The Kidneys

Kidney and its functions

Kidneys and its function:

The pair of organs called kidney situated at the back of the abdomen in the human body. It has the length of about four to five inches each.

The kidneys’ function is to filter the blood. All of the blood in our bodies passes by way of the kidneys a number of occasions a day. The kidneys take away wastes, management the physique’s fluid stability, and regulate the stability of electrolytes. Because the kidneys filter blood, they create urine, which collects within the kidneys’ pelvis — funnel-shaped constructions that drain down tubes known as ureters to the bladder.

Every kidney incorporates round 1,000,000 models known as nephrons, every of which is a microscopic filter for blood. It is attainable to lose as a lot as 90% of kidney function with out experiencing any signs or issues.

The kidneys perform 2 major functions.
1. Excretion of waste products of the body.
2. To control and regulate the excretion in urine of many constituents of the body so as to maintain the correct internal environment. In other words to ensure that the body fluids and tissues have the correct concentration of every constituent. Functionally the kidney is made up of units called nephrons.

Kidneys and their function

1. Nephron

Each nephron consists of the glomerulus and tubules.

The Nephron

Glomerulus

1. The glomerulus consists of arteries and capillaries formed by branching of the renal artery. There are about 50 capillaries in each glomerulus.
2. Each glomerulus is encased in a capsule which collects the filtered fluid from the glomerulus. (Glomerular capsule syn. Bowman’s capsule)
3. The liquid filtered through the glomerulus is called the glomerular filtrate. The amount of glomerular filtrate is approximately 170 liters/day. It contains water, Na+ K+. glucose and waste products like urea, uric acid, creatinine. The rate of glomerular filtration is called the glomerular filtration rate (GER).
4. The glomerular filtrate passes down the Bowman’s capsule to the tubules. The blood in the glomerular capillaries does not return directly to the renal vein, The capillaries form a network around the tubules, before returning to the renal veins.
5. If because of diminished renal blood flow, the GFR falls, waste products accumulate in the body—uremia.
6. GFR may fall because of fall in blood pressure, heart failure or fall in blood volume.

Tubules

1. The tubule is as important as the glomerulus.
2. During the passage downwards of the glomerular filtrate, the tubule reabsorbs substances. About 168.5 liters of water is reabsorbed out of the total glomerular filtrate of 170 liters giving a daily urine output of approx. 1.5 liters, If more water is drunk, the urine output is more. If less water is drunk, the urine output is less, The tubule, therefore, adjusts the urine volume.
3. The tubule also selectively reabsorbs the correct quantities of Na+, and other electrolytes, so that the body electrolyte contents are constant.
4. Glucose and proteins are also reabsorbed.
5. Urinary waste products are not reabsorbed.
6. The adjustment of acid-base balance of the body by the tubule is very important. By adjusting the excretion of acidic or basic (alkaline) salts selectively, the tubules adjust the acidity/alkalinity of the blood. The pH of blood is maintained at 7.2-7.4.
7. This is achieved by excreting selectively the salts of Na+ such as bicarbonates, phosphates, and also ammonia.
8. When the kidney fails following changes are produced:
(a) Waste products are retained—uremia.
(b) Acid-base balance is lost and the pH of the blood acidic.
(c) Water balance is lost.
(d) The patient ultimately dies.
9. Fortunately, patients with kidney failure can be rescued by using an artificial kidney called the dialyzer.
10. On a more permanent basis, the diseased kidneys are replaced by a kidney transplant
11. Most drugs are excreted by kidneys. In kidney failure, such drugs may accumulate in the body and cause toxicity. It is because of this, that some drugs may be contraindicated in kidney failure or the dose of these drugs reduced.
12. Some drugs can themselves damage the kidneys. They are called nephrotoxic drugs.
13. Certain diseases damage the kidneys, e.g. high BP, diabetes.
14. Kidney function may be assessed by estimating the level of waste products in the blood e.g. urea, creatinine. The glomerular function can be specifically determined by carrying out clearance tests which measure glomerular filtration.

2. Glomerular Filtration Rate (GFR)

Glomerular filtration determines the amount of water that passes down the Bowman’s capsule to the tubules. A small increase in GFR will give rise to a large increase in the volume of urine. A small fall in GFR will result in a substantial fall in the volume of urine. To prevent such variations there is an auto-regulating mechanism. The function of the autoregulating mechanism is to keep the GFR constant in spite of a rise or fall of blood pressure.

