Human Circulatory and Excretory Systems: Function and Health
1. The Internal Environment
The internal environment is the set of extracellular fluid found inside an organism. In humans, the internal environment is composed mainly of interstitial plasma and circulating fluids: blood and lymph.
- Interstitial Plasma: Originates from the blood that has been filtered through the thin walls of capillaries.
- Lymph: A liquid that forms from interstitial plasma circulating through the lymph vessels.
2. The Blood Circulatory System
Blood Vessels
- Arteries: Carry blood from the heart to various body organs. Their walls are very thick, strong, and elastic, so they can withstand the high pressure with which blood flows, driven by the heart.
- Veins: Carry blood from various organs to the heart. Their walls are thinner and less elastic than arteries. Blood flows through them at a lower pressure.
- Capillaries: Microscopic vessels forming a network connecting the terminations of the arteries with the start of the veins.
3. Blood
Blood is involved in various functions, including:
- Nutrition: Transports nutrients from the digestive system to all body cells.
- Respiration: Carries oxygen from the lungs to the cells and carbon dioxide produced by these in reverse.
- Excretion: Collects the waste produced by the cells and leads it to the excretory apparatus, so that it can be eliminated.
Blood Composition
- Plasma: A yellowish liquid, up to 90% water.
- Blood Cells: Red blood cells, white blood cells, platelets.
4. The Heart
Routes of Entry and Exit
The atria receive the veins. The left atrium receives the four pulmonary veins, two from each lung. The right atrium receives two vena cava, superior and inferior, with blood from around the body.
The ventricles give rise to the arteries. The right ventricle gives rise to the pulmonary artery, which branches into two branches that carry blood to each lung. The left ventricle gives rise to the aorta, which pumps blood to the body.
The output of blood from the ventricles is regulated by the semilunar valves situated at the beginning of the arteries. These valves open only as a consequence of the contraction of the ventricles and prevent the return of blood within them.
The heart, like the rest of the body’s organs, needs a blood supply that will provide oxygen and nutrients. However, this contribution is not made by the blood circulating through it, but rather it has its own system of coronary arteries and veins.
5. The Functioning of the Heart
The heart function is based on two types of coordinated movements, which occur in the atria and ventricles: one of contraction, or systole, and one of relaxation, or diastole.
Each cardiac cycle consists of various movements that occur regularly:
- Atrial diastole
- Atrial systole
- Ventricular diastole
- Ventricular systole
6. The Dual Circulation
The two circuits that are established are:
- Pulmonary Circuit: Established between the heart and lungs. In this circuit, the blood carrying carbon dioxide, collected in all organs of the body, reaches the right atrium via the superior and inferior vena cava. From there it passes into the right ventricle and out to the lungs through the pulmonary arteries. As it passes through the alveoli, the blood releases carbon dioxide and loads oxygen. Once the exchange of gases occurs, blood is carried through the pulmonary veins to the left atrium of the heart.
- Systemic Circuit: The blood supply to the heart from the lungs enters the left atrium, then passes into the left ventricle of the heart and out through the aorta. The blood from the lungs carries oxygen through the aorta and is distributed throughout the body, where the oxygen in the blood is consumed. After that, it returns to the heart.
8. Circulatory System Diseases
Arteriosclerosis
The term arteriosclerosis means hardening of the arteries. This can cause blood clots and blockages called thrombi, which sometimes break off and reach vital organs like the heart, brain, or lungs, blocking blood flow and leading to thrombosis.
9. Excretion and Excretory Apparatus
The different cells in our body produce a variety of waste substances that, if accumulated in large quantities, would be toxic to the body. These waste products first go into the blood cells, where they are transported to the various excretory organs, which are responsible for eliminating them.
Urinary System
- Kidneys: Two organs located in the back of the abdominal cavity on both sides of the spine. They filter the blood to remove waste products, resulting in urine.
- Urinary Tract: The tubes that carry urine from the kidneys to the outside. They consist of the ureters, bladder, and urethra.
10. The Functioning of the Kidneys
The kidneys are formed by the repetition of a structural and functional unit called the nephron, which filters and purifies the blood, thereby forming urine.
Each human kidney has about a million nephrons. In each of them, we can distinguish the following parts:
- Glomerulus: A crowding of capillaries through which blood flows, from which waste will be removed.
- Bowman’s Capsule: A kind of sac that surrounds the glomerulus. It produces plasma filtration.
- Renal Tubule: A long tube bent into a hairpin. It absorbs water and certain substances, and others are excreted.
- Collecting Duct: A common pipe for several nephrons, which carries urine to the renal pelvis.