Heart, Blood Circulation, and Excretory System Explained

Heart Anatomy and Function

Chambers and Valves

The heart is a muscular, hollow organ divided into four chambers:

• Upper Chambers (Atria): Left atrium and right atrium.
• Lower Chambers (Ventricles): Left ventricle and right ventricle.

Between the atrium and ventricle, the atrioventricular valve allows blood flow from the atrium to the ventricle but prevents backflow. The valve between the left atrium and ventricle is bicuspid, while the valve between the right atrium and ventricle is tricuspid.

Blood Circulation

Cardiac Cycle

• Atrial Systole: Atria contract, pumping blood into the ventricles through open atrioventricular valves.
• Ventricular Systole: Ventricles contract, sigmoid valves open, and blood flows into the pulmonary artery and aorta. Atrioventricular valves close to prevent backflow.
• Diastole: The heart relaxes and receives blood from the vena cava and pulmonary veins, filling the atria. Sigmoid valves prevent backflow into the ventricles.

Double Circulation

• Systemic Circulation: Blood flows from the left ventricle through the aorta, arteries, arterioles, and capillaries to all tissues and organs except the lungs.
• Pulmonary Circulation: Blood flows from the right ventricle through the pulmonary artery to the lungs, where gas exchange occurs in the capillaries surrounding the pulmonary alveoli.

Excretory System

The excretory system comprises the urinary system, sweat glands, liver, and respiratory system.

Kidneys

The kidneys are bean-shaped organs located at the back of the abdomen. They consist of:

• Renal Cortex: Outer layer
• Renal Medulla: Inner layer
• Renal Pelvis: Collects urine

Each kidney contains over a million nephrons, which filter blood and produce urine. Nephrons are composed of Bowman's capsule, the convoluted tubule, and the Loop of Henle.

Urinary Tract

• Ureters: Two tubes (approximately 28 cm long) connecting the renal pelvis to the urinary bladder.
• Urinary Bladder: Stores urine.
• Urethra: Duct leading urine outside the body. Contains a sphincter controlled by the micturition reflex.

Urine Formation

Urine consists of water, mineral salts, and waste products like urea and uric acid. Formation involves three steps:

• Glomerular Filtration: Filters water and waste products from the blood.
• Tubular Reabsorption: Reabsorbs essential substances back into the bloodstream.
• Secretion: Eliminates toxic substances into the collecting duct.