The Glomerulus and Capsule
1. If the BP falls, the glomerular capillary pressure could be expected to fall and consequently, the GFR would fall. This is prevented by contraction of the efferent arteriole which by blocking the escape of blood raises the glomerular capi11ary pressure, thus increasing GFR. The expected fall of GFRis prevented.
2. 1f the BP rises, the efferent arteriole re1axes lowering glomerular capillary pressure thus reducing the GFR which would otherwise have resulted from an increase in the glomerular capillary pressure. Thus, the GFR is kept constant.
3. The chemical substance which controls the state of contraction of the efferent arteriole is angiotensin II. Drugs which inhibit the formation of angiotensin Il (e.g. ACE inhibitors) may reduce the GFR and cause kidney failure. These drugs are contraindicated in patients who have reduced renal blood flow.
4. The volume of glomerular blood flow is regulated by the state of contraction of the afferent arteriole which is under the control of Various substances such as catecholamines and angiotensin II.
5. There is a collection of special cells next to the glomerular capillary which detects changes of pressure in the glomerular capillary. This is called the juxtamedullary apparatus.

3. Renal Mechanism for Excreting Dilute Urine

1. The total volume of urine which is excreted must be very precise so that the blood volume neither rises nor falls. Too large a volume of urine will reduce the blood volume and too little a volume of urine will increase the blood volume.
2. The volume of urine formed is regulated by the antidiuretic hormone (ADH) produced by the posterior pituitary gland in the brain. ADH inhibits excretion of water, by which mechanism it maintains a constant blood volume.

4. Renal Mechanism for Sodium and Potassium Excretion

1. A hormone called aldosterone produced in the adrenal gland controls the amount of sodium and potassium which must be excreted to maintain the body sodium and potassium concentration at a constant level.
2. Aldosterone increases excretion of potassium and causes retention of sodium in the body. By controlling the amount of aldosterone the amount Of sodium and potassium excreted can be controlled. This action of aldosterone is mediated via its effect on tubules of the kidney.

5. Renal Mechanism for Regulation of Acid-Base Balance

1. The pH (acidity/alkalinity) of blood must be maintained precisely at 7.4.
2. Any rise or fall may result in death.
3. The Principal organ for maintenance of the pH of blood at 7.4, is the kidney.
4. Normally there is a tendency for the blood to become acidic (because of metabolites). This is prevented by the excretion of an acidic urine. However if the blood tends to become alkaline, the kidney excretes alkaline urine to correct the pH of blood.
5. The 2 mechanisms by which the kidney adjusts the urine pH and thereby the blood pH are as follows:-
(a) The bicarbonate buffer system. The bicarbonate ion (HCO3) can be excreted by the kidney, either as H.HCO3 (carbonic acid) or as NaHCO3 (sodium bicarbonate — alkaline). By adjusting the excretion of bicarbonate either as carbonic acid or sodium bicarbonate, the kidney can excrete either an acid or alkaline urine, thereby maintaining a constant pH in blood.
(b) The phosphate buffer system. The phosphate ion (HPO4) can be excreted by the kidney either as Na.H.HPO4 (mono sodium dihydrogen phosphate acidic) or Na2HPO4 (disodium mono hydrogen phosphate alkaline). By this mechanism the kidney can excrete an acid or alkaline urine, thereby maintaining the correct pH in blood.

6. The Urinary Passage

1. The urine formed in the kidneys passes down the ureters—long tubes formed of smooth muscle. Ureters may be blocked by stones passing down from the kidneys. During their downward passage, the affected ureter may go into spasm and give rise to ureteral colic.
2. Gravel made up of oxalates, phosphates and other salts may also cause colic.
3. The ureters empty into the urinary bladder which is a muscular organ with a capacity of 300-500ml. When the bladder is full there is an urge to micturate or pass urine.
4. The sphincter (pinchcock) at the mouth of the bladder opens the bladder muscle contracts and urine is propelled down the last part of the urinary passage called the urethra.
5. The bladder is vulnerable to the direction (cystitis). It is also prone to cancer.
6. The important thing to remember is that the urinary bladder is under voluntary control. In injuries to the nervous system, control over the bladder may be lost. The bladder fills but does not contract and void urine. The urine dribbles out continuously, It becomes a great problem in nursing management.
7. The urethra is 8 inches long in males. It passes through the prostate gland. When the prostate becomes large it presses on the urethra and obstructs the free flow of urine. The bladder then Overfills and urine may dribble retention with overflow. The bladder lining may be irritated giving rise to frequency. Prostatic enlargement is common in men over 55 years. Treatment is essentially surgical.
8. In women, the short urethra (1 1/2 inches) makes them vulnerable to the ascent of bacteria (and infection) from outside into the bladder. This is why women suffer from urinary infections more than men.

Respiratory System
The special senses

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