Blood Composition, Heart Function and the Excretory System
Blood Composition
Blood composition: Blood is a connective tissue made up of plasma and different cells. Blood is known as the "sanguine tissue."
Plasma: Plasma is a liquid that makes up about 90% of blood. It contains a variety of dissolved substances: proteins, carbohydrates, lipids and mineral salts (such as O2, CO2 and nitrogen). Plasma without some proteins (such as fibrinogen) is called serum. When blood solidifies it forms a coagulum (clot).
Blood Cells
Red blood cells or erythrocytes: Biconcave cells that lack nuclei, are flexible and deformable. They contain hemoglobin, which transports oxygen and carbon dioxide.
White blood cells or leukocytes: Cells with a large nucleus that come in 5 types: neutrophils, eosinophils, basophils, monocytes and lymphocytes.
Platelets or thrombocytes: Fragments of cells without nuclei, produced from megakaryocytes. They intervene in the process of blood coagulation.
The Heart
The heart is a hollow, cone-shaped muscular organ. A myocardial infarction occurs when part of the heart muscle dies due to lack of blood flow.
The interior of the heart is divided into two halves: the right and the left. Each half has two chambers: the upper chamber, called the atrium, and the lower chamber, called the ventricle. The walls of the ventricles are thicker, because they pump the blood out of the heart.
The atria and ventricles communicate through the atrioventricular valves: the mitral valve connects the left atrium and left ventricle and has 2 leaflets, while the tricuspid valve connects the right atrium and right ventricle and has 3 leaflets.
Entry and exit routes: The veins that bring blood into the heart are the pulmonary veins (four, two from each lung) entering the left atrium, and the superior and inferior vena cava entering the right atrium. The arteries that take blood out of the heart are the pulmonary artery, which leaves the right ventricle and carries blood to the lungs, and the aorta, which leaves the left ventricle and distributes oxygenated blood to the body.
The output of the ventricles is regulated by the sigmoid valves, located at the beginning of the arteries, which prevent blood from flowing back into the heart.
Heart Operation
The heart performs two types of coordinated movements: systole (contraction) and diastole (relaxation). One complete sequence is called the cardiac cycle.
- Atrial diastole: Blood enters the heart into the atria.
- Atrial systole: The atria contract and blood passes into the ventricles.
- Ventricular diastole: Relaxation of the ventricles.
- Ventricular systole: The ventricles contract and the sigmoid valves open.
Dual Circulation
The circulatory system is called double circulation because blood passes twice through the heart, establishing two circuits. This prevents a complete mixture of oxygen-rich and carbon dioxide-rich blood.
Pulmonary Circuit
The pulmonary circuit starts and ends at the heart. Blood loaded with carbon dioxide leaves the right ventricle, travels to the lungs through the pulmonary artery, and returns oxygenated to the left atrium through the pulmonary veins.
General Circuit
The general (systemic) circuit starts and ends at the heart and passes through the different organs of the body. It carries oxygenated blood with nutrients from the left ventricle to all cells, and returns carbon dioxide-rich blood to the right atrium through the vena cava. Its route is longer, so it is called the greater circulation.
The Lymphatic Circulatory System
The lymphatic system collects the excess fluid that leaves the blood capillaries, returning it to the blood and transporting fat absorbed from the intestine. It is formed by the following structures:
- Lymph capillaries: Tiny blind-ended vessels finely distributed throughout the body that collect interstitial fluid.
- Lymphatic vessels: Structures similar to veins. These vessels drain into the blood circulatory system and are composed of valves.
- Lymph nodes: Small swellings located along the lymphatic vessels that filter foreign substances and microorganisms from the lymph. They are abundant in the armpits, groin and neck.
The interstitial fluid enters the lymphatic capillaries and is then called lymph.
Excretion
Excretion is the process by which the body eliminates waste substances produced by cell metabolism. The main excretory organs are the kidneys, lungs, skin (sweat glands) and liver.
The Urinary System
Kidneys: Bean-shaped organs located in the posterior part of the abdominal cavity, on either side of the vertebral column. They filter the blood to eliminate waste substances, forming urine. The cortex and the medulla make up the renal parenchyma.
Urinary tract: The urinary ducts transport urine from the kidneys to the bladder, the bladder stores the urine, and the urethra expels it to the outside.
Excretory Organs
- Lungs: Eliminate carbon dioxide produced by respiration.
- Liver: Eliminates waste products of digestion and excretes some drugs with bile.
- Sweat glands: Exocrine glands distributed throughout the entire skin.
Kidney Nephron Operation
The nephron is the functional unit of the kidney. Its role is to filter and purify blood and form urine.
- Glomerulus: A network of capillaries through which blood circulates and where filtration occurs, allowing waste to pass out.
- Bowman's capsule: A cup-shaped sac surrounding the glomerulus that collects the products of plasma filtration.
- Renal tubule: A coiled tube where water and certain substances are reabsorbed, while others are excreted.
- Collecting duct: A tube common to several nephrons that carries the formed urine to the renal pelvis.
Filtration and reabsorption are the two key processes in kidney function.