Kidney Anatomy / Function / Physiology

The kidneys are paired organs that lie outside the peritoneal cavity in the posterior abdominal wall, one on each side of the vertebral column. There is a deep fissure, the hilum, on the medial border of the kidney. The renal vessels and nerves pass through the hilum and in which lies the renal pelvis, the funnel-shaped continuation of the upper end of the ureter. The outer convex border of the renal pelvis is divided into major calyxes, each of which subdivides into several small calyxes. Each minor calyxe is cupped around the projecting apex of a cone-shaped mass of tissue, a renal pyramid. When the kidney is bisected from top to bottom, it is divided into two major regions : - an inner renal medulla and an outer renal cortex.

Each kidney is made up of approximately one million tiny units called nephron. Each nephron consists of a filtering component, called the renal corpuscle, and a tubule extending from the renal corpuscle.

The functions of the kidney are:

The glomerular filtration rate of kidneys varies with age and sex but is approximately 120 - 130 ml/min per 1.73 m2 surface area in adults. This mean that each day ultrafiltration of between 170 and 180 liter of water and unbound small-molecular-weight constituents of blood occurs . It would be necessary to ingest large amounts of water and electrolytes to stay in balance if these large volumes of ultrafiltrate were excreted unchanged as urine. This is prevented by the selective reabsorption or water, essential electrolytes and other blood constituents, like glucose and amino acids, from the filtrate in transit along the nephron. 60 - 80% of filtered water and sodium are reabsorbed in the proximal tubule along with virtually all the potassium, bicarbonate, glucose and amino acids. Further water and sodium chloride are reabsorbed more distally, and the fine tuning of salt and water balance is achieved in the distal and collecting tubules under the influence of anti-diuretic hormone and aldosterone. The final volume of urine is thus 1 - 2 litre per day. Calcium, phosphate and magnesium are selectively reabsorbed in proportion to need to maintain a normal electrolyte composition of body fluids.

Common Kidney Diseases and Their Presentation

A) Glomerulonephritis

Glomerulonephritis is a general term for a group of disorders in which the kidney are involved symmetrically by immunologically mediated injury to glomeruli. The pathogenesis of such immunologically mediated injury are deposition or in-situ formation of immune complexes and deposition of antiglomerular basement membrane antibody, both of which can activate secondary mechanisms that produce glomerular damage by complement activation, fibrin deposition, platelet aggregation, activation of kinin systems or inflammation with neutrophil-dependent mechanism.

It can present in the following ways :

B) Diabetic Nephropathy

Diabetes mellitus is a disorder characterized by chronic hyperglycemia due to relative insulin deficiency, or resistance or both. There are different types of diabetes. The most common ones are : type I, which is caused by an inability to produce insulin by the pancreas, and type II, in which the pancreas produces close to normal amounts of insulin, but the body is unable to use it properly.

Diabetes damages small blood vessels throughout the body, affecting the kidney as well as other organs and tissues including skin, nerve, muscles, intestines and heart. Diabetes is one of the leading causes of chronic renal failure in Hong Kong. The risk of developing diabetic nephropathy increases with the duration of diabetes.

There are some warning signs for the development of diabetic nephropathy :

(C) Hypertension

High blood pressure usually causes no symptoms at all. But some patients may experience dizziness, headache or nose bleeds. So it is important to have regular checkups to detect high blood pressure. High blood pressure can affect anyone at any age. It can affect children although it is less common.

High blood pressure can severely harm the kidneys. Severe high blood pressure causes kidney malfunction over a relatively short period of time whereas mild forms of high blood pressure can damage kidney over several years, with no symptoms evident until severe damage has already occurred.

(D) Urinary Tract Infection

It is common in women, uncommon in men and of special importance in children. Recurrent infection causes considerable morbidity and it may cause severe renal disease including end-stage renal failure. It is also a common source of life-threatening Gram-negative septicemia.

It is usually due to bacteria from the patient's own bowel flora. Transfer to the urinary tract is usually via the ascending transurethral route. It may also be via the bloodstream, the lymphatics or by direct extension.

The most typical symptoms are frequency of micturition by day and night, painful voiding, suprapubic pain and tenderness, haematuria and smelly urine.

(E) Hereditary Renal Disease

Renal disease can be inherited. One example is autosomal-dominant polycystic kidney disease (ADPKD). It usually presents in adult life and it is characterised by the development of multiple renal cysts, variably associated with extrarenal abnormalities. ADPKD is the most common inherited renal disease.

It may present as :

The natural history is one of progressive renal impairment and usually associated with the development of hypertension. The rate of progresson to renal failure is variable.

(F) Renal Stone

Renal stones may be single or multiple and vary enormously in size form minute, sand-like particles to staghorn calculi or large stone concretions in the bladder. They can be located within the renal parenchyma or within the collecting system.

Most people with urinary tract stones are asymptomatic. Pain is the most frequent symptom and the pain can be sharp or dull, constant, intermittent or colicky. The other manifestations include haematuria, urinary tract infection or urinary tract obstruction.

General Treatment

The underlying causes of the different kidney diseases should be treated accordingly to prevent their progression. For example, tight blood pressure and blood sugar control can prevent or retard the progression of diabetic nephropathy.

Before the patients having end-stage renal failure, there are medical therapies. Diuretics can be given to excrete the excessive body fluid and anti-hypertensive drugs can control the blood pressure. The renal patients also need to have diet control.

When the patients are in end-stage renal failure, they need renal replacement therapy. The aim of all renal replacement technique is to mimic the excretory functions of the normal kidney, including excretion of nitrogenous wastes, maintenance of normal electrolyte concentrations and maintenance of a normal extracellular volume.

There are different modes of renal replacement therapy :

(i) Haemodialysis

In haemodialysis, blood from the patient is pumped through an assay of semi-permeable membrane (the dialyser, often called an 'artificial kidney') which brings the blood into close contact with dialysate, flowing countercurrent to the blood. The plasma biochemistry changes towards that of the dialysate due to diffusion of molecules down their concentration gradients.

(ii) Peritoneal dialysis

Peritoneal dialysis utilizes the peritoneal membrane as a semi-permeable membrane, avoiding the need for extra-corporeal circulation of blood. A tube is placed into the peritoneal cavity through the anterior abdominal wall. Dialysate is run into the peritoneal cavity. Urea, creatinine, phosphate and other uraemic toxins pass into the dialysate down their concentratrion gradients. Water is attracted into the peritoneal cavity by osmosis, depending on the osmolality of the dialysate. The fluid is changed regularly to repeat the process.

(iii) Renal Transplantation

It offers the potential for complete rehabilitation in end stage renal failure. It allows freedom from dietary and fluid restriction, anaemia and infertility are corrected. The technique involves the anastomosis of a transplanted human kidney, either from a cadaveric donar or a living close relative, on to the iliac vessels of the recipient. The donar ureter is placed into the recipient bladder. The recipients need to take long term immunosuppressive drugs to prevent rejection.

Prevention of Kidney Disease