PHYSIOLOGY OF THE KIDNEY, Schemes and Mind Maps of Physiology

Download PHYSIOLOGY OF THE KIDNEY and more Schemes and Mind Maps Physiology in PDF only on Docsity! PHYSIOLOGY OF THE KIDNEY Dr. P. Stewart Sydney Australia The Functions of the Kidney l Regulation of the water and electrolyte content of the body. l Retention of substances vital to the body such as protein and glucose l Maintenance of acid/base balance. l Excretion of waste products, water soluble toxic substances and drugs. l Endocrine functions. Regulation of the water and electrolyte content of the body The kidney allows a person to eat and drink according to their habits without changing the composition of their fluid compartments. Renal Blood Supply is normally is about 20% of the cardiac output. Approximately 99% of the blood flow goes to the cortex and 1% to the medulla. The cortex is the outer part of the kidney containing most of the nephrons. The medulla is the inner part of the kidney and contains the specialised nephrons in the juxta-medullary region, immediately next to a 45 degree caudad angle (towards the feet) and slightly posterior angle. The angle of approach is important to avoid accidental intravascular or intrathecal injection. The needle is then advanced carefully until a paraesthesia is elicited. A click may be detected as the needle passes through the prevertebral fascia. This usually occurs at the superficial level. The use of a nerve stimulator with a special insulated needle is very helpful in confirming the correct placement of the needle and performing the interscalene block accurately. Correct stimulation produces twitching below the shoulder. Stimulation of the diaphragm indicates too anterior an approach. Once paraesthesia is obtained, the needle is stabilised and after negative aspiration for blood, 20 to 30mls of the local anaesthetic solution is injected slowly and carefully. Local anaesthetic solution Bupivacaine 0.375- 0.5% solution may be used safely in the volumes between 20-40mls, but the maximum dose of 2 mg/ Update in Anaesthesia24 kg should not exceeded. Other local anaesthetic agents like lignocaine or prilocaine may be used. Complications l Inadvertent epidural or subarachnoid injection is a potentially serious complication resulting from incorrect needle placement. l Vertebral artery injection, this can result in convulsions and loss of consciousness. l Phrenic nerve block is frequently produced, this complication precludes bilateral use of this technique. l Recurrent laryngeal, vagus, and cervical sympathetic nerves are sometimes blocked. l Pneumothorax is rare but can happen with deep placement of the needle and in unskilled hands. the medulla. These nephrons have a greater concentrating ability, the mechanism being explained below. The kidney is unique as it has two capillary beds arranged in series, the glomerular capillaries which are under high pressure for filtering, and the peritubular capillaries which are situated around the tubule and are at low pressure (figure 1). This permits large volumes of fluid to be filtered and reabsorbed. The Nephron: Each kidney consists of about one million nephrons. The nephron is made up of a glomerulus and its tubule (figure2). The tubule is Peritubular Capillaries Efferent arteriole Bowman's Capsule Glomerulus Afferent arteriole made up of a number of sections, the proximal tubule, the medullary loop (loop of Henle), and the distal tubule which finally empties into the collecting duct. Urine is formed as a result of a three phase process - simple filtration, selective and passive reabsorption and excretion. Filtration Filtration takes place through the semipermeable walls of the glomerular capillaries which are almost impermeable to proteins and large molecules. The filtrate is thus virtually free of protein and has no cellular elements. The glomerular filtrate is formed by squeezing fluid through the glomerular capillary bed. The driving hydrostatic pressure (head of pressure) is controlled by the afferent and efferent arterioles, and provided by arterial pressure. About 20% of renal plasma flow is filtered each minute (125 ml.min-1). This is the glomerular filtration rate (GFR). In order to keep the renal blood flow and GFR relatively constant hydrostatic pressure in the glomerulus has to be kept fairly constant. When there is a change in arterial blood pressure, there is constriction or dilatation of the afferent and efferent arterioles, the muscular walled vessels leading to and from each glomerulus. This process is called autoregulation. Autoregulation of GFR is achieved by Update in Anaesthesia 25 autoregulation of renal blood flow and a feedback mechanism known as “ glomerular tubular balance”. Glomerular Tubular balance. When there is a decrease in GFR, there is a resulting decrease in the fluid flow rate within the tubule. At the loop of Henle, there is greater time for reabsorption of sodium and chloride ions. Therefore there is a decrease in the number of sodium and chloride ions reaching the distal tubule which is detected by the macula densa. This in turn decreases the resistance in the afferent arteriole which results in an increase in renal blood flow. It also increases renin release from the juxtaglomerular apparatus which stimulates angiotensin II production causing constriction of the efferent arteriole. These both act to increase the hydrostatic pressure in the glomerular capillary bed and return GFR to normal (table 1). The juxtaglomerular complex consists of macula densa cells, which are special distal tubular epithelial cells which detect chloride concentration and modified smooth muscle cells, juxtaglomerular cells, in the walls of the afferent and efferent arteriole. These cells produce renin. Renin is an enzyme which converts the plasma protein angiotensinogen to angiotensin I. Angiotensin converting enzyme (ACE) which is formed in small Table 1 Proximal Tubule 'Bulk Reabsorber' Glomerlus (B) Loop of Henle Distal Tubule Hypertonic H yp ot on ic Urea H2O H 2 O H 2 O H 2 O H+ NaCI Na+ K+ (A) (C) Is ot on ic Nephron: Hypertonic H+K+ Na+CI- Collecting Duct Na+ Is ot on ic More hypotonic