Respiratory, Circulatory & Urinary Systems: Structure, Function, and Diseases
Respiratory System
Respiratory Structure
Formed by the lungs and airways.
Components of the Respiratory System
- Nostrils: Two cavities that communicate with the outside through the nares. They are separated from the mouth by the palate and communicate with the pharynx. The mucosa is coated with pituitary cells to heat and clean the air.
- Pharynx: Situated on the posterior wall of the mouth. Connects the nostrils to the larynx through the epiglottis, a membrane that prevents the ingress of solids or liquids into the airways. It is a common stretch of the respiratory and digestive systems.
- Larynx: A cartilaginous tube about 5cm long in front of the esophagus. Its epithelium has folds called vocal cords. The larynx is also known as an organ of phonation.
- Trachea: A tube of 12cm formed by semi-cartilage rings that prevent the closure of the airways. The trachea divides into two branches called the bronchi.
- Bronchi: Found in the interior of the lungs and in turn branch to form bronchioles. The trachea, bronchi, and bronchioles are made of ciliated cells, mucous cells, and prismatic cells that assist in the elimination of pathogens or harmful substances.
- Pulmonary Alveoli: Located at the end of the bronchioles. They are spherical expansions responsible for gas exchange.
- Lungs: Located in the chest above the diaphragm, cone-shaped and divided into three lobes on the right and two on the left to accommodate the heart. They are wrapped by a double membrane called the pleura, within which a fluid is formed that lubricates the lungs during breathing movements. Inside the lung are the bronchi, bronchioles, and pulmonary alveoli. The alveoli are wrapped in a complete network of capillaries where gas exchange occurs, depending on the concentration.
Mechanical Ventilation and Pulmonary Breathing
Breathing is not simply an exchange of gases in the lungs but a process that occurs in every cell of our body. Ventilation is a mechanical process by which atmospheric air enters the lungs through inspiration and the air in the lungs is expelled outward through expiration. Ventilation is based on the principle that volume and pressure are inversely proportional. In the lungs, breathing movements are due to the diaphragm and intercostal muscles. During inspiration, the diaphragm contracts and the levator muscles of the ribs cause a dilation of the ribcage, causing the lungs to expand. When the pressure inside the lungs decreases as a result of chest expansion, atmospheric air enters. During expiration, the chest increases the pressure and the air comes out.
Control of Respiratory Rhythm
The rhythmic activity is controlled by medulla neurons that act on the respiratory muscles. Factors influencing the rate of respiration include pH and oxygen concentration. Breathing is an involuntary activity but can also be controlled voluntarily.
Automatic Control
This control originates in the medulla and spinal cord and is responsible for transmitting the stimulus to the autonomic nervous system, which controls the normal rhythm of inspirations and expirations.
Voluntary Control
It originates from neurons in the cortex and sends information to the medulla and respiratory muscles. This mechanism can disrupt or modulate the respiratory movements.
Role of Alveoli
The blood is responsible for the exchange and transport of respiratory gases between the alveoli and cells of our body through simple diffusion. The concentration of oxygen is higher in the alveoli, so it is transmitted to the blood. Conversely, the carbon dioxide concentration is higher in the blood, so it goes to the alveoli. This exchange occurs due to three factors:
- Existence of a network of capillaries around the alveoli.
- The walls of the alveoli and the capillaries are made of very thin endothelium.
- High degree of humidity, which allows the rapid dissemination of CO2 and O2 dissolved in water.
Diseases
Common respiratory diseases include inflammation of the airways. Factors causing these diseases usually tend to be infectious microorganisms, but other factors that accentuate these pathologies are cold, dampness, and snuff.
Circulatory System
Heart
Located in the chest between the lungs and above the diaphragm. It acts as a pump that allows the movement of blood into the arteries and veins. The heart is enclosed in a membrane called the pericardium. It is approximately the size of a fist and weighs approximately 270 grams in an adult. It consists of four separate compartments: the right ventricle and left ventricle, and the right atrium and left atrium. The atria have thin walls, while the ventricles have thicker walls for stronger contractions. Atria receive blood from the veins, the right atrium from the superior and inferior vena cava, and the left atrium from the pulmonary veins. Blood passes from the right ventricle to the pulmonary artery and from the left ventricle to the aorta. Blood circulation through the heart chambers is controlled by valves.
Cardiac Cycle
The heart pumps blood through the blood vessels. When the blood reaches the atria, they contract, driving the blood into the ventricles (atrial systole). This is followed by ventricular contraction, which pushes blood into the arteries (ventricular systole). Then, for a few tenths of a second, the heart relaxes (diastole). The normal heart rate is approximately 70 beats per minute and depends on age, sex, and physical activity.
Blood Vessels
Blood vessels are responsible for distributing blood throughout the body. They consist of veins, arteries, and capillaries, all of which consist of three layers: the inner endothelium, the middle muscle cell layer, and the outer connective tissue layer. Arteries have a thicker muscle layer than veins, while capillaries are only made of endothelium.
Circulation
The body has a double circulation system:
- Pulmonary Circulation: Oxygen-poor blood passes from the right ventricle to the pulmonary artery and goes to the lungs, where gas exchange occurs. Carbon dioxide passes into the alveoli and oxygen into the blood. Oxygen-rich blood returns to the heart through the pulmonary veins and enters the left atrium.
- Systemic Circulation: Oxygen-rich blood passes from the left ventricle to the aorta and is distributed to all organs of the body. Veins collect oxygen-poor blood full of carbon dioxide and return it to the heart through the superior and inferior vena cava, entering the right atrium.
Urinary System
Metabolic Products
Metabolism generates toxic products that can be harmful if they accumulate in the cell. The major pollutants are carbon dioxide, urea, ammonia, and uric acid. Carbon dioxide is expelled through the respiratory system, while urea, uric acid, and ammonia are expelled through the urinary tract.
Structure
The urinary system is composed of two kidneys and the urinary tract, which includes two ureters, a bladder, and a urethra.
Kidneys
Two organs located in the abdominal cavity. Their function is to filter blood and produce urine, thanks to functional units called nephrons. Kidneys are well-supplied with blood and have a concave area called the pelvis where the renal artery and vein enter and exit. A longitudinal cut of the kidney reveals different layers: the capsule, cortex, medulla, and pelvis.
Urinary Tract
- Ureters: Ducts that connect the kidneys to the bladder.
- Bladder: An expandable sac at the base of the abdominal cavity. Its normal capacity is 350 ml but can hold up to 2 liters.