Human Cardiac Cycle and Excretory System Functions

The Cardiac Cycle and Heartbeat Phases

• Diastole: Blood enters the right atrium of the heart through the vena cava and the left atrium through the pulmonary veins. The atria dilate while the mitral and tricuspid valves remain closed.
• Atrial Systole: The atria contract, opening the mitral and tricuspid valves, allowing blood to pass into the ventricles.
• Ventricular Diastole: The ventricles swell to receive blood from the atria. The semilunar valves remain closed, preventing blood from entering the arteries from the ventricles.
• Ventricular Systole: The ventricles contract, and the mitral and tricuspid valves close to prevent blood from flowing back into the atria.

Dual Circulation in the Human Body

Circulation is described as double because, during a full cycle, blood passes through the heart twice, establishing two distinct circuits.

The Two Circulatory Circuits

• Pulmonary Circuit: This is established between the heart and the lungs. In this circuit, blood carrying carbon dioxide collected from all organs reaches the right atrium through the superior and inferior vena cava. From there, it moves to the right ventricle and is pumped into the lungs through the pulmonary artery.
• General Circuit: This is established between the heart and the different organs of the body. In this circuit, blood carries oxygen and nutrients to every cell to perform their functions; it also collects carbon dioxide and waste produced during cellular metabolism.

The Excretory System and Waste Management

The various cells of our body produce waste substances that must be expelled. These waste substances first pass into the blood, which is responsible for transporting them to organs that eliminate them from the body.

Excretion is the vital process by which waste substances resulting from cellular metabolism are removed from the blood.

The Urinary System

• Kidneys: These organs filter the blood to remove waste products, resulting in the production of urine.
• Urinary Tract: These are the conduits that carry urine from the kidneys to the outside of the body. They consist of the ureters, the urinary bladder, and the urethra.

Other Organs Involved in Excretion

• Lungs: These remove the carbon dioxide produced during respiration.
• Liver: This organ expels waste from digestion and certain drugs, which are excreted into the bile and eventually eliminated in the feces.
• Sudoriferous Glands: These exocrine glands are located throughout the skin. They produce sweat, a fluid consisting of water and waste substances.