Classified in Biology

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HEART: Is a muscular, hollow organ. It is divided in four chambers, the upper part is called atria and it can(be left atrium or right atrium). And the lower part is called ventricle (and it can be left ventricle or right ventricle). Between the atrium and the ventricle there is the atrioventricular valve. This valve allows blood to flow from the atrium to the ventricle but not vice versa. The valve between the left atrium and ventricle is bicuspid and  right atr. And ventri. Is tricuspid.

BLOOD CIRC.: Atrial systole: Atria contract and the blood is pumped through to the ventricles via the open atrioventricular valves.
Ventricular systole: The ventricles contract, the sigmoid valves open, and the blood flows out into the pulmonary artery and aorta. At the same time, the valves between the ventricles and the atria close.
Diastole: Heart relaxes and grows blood from the vena cava and pulmonary veins. This blood flows in the atria. The blood that is pumped into the arteries during ventricular systole does not flow back into the ventricles thanks to the sigmoid valves. These valves, close and prevent blood from flowing back to the heart.

Double circuit: Systemic circ.: Blood starts from the left ventricle and flows along the aorta which enters into arteries, arterioles and finally capillaries. Bood goes to the alls tissues and organs except the lungs. 
Pulmonary circ.: Blood starts in the right ventricle when the pulmonary artery which goes to a lung. Then pass a dense network of capillaries surrounds the pulmonary alveoly. This gas exchanges takes places there.

EXCRETORY SYS. Is made up of the urinary system, sweat glands, liver and respiratory system. 
The kidneys: have a bean shaped form, are at the back of the abdomen. External part: have the renal cortex and the internal part is the renal medulla and the renal pelvis.
Urine is formed in the cortex and medulla. Each kidney is made up of over a million nephrons, that filter  the blood and produce urine. Neprhons are made up of Bowman's capsule, Convoluted tube and the Loop of Henle.

Urinary tracts. Ureters: There are two tubes around 28 cm long, that connect the renla pelvis of the kidney to the urinary bladder. The nervous stimulus that expels the urine is micturition reflex.
Urethra: This duct leads to the outside of the body. At the begining there is an sphincter which stays closed as long as the micturition reflex does not occurs. And in men the urethra is conected to aduct in the reproductive system.
PofUrine: Is made up of water, various amounts of mineral salts, and waste products like urea and uric acid. There are 3 steps, the glomerular filtration that filters the water and eliminates waste products. Tubular reabsorption, that filtered susbtances are reabsorbed. And the Secretion, that eliminates toxic substances are going to be secreted in the collecting duct.

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