblasts. These cells produce erythropoietin, the growth hormone for erythrocytes when the oxygen in the blood is low.
There are two types of Nephrons:
- Cortical Nephrons:
- Glomeruli in the outer cortex
- Short loops of Henley that extend to the outer Medulla
- involved in solute reabsorption
- No vasa recta
- 85- 90 % of all nephrons
- Juxtameduallry Nephrons:
- Glomeruli in the inner cortex
- Long loops of Henley that extend to the inner medulla
- Responsible for the development of the osmotic gradients in the renal
- Medulla, which are used to concentrate urine, as it is surrounded by the vasa recta
- Involved in solute reabsorption
- 10-15% of all neurons
Structure & Histology