The Anatomical Structure, Physiology, Endocrine And Clinical Function Of The Kidney
Position Of The Kidney In The Human Body
Diagram Of A Pair Of Kidneys
Location Of The Kidney In The Human Body
The Anatomy Structure Of The Kidney
The Kidney play a major role in maintaining the internal environment (mileu interior) of the body. The advent of electron microscopy, immunofluorescence studies, dialysis and renal transplantation have considerably advanced the knowledge about renal structure and function in health and disease. The Kidney measures approximately 12 cm in length, 6cm in breadth and 3 cm in thickness and it ranges in weight from 120-170g. Section reveals an outer zone which is the medulla. The cortico-medullary junction is indistinct. The renal papillae project into the renal pelvis. The pelvis takes origin from the major calyces which are formed by fusion of 2 or 3 minor calyces. Each Kidney consists of about a million nephrons which are the functional units. At the hilum, the renal artery enters the kidney and the renal vein and lymphatics leave the organ.
The spatial arrangement of different parts of the nephrons in different zones of the renal substance plays an important role in their function. On a functional basis, the nephrons may be divided into the glomerulus, proximal convoluted tubule and collecting ducts.
The glomerulus: It consists of a central mesangial stalk surrounded by a capillary network. The Bowman’s capsule encircles the capillary network all around except for the space for the entry and exit of the afferent and efferent arterioles. The wall of the capillary loop is lined by endothelial cells, basement membrane and specialized epithelial cells which have projections called foot processes. Under the electron microscope, small fenestrations can be identified in the endothelial cell cytoplasm. The foot processes of epithelial cells seem to rest on the epithelial side of the glomerular basement membranes. The glomerulus acts as an ultrafilter, filtering about 120 ml of filtrate from nearly 1500 ml of blood passing through it every minute. The ultrafiltrate contains all the solutes in the blood such as glucose, sodium, potassium, calcium, bicarbonate, phosphate and the small and medium molecular weight substances such as urea, creatinine, uric acid, aminoacids etc.
The Proximal tubule: It is lined by columnar cells with brush border towards the lumen. The cells contain several mitochondria and other organelles and are metabolically very active. About 60-70% of the filtrate and solutes are reabsorbed here. Glucose, bicarbonate, amino acids and phosphage are nearly completely reabsorbed.
Physiologic And Endocrine Functions
From previous article microscopic components of the kidney such as the glomerulus and proximal convoluted tubule have been discussed. This segment will focus on the limbs and the endocrine function of the kidney.
The descending limb of the loop of Henle: It is lined by cells which are freely permeable to water and urea.
The ascending Limb: It is lined by small columnar cells which actively pump out sodium and chloride from the lumen. This segment is impermeable to water. Isotonic fluid which enters the decending limb of the loop of Henle becomes hypertonic by the time it reaches the tip of the loop. This is due to the water reabsorption. As it traverses the ascending limb, the active chloride and sodium transport makes the fluid markedly hypotonic. This hypotonic fluid enters the distal tubule. On account of these phenomena, the medullary interstitium is maintained in a hypertonic state and this plays an important role in concentrating the tubular fluid. The cells of the distal convoluted tubule do not have brush border. The reabsorption of sodium and water in the distal convoluted tubule are under the control of aldosterone and antiduiretic hormone respectively.
The distal tubule also secretes ammonia into the tubular lumen thus helping to acidify the urine and excrete acid ions. Reabsorption of water and final adjustments in the concentration of urine are done in the distal tubule and collecting duct.
Out of nearly 120ml of ultrafiltrate, only 1ml is excreted as urine. Water, solutes and other substances necessary for metabolism are reabsorbed whereas waste products such as urea, creatinine and uric acid are excreted.
The justaglomerular apparatus: It is a functionally important component of the renal vasculature. It consists of the afferent and efferent arterioles adjacent to their respective glomeruli, the macula densa of the corresponding distal tubule, and a collection of cells that lie near the glomerular hilum between the afferent and efferent arterioles. The myoepithelial cells are the major source of rennin which is released from them. Renin is concerned with the regulation of blood pressure.
The kidney also functions as an endocrine organ and is responsible for the production of hormones which have local and systemic effects. In addition to rennin, four other renal hormones are recognized. The conversion of 25- hydroxycholecalciferol (25 OH D3) to its active form 1,25-dihydroxycholecalciferol (1,25 (OH)2 D3) occurs in the kidney.
Erythropoietin produced in the kidney is a powerful stimulant for erythropoiesis. The renal prostaglandins and kallikreins are the other important putative hormones of renal medullary system.
© 2014 Funom Theophilus Makama