EXCRETORY SYSTEM

The Excretory or Urinary system deals with the removal of waste products from the body, which if they are accumulated would prove harmful.

The Urinary system consists of

2 Kidneys – made up of Nephrons and collecting tubules

2 Ureters convey urine to bladder

1 Urinary bladder to collect and store urine

1 Urethra through which urine passes from bladder to the outside

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THE KIDNEYS

Each kidney is a bean shaped organ approximately 4.5" long, 2" broad and 1.25" thick. They are made up of three areas, the fibrous capsule surrounding the kidneys, the cortex which is a layer of tissue immediately under the capsule and between the pyramids and the medulla ( the innermost layer consisting of the renal pyramids ). The kidney tissue is made up of numerous renal tubules called nephrons. Each nephron consists of a structure called the capsules of Bowman . It is into these capsules that a large quantity of water, salts, urea and glucose is filtered.

Each capsule consists of :

A BOWMANS CAPSULE containing a network of blood capillaries (glomerulus). This leads into the FIRST CONVOLUTED TUBULE, the LOOP OF HENLE, a long U shaped tube; the SECOND CONVOLUTED TUBULE leading into the COLLECTING DUCT which received the urine from several tubules.

The BOWMANS CAPSULES and CONVOLUTED tubules are situated in the outer part of the kidney called the CORTEX.

The LOOPS OF HENLE and COLLECTING DUCTS are situated in the inner part of the kidney called the MEDULLA, arranged to form the PYRAMIDS.

The COLLECTING DUCTS all open into the smaller cavities called CALYCES and these pass the urine into THE PELVIS and down the URETERS into the BLADDER.

The kidneys have a good blood supply . Approximately 1200 litres of blood pass through the kidneys daily and about 110 litres of filtrate is formed. Nearly all of this is reabsorbed leaving just 1 litre of urine which is the average amount of urine passed each day..The blood is supplied via the Renal artery and within the kidney each tubule receives a good blood supply via smaller ARTERIOLES and CAPILLARIES. Blood passes out of the kidney in the RENAL VEIN. This VENOUS blood will contain no urea, less water and salts.

THE RENAL PELVIS is the funnel shaped structure which acts as a receptacle for the urine formed by the kidneys. It has a number of branches called calyces at its upper end. Urine passes through the renal pyramids into the calcyes before passing through the pelvis into the ureter.

Nerve supply to the kidneys consists of sympathetic and parasympathetic nerves. ( Autonomic nervous system ). Eg. Sympathetic nerve supply results in muscle tone of urethral sphincter being increased inhibiting micturition ( passing of urine ). Parasympathetic nerve supply results in relaxation of urethral sphincter muscles resulting in micturition.

The functions of the kidneys are to:-

Maintain the water balance of the body

Maintain the alkalinity of blood

Excrete toxins and drugs

Form urine

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URETERS

These are about 25-30 cm long, 3mm in diameter and continuous with the renal pelvis. They pass through the posterior wall of the bladder. They convey urine from the kidneys to the urinary bladder by peristaltic contraction of the muscular wall. These contractions occur at about 10 second intervals, sending little spurts of urine into the bladder.

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BLADDER

The bladder lies in the pelvic cavity and its size and position vary depending on the amount of urine it contains. When distended, it rises into the abdominal cavity. Where the Urethra commences. A thickening of smooth muscle layer acts as a sphincter and controls the passage of urine from the bladder into the Urethra. The urinary bladder is a resevoir for urine. When 200/300 mls of urine have been accumulated autonomic nerve fibres in the bladder wall, sensitive to stretch, are stimulated. In a child this results in micturition. In an adult, Micturition reflex is stimulated but sensory impulses pass upwards to the brain and although there is a desire to urinate, by conscious effort reflex contraction of the bladder can be inhibited for a limited amount of time.

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URETHRA

The urethra is a canal extending from the neck of the bladder to the exterior. Its length differs in males and females. The male urethra is aproximately 19 – 20 cm long, passes through the prostate gland and eventually at the slit in the top of the penis. In the male it is also the common passage for semen. The female urethra is aproximately 4cm long and runs down behind the pubis symphysis and opens in front of the vagina. The fact that the female urethra is so short and opens into a relatively exposed area, explains why women are prone to urinary infections.

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HOW URINE IS FORMED

Glomerulus in Bowman's Capsule Brings blood from renal artery to each tubule.

Bowmans Capsult Urea, water, salts and glucose are filtered out from the plasma through the walls of the capsule into the 1st Convoluted tubule.

1st Convoluted Tubule Selective re-absorption takes place of any glucose, amino acids, some salts, water and Vitamin C into the plasma.

Loop of Henle Re-absorption of water as required.

2nd Convoluted Tubule Further re-absorption into plasma of water and salts as required to maintain a balance in the blood. This is controlled by the hormone ADT from the pitutary gland. At this point, the filtrate is now urine.

Collecting Duct Receives the urine from several tubule and passes it via the pelvis to the ureters which open into the bladder.

The composition of urine is :-

Water - approx. 96%

Urea - approx 2%

Uric acid (Ammonia ) - approx 2%

Various salts)

The salts consist of mainly phosphates, sulphates and sodium chloride. The analysis of urine can indicate many imbalances in the body. Eg. Diabetes, pregnancy and alcohol levels.

Summary of waste products.

PRODUCTS SOURCE HOW REMOVED

Excess heat Cell respiration and Faeces,urine, sweating

Muscular activity Expired air

Carbon Dioxide Cell respiration Expired air

Excess water Cell respiration Urine, sweat,expired air

Intake through food

and drink

Nitrogenous Breakdown of unwanted Urine

Waste e.g. Urea amino acids in Liver Some in sweat

(Deamination)

Salts Intake of food Urine,faeces,sweat

Faeces may contain some nitrogenous waste products but the bulk is undigested and/or indigestible food material.

Thus the Respiratory System, Skin and Urinary system play an important part in the Excretion

The Urinary System is a very important system to balance the composition of the blood and the tissue fluid. The urinary system has the following functions.

  1. To regulate salt and water balance
  2. To remove nitrogenous waste e.g. urea

1.To remove excess salts (sodium chloride) and other minerals from the body.

DISEASES AND DISORDERS OF THE URINARY SYSTEM

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Cystitis – Inflamation of the bladder, more common in women.

THE REPRODUCTIVE SYSTEM

The female reproductive system consists of :-

The Ovaries ( 2 ) – Made up of numerous small sacs ( the ovarian follicles ) each one containing and ovum or egg.

The functions of the Ovaries being to:-

Develop and expel the egg during a females reproductive life.

To produce the hormones Oestrogen and progesterone .

The Fallopian Tubes – To transport the Ova from the ovaries to the Uterus.

The Uterus ( or womb ) - The Uterus is wholly under the control of the sex hormones.During menstruation the lining of the Uterus is expelled, during pregnancy the Uterus grows out of the Pelvis to occupy much of the abdominal cavity and after delivery of the baby once the placenta has been expelled, the uterus returns to its normal size and position.

The Vagina – This is the passage leading from the Cervix to the Vulva, it is a sensitive organ containing erectile tissue.

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The Male Reproductive System consists of:-

The Testes – The male reproductive glands

The Scrotum – The bag like organ containing the Testes

The Penis – The male organ of coition containing the Urethra which is a dual organ with the Urinary system.

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